ECU Diagnostics – part 2 : ECU, OBD and CAN


Caterhams, like all modern cars, have an Engine Control Unit (ECU) – a black box full of electronics, controlled by a microprocessor that manages how the engine runs.  And because it has a microprocessor it means it runs some software to control everything. It also connects to a bunch of sensors, like temperature, pressure, engine speed, air-flow, lambda etc which it then uses to set the engine’s inputs, things like ignition advance/retard, air/fuel mixture and other stuff.

The ECU on a factory, or self-build car, is manufactured by MBE Systems and seems to be supplied to Caterham through SBDMotorsport in the UK. The model used by Caterham is the 9A4 that runs Caterham specific software and mapping. It is also “locked” – meaning that there is no user accessible way of modifying the software or the map. We’ll talk about maps and software in a later post.

What these MBE ECU’s do allow though, is access to the internal sensor information used by the ECU to run the engine. There are many, many parameters used by the software to make an engine run properly and we hoped to be able to see all this information in “real time” – meaning we can see the data change in front of our eyes, hopefully being updated many times per second.

It’s also important to understand that there is both primary data (taken directly from physical sensors) and derived data (secondary, extrapolated or calculated data derived from the primary data). We’ll hopefully be able to see both.

The standard location for the ECU is underneath the battery in the engine bay:

MBE 9A4 ECU located under the battery and behind the grey plug connecting it to the wiring loom


In order for us to be able to interrogate the ECU and extract all this data, we need some way of connecting a computer into the ECU. Handily, legislation has been in place for a number of years that requires car manufacturers to fit a car with a diagnostic connector. This is known as the OBD port and looks like this:

OBD connector located above the ignition switch on the steering column

It’s through this connector that we’ll be getting access to the ECU and hopefully seeing what’s going on. The OBD port on a Caterham is located above the steering column underneath the dash. On my 2017 car it is loosely mounted, dangling on a short length of electrical cables that run to the connector. So you need to be careful when handling it so you don’t pull any of the cables out of the connector.

There are a bunch of standards that define the physical and electrical OBD connector used on cars. The data that’s sent through this connector (I’ll also call it a port) is mostly open to the car manufacturer as to what gets sent and what standard transmission protocols are supported. It’s also very common that the data sent on these ports is proprietary to the particular ECU manufacturer, each one having their own protocols.

For those reading this that aren’t quite so into communications protocols – a protocol is like a language. Computers and tech stuff communicate between each other using protocols. These protocols define what gets sent between two computers and how to interpret it. For instance, there are now over 8000 protocols that define how devices communicate over the internet, and more are being added every day. Check out the IETF RFC pages to learn more about those protocols and standards.

To make things more complicated, manufacturers of different equipment may interpret the protocol definitions (known as “standards”) differently and so end up with dialects of these languages. So the protocols differ from one manufacturer to another just like different dialects of French or English.

Fortunately, there is one protocol that does seem to be reasonably well followed by most car manufacturers and that gives us something to start with. That protocol is often known as OBD-II (OBD 2). We’ll go into more detail about the protocols supported by the MBU 9A4 in a future post.

As we started this investigation, learning about a Caterham’s ECU diagnostic port, we knew only a few things:

  • SBD Motorsport supplies a piece of Windows software (Easimap) that can get all the available data points from the car
  • OBD-II Scanners can access some data from the car (though not as much as Easimap)
  • People have tried to access the diagnostic data on a Caterham before and not been able to find much

So, we started with a general assumption that because the Easimap software could interrogate the car for lots of data, then hopefully we could too. Though one thing on our minds as we started out on this journey, was whether the data exchange (protocol) used between the car and the Easimap software was encrypted – that would cause a whole further level of investigation and might lead to all this effort being fruitless.

Early on in our investigations we wondered if Caterham and MBE were doing something special (in an electrical sense) on the OBD port that was proprietary. However, a quick check under the dash and we can see that Caterhams have only 4 wires connected to the OBD port. There is 0v, 12v, CAN-L and CAN-H. CAN-L and CAN-H make up a twisted pair, see CAN bus below for more on those signals. So, we could see that none of the reserved connections on the OBD port were being used for anything special.

OBD Connector. 2 x 0v (black), 12v (purple), CAN-L and CAN-H


There’s another twist to the OBD port. It’s one thing to know what it physically looks like, what its electrical connections are and how to connect to it, but its another to be able to understand the data that’s flowing through the connector.

The first layer of that puzzle is to know that two of the twelve electrical connections on an OBD port support a standard called CAN bus. CAN stands for Controller Area Network and is used in industrial and automotive systems. It’s called a network because you can have many CAN bus devices connected in a network. In computing terms a “bus” is something that transfers data from one device in the network to another, we often say that a “bus is shared” between the devices. Modern cars can have more than 50 devices in their CAN bus network; controlling everything from the engine, gearbox, infotainment systems, door locks, windows, lights, brakes and everything in between. My 420R 2017 Caterham has only one CAN bus device, the ECU. It seems that other cars (such as Sigma engined cars also have the rev-counter connected to the CAN bus – but not on my Duratec car).

Most modern cars also have multiple CAN busses: private ones, not directly connected to the under-dash OBD port, for the safety critical components and public ones that are connected to the OBD port and more readily accessible to people like us. These private and public CAN busses are often connected together via a CAN bus bridge. There’s no evidence so far that there is any more than one CAN bus on a Caterham, and therefore no bridges.

CAN bus defines both an electrical connection (wiring, what each wire does, voltages, termination impedances etc) but also a transmission protocol… i.e. what sequences of voltage changes mean (how fast are the changes (bitrate), is it digital or analogue, is it serial or parallel communication, etc.).

So the CAN bus standard defines all this with the following key points for Caterham tinkerers:

  • There are two electrical signals making up a single serial data channel. The two signals are differentially coded, meaning you take the two electrical cables as a pair and use the voltage difference between the two to give you a single signal. The two signals are called CAN-L and CAN-H (CAN Low and CAN High). And Soto measure the data in the bus you measure the voltage difference between CAN-L and CAN-H.
  • When there’s no data being transferred on the bus the two data lines, CAN-L and CAN-H sit at about 2.5V. It’s one of the main ways you can find a CAN bus on a car… looks for pairs of wires that have a voltage of about 2.5V on them, there isn’t much other wiring at that voltage on a car.
  • The two CAN-L and CAN-H signals combine to create a single serial data stream – meaning digital bits are transferred one at a time in succession.
  • CANBus can run up to 1Mbps (mega-bits-per-second). On a Caterham the speed is set to 500,000 bps.
  • There are three main types of CAN bus signalling. Standard addressing (using 11 bit identifiers (IDs)), Extended addressing (using 29bit IDs) and CAN-FD (or Flexible Data – sending larger amount of data in one go). Caterham’s use Extended addressing and not FD.
  • A stream of bits being sent on the bus forms a frame.
  • Each frame contains information about:
    • when the frame starts
    • what type of frame it is (standard, extended, error, etc)
    • how much data is in the frame (up to 8 bytes)
    • error checking
    • acknowledgement bits and
    • end bits
  • Data on the CAN bus can be either:
    • Broadcast: a device on the bus continuously sends data at pre-determined intervals. The 9A4 ECU does broadcast some information on the bus (I’ve called this protocol “MBE-Broadcast”) and I’ll talk about this in a future post
    • Request/Response: This is how the OBD-II protocol works and how Easimap communicates with the car. A device that want’s data (Easimap on a computer or a sniffer/scanner) “requests” data from the ECU and the ECU “responds” with the data. We’ll talk about the “OBD-II” and “MBE-ISOTP” protocol in later post.

For those interested, the format of a CAN bus packet is shown in the diagram below. Later on we’ll see that all we’re really interested in is the ID, Extended ID and the Data Field. The rest of the protocol is either handled by the CAN bus chipset or by the low level software (in my case that’s the Linux SocketCAN kernel driver).

Extended 29bit CAN bus packet format. From the Microchip MPC2515 datasheet. 

If that diagram isn’t making sense then perhaps this paragraph is for you: Each of the oblong boxes running across the centre of the diagram represents a single bit transmitted on the CAN bus. Therefore, time runs from left to right, so the leftmost oblong (bit) on the diagram is sent on the CANbus first, followed by the second, etc etc until the last bit represented by the rightmost oblong on the diagram is sent last. All those bits together are a CANbus frame and can contain up to 8 bytes of data on the MBE 9A4 CAN bus.

If you want more info on what all the stuff means in the diagram then I’d recommend heading over to the CANbus wikipedia link, I can’t add much more to what they say there.

There’ll be a post later in this series about me connecting my Logic Analyzer to the CAN bus and hopefully the screenshots there will explain a little more about how data is sent on CAN bus.

Arbitration Fields, CAN bus IDs and Addresses

Having said that, I hope I can clear up one thing that caused me confusion at first. The diagram above and the wikipedia page shows a field called the “Arbitration Field”.

At the physical level (i.e. the 0’s and 1’s on the electrical wires) the arbitration field controls who gets access to the bus. It does this because if two devices start to transmit on the bus at the same time then the one with the lowest numeric around bitration field code “wins” and continues to transmit, while the second device with the higher arbitration code backs off and tries again later. However, at a higher level, such as SocketCAN (see below), these arbitration fields are also like addresses, in computer terms, and different devices will listen for different arbitration fields which are known as ID’s (identifiers).

And for even higher software levels such as ISOTP (see a future post) they are also known as transmit and receive addresses. 

That’s all a little confusing, but all you really need to know is that Arbitration Fields, CAN bus ID’s and the ISOTP Receive and Transmit Addresses are all the same thing.

CAN bus Software

We’ll be talking some more about the CAN bus software in a future post but for the moment it’s worth pointing out that there seem to be a few ways of accessing CAN bus data for writing your own code on a computer.

I’m mainly interested in using Python as a programming language to do my tinkering and in my opinion the only way to do that is with a Unix derived system. And that means either Linux (RedHat, Ubuntu and their derivatives) or MacOS (BSD based).

… and for those operating systems and for Python there are a couple of ways of poking at the CAN bus.

  1. SocketCAN: this is the way I went. For me it gives closer access to the bus and allows me to use packet capture software like Wireshark. The CAN bus drivers are included in modern kernels and there is good support for other kernel mode protocols like ISOTP that we’ll need later. The CAN bus kernel module creates an interface that makes the CAN bus look like a network socket and so turns CAN bus frames into packets like an IP packet. These packets are then available to software applications like Wireshark so they can be captured and viewed. Python has a good wrapper for SocketCAN which is simply called python-can.
  2. ELM327: Another way to get CAN bus frames into a computer is to use a ELM327, or similar, device. They can connect to phones and computers using Bluetooth or Wifi and have python support through the python-OBD project. They’re good because they mean your computer can be remote from the car. But I’m not a big fan of Wireless for critical connections and I’m hoping that my future OBD dashboard will be critical. They also have to work in “user space” (i.e. not in the kernel) and so can suffer from Linux’s susceptibility to not being as “real time” as some other OSes.

Yet another route for accessing CAN bus is through even smaller computing devices. I’ve spent a lot of time playing with the Arduino and Adafruit Feather devices over the years and I’m thinking that at some point I might switch from the Linux environment to Arduino for this project. These Arduino derivatives have their own programming environment and software drivers for things like CAN bus and they are also more “real time” than OSes like Linux and especially Windows. There are CANbus “shields” for Arduino and even Feather CANbus boards for the Adafruit feather line. But for the moment, Linux is a great way to figure out what’s going on with the car and to get rapid development done.

Example CAN packets

We’ll get into discussing the protocols used on a CAN bus in much more detail in furture posts, but for the moment here’s what three CAN bus frames look like to and from the car. The first two packets in the diagram below are to the car from Easimap and the third is the car’s response back to Easimap. The frames below are how Wireshark (a packet sniffer software tool) displays the frames once captured from the CAN bus, it takes the serial stream of bits and decodes them into what each bit means, displaying them in a more “human readable” form…

Frame 626:
Controller Area Network
...0 1100 1011 1110 0001 0001 0000 0001 = Identifier: 0x0cbe1101
1... .... .... .... .... .... .... .... = Extended Flag: True
.0.. .... .... .... .... .... .... .... = Rem. Tx Req. Flag: False
..0. .... .... .... .... .... .... .... = Error Flag: False
Data: 10 0a 01 00 00 00 00 12

Frame 627:
Controller Area Network
...0 1100 1011 1110 0001 0001 0000 0001 = Identifier: 0x0cbe1101
1... .... .... .... .... .... .... .... = Extended Flag: True
.0.. .... .... .... .... .... .... .... = Rem. Tx Req. Flag: False
..0. .... .... .... .... .... .... .... = Error Flag: False
Data: 21 66 67 a8 a9 00 00 12

Frame 628:
Controller Area Network
...0 1100 1011 1110 0000 0001 0001 0001 = Identifier: 0x0cbe0111
1... .... .... .... .... .... .... .... = Extended Flag: True
.0.. .... .... .... .... .... .... .... = Rem. Tx Req. Flag: False
..0. .... .... .... .... .... .... .... = Error Flag: False
Data: 05 81 aa aa 16 00 12 66

CANbus packets will be printed differently depending on which software you use to look at them. In this particular example from Wireshark:

  • 0x0cbe1101 and 0x0cbe0111 are CAN bus identifiers (ID’s) or Arbitration fields written in hexadecimal (the 0x means hexadecimal). From the discussion above about arbitration, the device sending the ID of 0x0cbe0111 would take priority over the device sending 0x0cbe1101, because 0xcbe0111 is less than 0x0cbe1101 . The first two frames with an ID of 0x0cbe1101 are sent from Easimap and the last frame with an ID of 0x0cbe0111 is the response from the car’s ECU back to Easimap
  • There are various flags shown, extended flag (meaning 29bit IDs), Remote Transmission Request Flag and the error flag.
  • The hexadecimal numbers on the Data line are the data, ie the stuff we’re really going to be interested in like engine speed and fuel mixture. 

Well that’s about it for the basics of ECU’s, OBD and CAN bus. We’ll build on all of this in future posts.

ECU Diagnostics – part 1 : Introduction

I’d been planning to have a look at the car’s On Board Diagnostic (OBD) port for over a year, but hadn’t really got into the project. I’d bought the required cables and some test equipment, but other stuff always seemed to be higher priority. That was all about to change with a post from Mark (CtrMint) on‘s Blatchat.

The thinking behind my project was to create a “device” that could plug into the diagnostic port on my car and provide a readout of what the car was thinking… perhaps that might turn into a simple project like change-lights or a race-computer – but a bit more high-tech than a few LEDs on a black box.

So that was my motivation, a home brew race-computer come dashboard come change-lights.

But back to Blatchat… Mark had asked for help looking into ECU Diagnostics and had started a long thread on what he thought was going on (here’s the thread link for club members). Seeing as it was something on my list too I thought I’d pitch in. I hadn’t appreciated how much I’d get sucked into the project, but now, about a month later and many, many hours of work I think I know enough to provide a few posts on the subject.

By the way, I know I’ve plugged Mark’s website before but he does a great job of blogging his Caterham life.

In all though, I think this is going to be a bit more than a few posts… we’ve been busy. It’ll be more like 15+ posts I think.

There’s a lot to talk about and I want to break it down so others can dip into the subjects and not have to plough through one monstrously long post. Some readers will want to get into all the nitty gritty and others will just want to browse. I’ll try and summarise things at the end for people who just want the conclusion.

We’ll be getting into the details of the protocols being used by the ECU, how we decoded those protocols, what we learnt, what software we used, and what software we had to change and/or write, and what we’re hoping to achieve with what we’ve learnt.

My intention with these posts is not to repeat what you can find in other places on the internet about car diagnostics. This is meant to be about diagnostics on a Caterham and where appropriate to point you at other resources to find out more detail, or even the basics. Hopefully it will pull a lot of things together to show what we did, how we did it and what it means for people wanting to find out more about Caterham ECU diagnostics. I hope that makes sense.

I’ve written the posts to be read by someone who is technically minded (aren’t all Caterham owners? 🙂 ) but who may not be steeped in internet knowledge. Hopefully those of you that are internet nerds won’t mind the extra words and can also find what you need without despairing at my efforts to keep everyone up to speed.


So here we go for a marathon series of posts. I’ll update this page with links to the completed posts as they come along…

  1. Introduction
  2. ECUs, OBD and CAN
  3. Test Setup
  4. Wireshark Patching and OBD-II Results
  5. The Correlator Dead-End
  6. Reading Material
  7. ECU Maps and Mapping
  8. Easimap uses ISTOP (sort of)
  9. The Easimap Protocol Theory
  10. Decoding EC2 Files
  11. Logic Analyzer on a CAN bus
  12. OSI 7 Layers for Caterham Diagnostics
  13. Three Diagnostic Protocols in the MBE 9A4 ECU
    1. Protocol 1 – MBE-Broadcast
    2. Protocol 2 – OBD-II
    3. Protocol 3 – MBE-ISOTP
  14. Software Framework
  15. How To: Raspberry Pi and Software Setup
  16. Where Next

Visit to Millwood for Flat-floor and Stalling Problem ECU Reset

So, after over 2000 miles in the car, it was time to address one of its biggest niggles… low temperature stalling.

Ever since I’d built the car it would stall at low engine temperatures. It would stall as I sat on the drive putting my harness on, it would stall at junctions for the first 4 or 5 miles and it would stall if gear changes were taken too leisurely. It needed a good 90-95C showing on the temperature gauge before it would idle anything close to ok. And even then it would occasionally stall when I de-clutched at junctions or changing gear.

The car was a real PITA to drive for anyone else and the final straw came when a co-pilot had a minor shunt after he’d stalled when pulling away from a junction – new rear wing needed! I’d got a few other events in 2019 set where other people would be driving the car and so it needed to get sorted.

What Needed to Get Done?

By this time I’d convinced myself that the car needed an ECU remap, it seemed as though the ECU was failing to “catch” the drop in revs as the engine speed dropped on a de-clutch when it was cold. But it also stalled when started from cold, so something was afoot!

I had the tick-over set at about 900 rpm, so I didn’t think it was that. I could certainly stop the stalling by increasing the tickover to something more like 1200 rpm, but that seemed like too much of a workaround. What seemed to make things worse was the light flywheel action… revs seemed to drop too quickly for the map to react to the drop.

Now, Caterham’s supplied ECU mappings don’t get a good rep on the internet forums or Facebook. And because the ECU map is locked then you need a new ECU if you want it remapped. That was going to set me back at least £500 for an ECU plus the cost of getting it mapped, probably on a rolling road. And of course all the talk on the internet had convinced me that this was what was going to be needed.

Then in a rush of common sense, I decided it would be a good idea to get someone who knows what they’re doing to have a look. But who?

Millwood Caterham

I don’t have a great deal of experience with Catherham servicing so I turned to the Internet for help. It seems that The Two Steves, Dave Gemzoe or Northampton Motorsport would be good places to go, especially for a remap. But I wanted something closer to home and hopefully less expensive.

I got the car from Williams originally and they had done the Post Build Check. I don’t have anything against Williams, but the fact that this stalling problem had not been picked up at the PBC and the desire to try someone new, led me to Millwood in Dursley, Gloucestershire. I gave Eric a call at Millwood and the car was booked in about 2 weeks later, early June 2019 (Eric was away for part of that time so couldn’t be any earlier).

Here’s their promo picture from their website (…

Millwood Caterham (courtesy of their website)

I turned up to Millwood after a quick early morning run up the M5. Millwood’s showroom is not particularly difficult to find but it’s tucked away behind a petrol station. The showroom probably holds about a dozen Caterhams and has a single bay workshop at the rear of the place.

Eric was working on another car when I arrived but was straight out to meet me and we got into talking about what needed to be done.

Because the stalling was only really happening when cold we decided to have a go at a flat-floor setup to let the car cool down a bit before looking at the stall problem. So, we rolled the car into the workshop bay and onto the 4 mobile weight scales.

Eric doing some measuring before putting the car on the scales

Flat Flooring

Now, of course, I’m new to all of this, so if this is all old-news to you then you might want to skip ahead. Or, if I’ve got it wrong then let me know.

But for the novices amongst us, the idea of a flat-floor setup is to get the weights of each corner of the car about equal when you’re sat in it. It’s a bit of a misnomer that this has to be perfect, as the weights are going to change as you corner and the desired setup’s going to need to change if you’re tracking the car, depending on the circuit and for instance whether it is a clock-wise or counter-clock-wise circuit (more rights or more lefts respectively).

Eric was working on getting the left and right sides the right height and then diagonal weights about equal. With me sitting in it, clearly the LF/RR diagonal was going to be heavier than the RF/LR diagonal, but he thought my car was about as good as we were going to get. He changed a couple of the damper settings a little, but not that much in the end. It didn’t seem as thought Eric has a particular set of ratios or settings in mind, he looked at what the first set of readings were and worked from there. Though maybe I’m doing him a disservice!

He did give the front OS a 0.5 degree toe-in which he says helps with road driving and steering feel a bit, but I might not notice much. I also had about 7Kg of tools and “emergency” stuff in the boot which we had to remove.

Here are the eventual weights:

Top set are with me in the car and the tools still in the boot. Middle set of weights is without me sitting in the car, bottom set with me in it (both middle and bottom set have the tools removed). There’s around 20L of fuel in the car.

and here’s the machine:

Weigh Scales

Now, to interpret the (middle set) of weights, the LF, RF, LR and RR are obviously each corner weight. The “Left 49.3%” is the LF and LR added together and taken as a percentage of the total weight of the car:

(148.0  + 156.5)  / 617.5 = 49.3%

It’s slightly heavier on the right… not a surprise, the exhaust system and steering column are on the right for a start. The “Rear 50.6%” is the Rear/Front split, which is a lot more even than I thought it would be, I thought it might actually be front heavy, but the engine is quite a way back in the frame I guess. Then the “CR 50.7%” is showing that the LR/RR diagonal is 50.7% of the total weight.

I was a little surprised at the 617.5Kg total weight. With about 15Kg of fuel (20L) and we assume 210bhp, then we’re actually at about 350bhp/tonne. Still respectable but not the headline 420bhp/tonne that Caterham would like you to think. On my car its the heater, leather seats, carpets etc that are all adding up.

It’s also interesting that with me sitting in the car, 75% of my weight (67Kg/89Kg) goes to the rear of the car. That makes sense of course, I’m sitting way closer to the rear wheels than the front, but its interesting to put a number on that. And almost the same can be said of my weight being added to the right (63/89 = 70%), again making sense but interesting to see the numbers.

After a few tweaks that was the suspension set up. Eric only dropped the front slightly in the end (about 5mm) and adjusted the dampers slightly to get the diagonal weights as seen in the last set of readings. It would take the ride home in the car to tell if anything felt different.

Stalling Problem

Now onto the stalling problem. Eric sat in the car and fired it up. It died. He fired it up again and it died again.

So Eric has seen on ton of these and he went straight to what he thought the problem was. He set about adjusting the tickover to be just below 1000rpm. Doing this reasonably quickly so as not to warm the engine up. That took a minute maybe.

Next Eric sat in the drivers seat and turned on the ignition, disarming the immobiliser and proceeded to repeatedly turn the ignition from position 1 to position 2. I’m pretty sure it was 1 to 2 and back again, rather than 0 to 2 and back, but I’ll have to check that.

As he was doing this he explained that the ECU often gets the “zero” position of the throttle potentiometer incorrectly set. If you play with the idle screw or if it becomes mis-configured (he didn’t go into why that might happen) then you need to go through this procedure again:

So the ECU reset procedure for a factory Duratec should be:

  • Set the idle tick-over to just less than 1000rpm (I’ve heard that Catherham suggest 950)
  • Turn ignition to position 2 and immobilise the car
  • Repeat ignition position 1 to position 2 and back again 10 times

This procedure tells the ECU to remember this new potentiometer value to be the zero “home” position for the throttle. It may also do other things like reset the TPS sensor and do things with the Lambda sensor but I’ve found no discussion on the internet yet about that for Duratecs. Also note that this seems to be a different procedure than for K-Series cars – with those there’s a procedure that involves depressing the throttle 5 times while the engine is off but the ignition is on (see Google for exact instructions).

Once Eric had done this he fired the car up again and it didn’t immediately die. It tried to a couple of times but after a couple of quick blips to the throttle it sat idling, better than it had ever done cold.

And that was it. 

While all this was happening, Eric and I had drunk coffee and talked a lot about the history of Millwood: how they used to be the Caterham dealership until Williams took over, how Eric used to be the 2nd for Burns in the WRC and how he and Jon had come to an agreement that Eric would work for Millwood after giving up his separate servicing business (seemed to be a lot of running of various rally cars for himself and for privateers since leaving Subaru).

We finished up and I set off home again after being there for about a couple of hours.

It’s also interesting to note that while I was there, Eric took a couple of telephone calls and helped those people out. There was also another customer that was negotiating a crisis that showed up and Eric helped out while I was there too. There didn’t seem to be any formalised process for this, just helping customers with their cars.


As I write this, I’ve put another 700 miles on the car in about 2 months since visiting Millwood. The car now feels more “sure footed” in corners and less dithery on the straight ahead, that was noticeable within the first 200m of leaving Millwood. I suspect that’s the toe-in that Eric didn’t think I’d feel much. The steering is probably a little heavier and there’s more confidence in how it drives and on straights it seems to stay on-track rather than wandering around like it did before.

As for the stalling. It’s much, much better. But still not perfect. It takes a couple of minutes of warming up in the mornings now before it will not die at a junction. Also the occasional stall at a junction and between gears has almost completely gone. I’m going to try the reset procedure again myself when the engine is really cold, see if that makes difference.

It’s also so much more pleasant to drive. I hadn’t appreciated how much low down go the car has. I can now happily do a rolling drive through a junction without having to heel-and-toe and slip the clutch. It pootles around town now without the kangarooing I was getting and I often short shift, missing a gear now from 2nd to 4th or 1st to 3rd, because its so much easier to drive.

This all now makes complete sense, if the ECU doesn’t know where the throttle is then it’s not going to be able to set the correct mapping.

For the moment, the need for a remap has gone. I still find it a bit unnerving how the engine revs drop so quickly when de-clutching, but at least the ECU catches the drop now before a stall. The light flywheel is also a pain if you want to pootle – gearshifts have to pretty quick not to need a blip going up the gears and if you’re too quick you’ll beat the gearbox and crunch a gear.

The clutch is also still pretty fierce and there’s not much to be had from starting off in 2nd (unless you’re on a down-hill) seeing as you’ll burn the clutch trying to make a clean getaway. So, for me, always start off in 1st, make sure your gear changes are quick, but not fast, and keep it in a higher gear around town. A quick point about that, in Bristol where I live, almost everywhere is set to 20mph, there’s very little point in attempting any spirited driving or else you’ll loose your license. I’ll leave the spirited stuff to some safe open roads and the track.

FWIW, I always heel-and-toe the throttle on a downshift – with that engine burble, why wouldn’t you. It’s also how I drive all manual cars, so why not this one.

In the end less than a couple of hundred quid for some time with an expert, a coffee and a chat was well worth it from my perspective.

Millwood are highly recommended.

[ PS: Thanks to Mark from for spurring me into action and getting some more posts out. He’s got a great site by the way]

Caterham 420R Build Tools and Trinkets

I thought I owned them all until I built a Caterham.

Sorry to all you non-builders, another technical post I’m afraid.

I’d been planning to do this post since finishing the build but I only finally got it over the line after being contacted by a number of prospective builders all asking me about tools. There’ll also, hopefully, be another one soon about “consumables”.

How Many Tools are Enough

Clearly any discussion about the number of tools needed is going to end up showing the total to be N+1, where N is the current number of tools you own. The same rule applies to all sorts of things… bikes (both kinds), t-shirts, books, car magazine subscriptions… and oddly enough – unicycles!

Having said that, I did have almost all of the essential tools, and probably could have got by without buying many at all. But I did end up buying a number of additional tools, of course! Some were just to help make things easier, some were to speed up the process. And some were because the one’s I had were a bit tired and I liked the idea of an upgrade. In some cases, like a socket set, I was missing a few sockets and decided now was the time to buy afresh. One or two tools I bought and didn’t turn out to be useful at all – but surprisingly few actually!

Is This List Definitive?

The discussion below is not meant to be definitive, exhaustive or for that matter definitively instructive. It’s what I used and may give you food for thought if you’re thinking about building a Caterham.

The tools listed are just for the build. As time goes on you (I) will need other tools like an oil-filter wrench and feeler gauges etc etc. But here goes for the build tools…

To the Tools

I come from the school of thought that says that:

At a wedding, there should be just one photo taken that includes everyone attending

It seems I think the same way about tools (I hope nobody was at the bar when it was taken)…

Almost all the tools I used to build our Caterham 420R

The image above contains all the main tools (I think). Of course I also have others that got used that are not shown but hopefully there’s nothing major missing.

For instance I have a lot more screwdrivers, pliers, cutters, hammers, many spanners, etc etc, that are not shown. The ones that are shown are, I think, indicative of what’s needed. I also tended to use socket sets instead of dedicated tools… I’d often grab the 1/4″ socket set for: allen-head bits, posi-head bits, small sockets. I always seemed to be pulling that set out and diving into it. The 1/4″ socket set (blue lidded socket set in the middle of the picture) was ideal because it had just about everything in it I needed and it was small enough to fit in, and around, all the tight spaces of the car. Having said that, if I had to have only one socket set then it would be a 1/2″ set in both metric and imperial.

The only major things missing from that picture above are the axel stands I used (highly recommended for small garages or where you want to get the car outside to work on)…

Front ans rear CJ Autos Axel Stands – I trimmed the excess of the cross bars after taking the picture

… and of course there was the engine hoist (borrowed)…

Engine hoist and leveler in use

You may disagree with how I classify the following categories, it is just my opinion in the end.

I’ll give a quick list and then discuss them in more detail below.


Here’s a list of the really useful stuff. If something isn’t critical then it’s listed elsewhere:

  • Allenkey sets
  • Axle Stands
  • Brake bleeding kit
  • Cordless Drill
  • Dolly
  • Dremel
  • Drill bits
  • Dust mask
  • Ear defenders
  • Engine hoist and leveller
  • Eye protection
  • Funnels
  • Files
  • G-clamps
  • Grease gun
  • Grips
  • Hacksaw
  • Hammers
  • Hole punches
  • Low profile Trolley Jack
  • Multi meter
  • Petrol can and/or jerry can
  • Pliers
  • Revolving Punch Pliers
  • Rivet tool
  • Scissors
  • Screwdrivers
  • Side Cutters
  • Socket Sets
  • Soldering Iron
  • Spanners
  • Special Sockets
  • Stanley knife and blades
  • Tape measure
  • Torch
  • Torque Wrench
  • Vice

You May Need These

  • Brake Piston/Caliper Windback Tool
  • Circlip pliers
  • Tap and die set

The Time Savers

The items below would sit squarely on my essentials list if I were to do the job again. That’s not to say you need them, I just thought they saved me a bunch of time and they were worth the expense to me and I had many of them already anyway.

  • Crows Foot Spanner’s
  • Creeper/Stool
  • Digital calipers
  • Drill Rivet Attachment
  • Hot Air Gun
  • Hot-glue Gun
  • Laser Measurer
  • Rotating Hole punch – highly recommended
  • Scissor or low profile bottle jack
  • Tweezers

The Trinkets

Some of the tools I bought were more to test things out than because I thought they were going to be really useful. I just wanted to see how that worked or it was clearly going overkil. I thought this list would be longer when I created this section…

  • Rivnut tool
  • Tablet in the garage

Wouldn’t Recommend

Some stuff I bought I wouldn’t recommend. Other’s might find useful but I didn’t.

  • Pipe bender – for brake pipes. Some people may find they have the time to bend brake pipes with a pipe bender but I ran out of patience. It was much more fiddly than the usual central heating 15mm pipe bending I’ve done for household plumbing. In the end I formed the rear brake lines around whatever cylindrical item I had to hand and that I thought would create the right radius.

Could Have Needed But Didn’t

  • Ball joint splitter

More Detail

Here’s the list again in more details and with some comments. I can’t guarantee that all the links will work forever but where they do I hope they’re helpful – it’s amazing how far you can go back and see your Amazon purchases, I bought some of those tools over 10 years ago.

[ Note: you can click on the pictures find out more info on Amazon ]


Item Comments Link
Allenkey sets Various sizes, lengths and config: loose, socket sets, t-bar set.

Special: 9.5mm (3/8″) for engine mounts. The only other odd allen-key that caused me problems was the diff fill-plug – short 14mm.

When I bought these they came in a different black case… but these seem to be what I bought…

Axle Stands Min 3 regular but I used AJ Autos Mobile Axel Stand and replace steel wheels for “rubber” wheels Mobile Axle Stands

And then I switched the steel wheels for these…

Brake bleeding kit In the end I used Sealey  VS820. I tried a few different brake bleeding options (mostly cheap and not so cheerful) but this did it for me in the end.
Cordless Drill Any drill really, but cordless always works better for me and I’m caught in the Dewalt eco-system, so I use a Dewalt (DCD996) and which is my bestest of friends around the house
Dolly 4 wheeled home made dolly (40x50cm). Useful for moving engine/gearbox around on (not shown in the picture but it’s in the blog in a few places). The dolly linked here is similar to the one I made.
Dremel Used for grinding and cutting on many occasions. Through a quirk of history I actually own two Dremels. I keep one with the flexible extension shaft on it and the other just with a cutter installed. That’s way overkill to have two Dremels but I think one (or similar) is essential to fettle bits that don’t quite fit.

My Dremels are an older generation than the one linked.

Drill bits Metric (1mm – 10mm) and imperial (1/16″ – 1/4″) (pretty essential for some of the rivet holes that need the right hole size or else the rivets won’t fit or pull out)

Dust mask For fibreglass cutting, at the least. I’ve got on well with the 3M mask linked, but your face shape may work better with something else.
Ear defenders Sometimes used these when doing a lot of drilling/grinding. I use the Peltor ear defenders, your mileage may vary
Engine hoist and leveller I’m going to call the leveller essential but you could get away with straps… but I didn’t
Eye protection Of course. I use all sorts of clear and tinted safety glasses… too many to link to.
Files Various files. Needle files are very useful bot tricky to get at places
Funnels For various fluids and one with long neck for fuel fill
G-clamps I mainly use 6″ QuickGrips but also used some old school steel G-clamps.
Grease gun Prop shaft universal joints
Grips Slip Joint Pliers – I know them as “grips”
Hacksaw Useful for creating custom allen keys or cutting down bolts I provide that were that are too long
Hammers Leather/copper hammer was useful persuader
Hole punches See optional leather/material punch
Low profile Trolley Jack The one I got just about worked. It struggles to get under the exhaust system on the RHS and needed a wooden block to lift the rear (using the cross member at rear of prop shaft tunnel) Trolley Jack
Multi meter Checking voltages and continuity
Petrol can and/or jerry can Need something like 20l to prime system so something bigger than a 1G container can save some trips to the petrol station. Carrying petrol around is dangerous and I haven’t bought any containers online so I’m not going to link to anything I haven’t tested. No Link
Pliers Thin nosed most useful for me. The ones shown are electrical but any will do.

I do have mole-grips but try to avoid using them. They’re fine for holding something that might get hot (when grinding perhaps) but I rarely use mole-grips.

Revolving Punch Pliers 2.0-4.5mm. Highly recommended for rear wing rubber trim etc
Rivet tool A hand riveter is essential and I found a drill attachment to be a real time saver
Scissors Amazed how often I was reaching for a set of scissors
Screwdrivers Flat, posi and torx (wing mirrors)
Side Cutters For wire, IVA trim, cutting misplaced zip-ties etc. The ones I have are electrical ones but they’re good quality and I can’t stand poor side cutters.
Socket Sets 1/2″ is essential in metric (not linked, there are too many options but going with a reputable brand would make sense to me).

Used the 1/4″ set most of all

3/8″ used less.

I do have a 3/4″ socket set but didn’t need that in the end.

I also bought converter bits to allow me to put, for instance, a socket from one set onto a ratchet from another.


Soldering Iron

I use a simple 25W hobby iron
Spanners Lots. I have many metric from 5mm up to big. 24mm for headlamp locking nut adjustment, thin 15mm Spanner and a 32mm for the oil lines.

Don’t forget you often need two to do up a nut and bolt. Sometimes it works that a socket and a spanner will do but sometimes you need two sockets or two spanners. Doubles are only really needed for the common metric sizes – 10, 13, 15, 17 and 19.

I do own adjustable spanners, but I use them only as a last resort… and only use ring spanners where at all possible – much more reliable than adjustable or open ended spanners.



Not a complete set, but a start…

Special Sockets 41mm for rear hub nuts
Stanley knife and blades Always useful
Tape measure Tended to use digital calipers more to measure bolt lengths and the like, but sometimes need some extra reach
Torch To be honest I used my iPhone light more often than not when reaching for a torch. But I did also have a rechargeable magnetic torch/bulkhead light.
Torque Wrench’s – x 3 Norbar 13658 3/8-inch 8-50NmTorque Wrench
Norbar 13441 1/2-inch 20-100Nm Torque Wrench
Norbar 13445 1/2-Inch 60-300 Nm Torque Wrench
Obviously used the 20-100 the most but 8-50 useful for things like brake lines and 60-300 needed for rear hubs, though many leave those for PBC to complete.

Vice Regular bench vice No Link

You May Need These

Item Comments Link
Brake windback tool I ended up fiddling with the rear brakes a lot to try and improve feel and still not sure if applyig handbrake before bleeding was the cause.
Circlip pliers I took the rear brake calipers apart but can’t think of another use
Tap and die set Metric. Questionable essential, but was for me. Seat belt harness but also when cutting down bolts to clean up occasional thread.

Time Savers

Item Comments Link
Creeper/Stool Not being the spring chicken I once was, I found this convertible creeper/stool to be invaluable whizzing around under the car and sitting on it when working on suspension stuff
Crows Foot Spanner’s Metric and imperial. The metric ones came in handy a couple of times like the dry-sump to radiator oil lines. 32mm for oil lines

Digital calipers Cheap as chips these days and I think almost essential
Hot Air Gun I used one of these to shrink all the heat shrink – gotta be careful not to get too hot though. Otherwise a soldering iron works too.
Hot-Glue Gun Used hot glue to tack a few things down when I needed a light tack before final fixing.
Rivet Drill attachment Real time saver completing the riveting for the internal trim
Laser Measurer Found it useful instead of a tape measure but by no means essential. The one shown is a bluetooth version that I picked up for another project – Bluetooth is overkill
Low Profile Scissor or Bottle jack Sometimes needed to lift something where the trolley jack couldn’t get to. I had an old scissor jack that I used along with a hydraulic scissor jack that I had to modify to fit under the rear A-frame (ground the recess at the top down).
Multi-Tool Leatherman Wave. I’m a fan of having something quick and dirty on my belt while I’m working in the garage. Gotta be careful not to butcher whatever I’m working on though, there’s no substitute for the right tool.
Quick Grips QucikGrips useful when you only have one pair of hands. My Quick Grips are over 20 years old but I think the linked Irwin ones are the same. I also have the 18″ ones but don’t think I used them on the Caterham build.
Tweezers Good for pulling wires out of holes in tubes
Dremel Flexible Shaft As mentioned elsewhere I had the luxury of two dremels one always fitted with the flexible extension


Item Comments Link
Rivnut tool Thought I’d use it more but only ended up using for the lambda sensor cable under the driver footwell and attaching the oil catch bottle to the frame.  

Tablet in the garage Many times flitted between build blogs looking for a picture that helped me decipher the old build manual. New manual much better in that respect but as of mid 2018 the new manual is still not good enough to do the whole build. I use iPads… other tablets are available.

Wouldn’t Recommend

Item Comments Link
Pipe Bender I thought I’d be able to create lovely neat rear brake pipe install with this but I didn’t have the patience to “get the knack” of it.

Could Have Needed But Didn’t

Item Comments Link
Ball joint splitter Might of needed this if I’d needed to strip back the front suspension but I got that right and so didn’t need one.

So that’s a lis of all the tools I used. I toured my garage a few times trying to think if there’s anything I missed, and of course I added some items as I did those tours. But hopefully I got most of them.

Next up I hope to have a similar list of “consumables” – things like masking-tape, extra zip-ties, jubilee clips, rivnuts, adhesives etc etc.

The Wiper Motor Fuse… and Test Rig Overkill with a Logic Analyzer

Drip, drip, drop, click, bugger!

… or in other words… rain… followed by another wiper fuse blowing.

The Story So Far:

For those of you that need a recap, I’d tested the wipers in December, prior to the IVA (Individual Vehicle Approval) test, and they had also been tested by the dealer that sold me the kit, Williams, at the Post Build Check. However, on the morning of the IVA, at 7:30AM, just as the heavens opened, and as I was pulling off our driveway, they stopped wiping and just sat still. I had to drive to the test centre, getting wet and knowing that with the wipers broken I was heading for a certain failure.

We did get them going again during the IVA but only with the help of a very considerate examiner and a spare fuse.

Driving in the rain with no wipers and my head hanging out the side of the car

Since then the wiper fuse has blown a total of 4 times, not a big problem, mainly because I was intrigued as to what was going on and was increasing the fuse size as each new fuse blew. A couple of the times it blew was when I was testing, but it also blew a couple of times when it mattered and I had to drive in the rain with no wipers. But fortunately, I had decided to carry spare fuses after my school-boy-error of not taking any to the IVA test.

Blown 10A Wiper Fuse

Well, of course, the standard response to any fuse failure is to adopt the age old course of action: throw in a bigger fuse and see what happens!

As the fuses kept blowing I’d managed to “do in” two 10As (factory fitted), two 15As and then finally found that a 20A held. I was still rather in the dark as to what was actually going on though. Was there a current spike that was “just about” holding with a 20A? Or was it more of a sustained current draw that was within the limits of this 20A… and of course, why do I need a fuse that’s double the factory fit item?

2018 Catheram 420R Fuse Box and Relays – there’s a 15A fuse in the topmost wiper motor fuse position

[ Note: the image above has an RDX relay where the standard indicator relay should be – that’s another story but relates to new LED indicators, brake, reversing and fog lights… but that’s another story ]

Now, sticking in a 20A is not quite the bodge-it-and-scarper approach that it would be to use a piece of tin-foil across the fuse terminals… as had been suggested by some, but it was close.

I went through a bunch of possible options that might be causing the problem. Clearly there was too much current being drawn as the wiper motor started. But I couldn’t imagine Caterham fitting a 10A fuse when they knew there was a startup problem with the motor… or could they? Of course this could be a defective motor, or there was some inrush current protection missing from my car.

Other options I thought of mainly revolved around the mechanical linkages getting “bound” and causing a higher than normal stall current in the motor. If that were happening then it would probably happen at some point no matter how long the wipers had been running… i.e. it could bind at a particular point in the cycle but it could take a few cycles before the problem showed itself.

That didn’t seem to be what was happening…. the fuses seemed to all blow when the wipers were first switched on… and more than that, at the very point the wiper switch was flipped. It did catch me out once when I thought that wasn’t the case, but in the end I decided that I had gone from slow-wipe to fast-wipe and the wipers had stopped mementarily in between those modes – so inducing a “start condition”. It seemed to me that if it was a mechanical bind then at some point in the wiper motion there would be a sticky point and the current would increase there. But it doesn’t feel like that’s what’s going on.

Ok, so it’s probably something to do with startup current. But how am I going to see exactly what’s going on. Hmm. A volt-meter or ammeter isn’t going to be able to tell the story at the split second when the wipers start up. I could use an oscilloscope but the only one’s I’ve got are 15+ years old and are not the sleek slim LCD units you can buy these days. I’ve also got some similarly old logic-analyzers that I could have broken out, but they too are bulky and difficult to extract data from to include in this blog. Very Web 1.0.

Bring on the Salaea Logic Pro 16. My modern way of doing oscilloscope and logic analyser work is to use the logic/analogue PC dongle from Saleae. They come in different sizes, i.e. inputs, speed and colors. I have the 16 channel Pro version in a fetching anodized red.

Saleae Logic Pro 16 – In Red

Salaea can be found at

The Salaea 16 Pro is an great bit of kit. It plugs into a USB socket on your PC and can deliver up to 500Ms/s (mega-samples per second) logic acquisition and 50Ms/s analogue acquisition all from a box that fits in the palm of your hand. Each of the 16 channels can be set to logic or analogue acquisition and the whole thing also takes its power from the USB bus, so only needs a single cable. I only need a single analogue input for this investigation, but nice to have 15 backups!

I think for the higher acquisition rates you need to have a USB 3 port on your computer but for what I needed in this project, USB 2 was going to be fine. It’s also really tiny so hooking it up to a laptop meant that if I wanted to I could “go mobile” and connect up the rig to the car as I drove around. In the end I didn’t need that, but it was always an option.

On the PC side of things… I’m a Mac sort of person, so I had the Salaea hooked up to a Mac Book Pro. That’s way overkill for what I needed but if I wanted to “go mobile” I would have broken out an old MacBook Air or something.

Salaea are beta’ing (as I write this in April 2018) a live capture mode too… the current release of software only lets you see results once a capture has run. Again, that’s fine for this project but I’m looking forward to finding other projects where I can use that feature.

Logic Analyzer Setup

That’s the tool sorted, now how do I connect it to the car.

Hmmm… So… I’m looking to get current measurements of the wash-wipe circuit. I either need to put an ammeter type device in series with the circuit or I need a current probe. I don’t have anything I can connect to the Saleae that is like an ammeter or that I could construct… to do that I would need an accurate and low resistance that’s in series with the motor and that I could then measure the voltage drop across. We’re talking about 10 or more amps here so the resistance would have to be very low – or else I’m going to be affecting the measurements by adding more power draw and voltage drop. I didn’t have those sort of components to hand.

My solution was a current probe. I have a 0-20/0-60Amp dual range current probe that generates a 10mV/Amp and 100mV/Amp in it’s two ranges. For a load of less than 20A (my 20A fuse is holding, so I’m interested in less than 20A draw) the lower current range of 100mV per amp should do nicely.

For those not familiar with current probes, they measure the net current flowing through any wires passing through their jaws. The effect is an electromagnetic effect that we won’t go into here. However, the result is that the net current flow produces a proportional voltage response on the probe’s output.

Pico TA018 Current Clamp

My next problem was that the current probe has a BNC connector on the end. That’s fine for the old-school oscilloscopes but not what the Saleae needs. It uses 1/10” headers for its input and has breakout leads that can hook up to 1/10” headers or at a push bare wires. I was unlikely to need the BNC connection type again, so… off with its head!

I could have made a BNC to 1/10” converter cable, but I was feeling lazy. That being said I also couldn’t just hack off the ends, I did need to at least provide something a little more professional – OCD, moi?!

BNC Probe connection replaced with 1/10th” header connectors

Back on the Saleae side of things, it’s analogue inputs are configurable up to a maximum input voltage of 3.3v. So, with 20A and 100mV/A, I need a voltage range of 0-2V – that’s perfect for the Saleae. Bostin!

Now I need to figure out how to get the current probe clamped around the wiper circuit input. Hmmm (again)…

I dangled myself into the passenger side footwell hoping that the wiper motor wires were accessible – they weren’t. The wiper motor is tucked up under the passenger knee panel. Hmm. I could of taken the knee panel off but I was feeling lazy, as discussed above.

This is what a Caterham Wiper motor location looks like

What about getting the probe on the back of the fuse panel. That had been the plan all along. But there wasn’t a lot of room there either. I’d probably have to unbolt the fuse panel to be able to get at it. Hmm.

Then came a eureka moment. I’m sure this is something that old hands know all about and use all the time. But I was pleased I worked it out myself… Use… a… modified… fuse (as pictured above).

You’re regular automotive fuse is essentially a plastic molded holder for the fuse wire. My plan was that I could nibble away at the end plastic of a blown fuse and insert a wire loop that I could get the probe onto. It has to be a “blown” fuse or else my wire loop will only be in parallel with the fuse wire and I won’t “see” the full current in my probe. Then, once I have this fuse Frankenstein, I can pop it into the fuse socket for the wipers and attach my probe.

Here’s the modified fuse…

Fuse with added wiring loop.

I could probably do a neater job than this second time around if I had another go – but it will do. I had to file away some of my soldered connections so the fuse would fit back into its holder and the wire loop got caught on the soldering iron at one point and so I lost a bit of the insulation – it’ll do!

And I also probably would have got the measurement done faster if I’d have taken the knee panel off… but lazy doesn’t mean lazy in all respects. When there’s a Eureka plan to hatch then lazy goes out of the window!

And here it is in the car with the probe attached.

Current probe attached to (wrong) fuse position

Now to take some measurements.

After a false start where I actually started to probe the wrong fuse, I finally found the right fuse (it’s tight up in that fuse box), and got to taking some measurements. I set the analyzer to 5ks/s and the full 3v3 input range. I didn’t need a lot of either time or voltage resolution… I was sure that anything that was going on was happening at the ms (millisecond) scale and not at the nanosecond scale. I was mostly right.

Scope output – ignition on then 4 cycles of wipers

The image above is a screen shot from the Saleae with the probe being connected into the wider fuse socket. The first spike is the ignition being turned on. Then we see a big spike where the wipers are turned on. There are then 8 “bumps” showing the wipers making 8 sweeps (4 cycles of left then right). It’s not totally obvious to me why there’s a spike associated with the ignition switch being activated – though I assume its something to do with the wiper reverse/park, there’s a momentary energising of the motor as the park circuit decides that the wipers are in deed parked.

But we’re interested in the big spike…

Initial wiper motor in-rush spike – medium zoom

We can see that the initial in-rush spike is large but also drops very quickly only to be followed by a wider almost the same amplitude second wave of current.

If we zoom in on the initial spike we can get a current reading…

Wiper motor in-rush spike – high zoom

The 1.657V shown on the image indicates 1.657 X 10A (100mA/V), or 16.5A.

Ok. Problem identified. The motor is drawing some 17A for about 0.6ms then almost the same for a much longer period. The shape of the curve is a little odd, there’s clearly a decay going on here followed by a cut-off, so I suspect the current peak is a lot higher than what I captured. This is firmly in line with the stall current theory, the magnetic fields build quickly and the current would be infinite if it wasn’t for the resistance of the windings. But in a real motor the currents aren’t infinite – luckily. Phew… my 3 years of university motor and electro-mag theory are paying off! Time to dissect the time… and up the sample rate a bit.

So, we’ve got a current spike when the wiper motor starts. That’s normal. And at the time scale that a wire fuse is going to be worried about (millisecond duration) the current draw is around 17A. The question is still: why is it so high?

There’s some writing on the side of the motor that shows what looks like a “14W”. I’m not sure if that’s relevant but I’m clearly going to need to do some more investigating. Especially if the motor is 14W… the 17A startup current and the ~7A running current are way bigger than 14W. For those interested, 17A x 12V = 204W. You can do the other calculation if you like, consider it homework. That wold be 1/3 of a horsepower at stall… while that’s feasible, that’s not going to be needed to drive two small wiper arms I don’t think.

One of the causes of the large spike, could be that the motor is faulty or at least the windings are at the low resistance side of a manufacturing tolerance. I’m certainly not going to be taking the motor apart to dissect it, but I might put a ohmmeter across the terminals if I take it out.

Perhaps I still also need to look at the mechanical side of things. The wiper assembly shows the motor attached to a gearbox and then through a bar to two worm gears that drive the blades themselves. Maybe the gearbox is dry or the worm gears need lubricating. It should be simple to get some lubricant on the worm gears but getting the motor and gearbox out will have to wait for another day.

The only other option I can think of at the moment is that there should be some sort of shunt across the motor to dampen any inrush – perhaps that’s faulty, missing or not designed for.

Next steps are to see if this is a common problem that I hadn’t been able to Google on Blatchat… and to do some digging to see if there are any clues on the Caterham supplied wiring diagrams (though I’m less than confident that they reflect how my car is actually put together).

So, what’s going on?

Well, I guess I still don’t know. Though I know more detail on what’s going on at the current level.

And of course leaving the 20A fuse in place isn’t really a great option. The fuse is there, obviously, to protect against over-current. And with the stall current being below the fuse rating I have in there now, then a stalled motor isn’t now going to blow a fuse. I guess if the motor does truly stall then it’s going to get rally hot too… that’ll reduce the resistance of the windings enough perhaps to increase the stall current and blow even the 20A fuse. But that’s perhaps just wishful thinking… there’s a lot of metal in a wiper motor and it will take a long time to heat up and even then I doubt the winding resistance will fall low enough to blow the fuse. If I had a spare wiper motor I’d probably try that out – sounds like an interesting experiment.

More to come…

PS: This post was written in April of 2018 but for various reasons has only been posted now (June 2018). In the intervening time I’ve not had a chance to look any further at the Wiper Motor problem but my 20A fuse has held. Hopefully I’ll get a chance to look some more at this at a later date and report back.