Currently not Caterhams biggest 'Fan'

Updated: Feb 22, 2019

Building my Caterham 7 has certainly been a mixed bag of emotions, and sadly it's included a significant amount of frustration which I feel towards Caterham.


Recently I'd started to think the build had progressed to a point where there was little chance for further frustration. Oops my mistake, Caterham managed to drop the ball again, and I feel in a critical area.


If you've been following my blog you'll know I recently started the engine, the covering blog is still to be published along with all the details, including the challenges with the radiator fan which no matter what wouldn't kick in. Well over the last week I've been working to resolve many electrical issues, and today I spent a good 4 hours trying to resolve the fan. I got there, and what I discovered was very frustrating.


So what was wrong, well during the engine start process, you need to get the car hot enough to open it's thermostat so you can bleed the coolant system and then ensure the fan starts. It requires running the car up to a high temperature which isn't fun and very unsettling, made only worse when something like the fan doesn't want to play ball. To make matters worse the documentation surrounding the Duratec engine, its supplied thermostats and fan operation is appalling. There is thread after thread online debating the topic, it seems complete shambolic.


I'll leave the engine start details to the pending blog, needless to say, I'd concluded there was an issue with my fan during the cycle. But what exactly I didn't know.


On the day of the engine start, I'd verified the fan was fine by connecting 12v to it, and it ran fine. I also connected a multimeter to test the loom fan plug and it was ground and open circuit, again to be expected. The fuse, a 30A was also checked and confirmed good. I also received information stating the ECU temp sender on the hot side of the block near the submarine U bend would also trigger the fan if removed when the car was warm. I tried that, still nothing.


Today I checked the fan's relay by removing it and validating the high current side switched in response to input on the low voltage side. I used a bench power supply to apply a voltage to the low current side, sure enough, you could hear the relay click. All good there then!


At this point I started to run out of ideas on how to proceed. In the end I took the decision to trace the entire circuit. I'd been informed via BlatChat the fan is switched via the ECU, basically, the temperature sender information is feed to the ECU which has a connection to the aforementioned relay, when the required temperature is reached the ECU then switches the relay and in turn operates the fan. My plan was to split the ECU PCM plug to reveal each of the connections to the relay and trace those back to the relay. I also needed to trace the relay to the fan. Once I split the PCM plug and looked at the wiring diagram I realized working out which pin was which wasn't that easy. I decided to go have a look at the relay wiring.


The relays in the Caterham are under the passenger side bulkhead and on my car the relays were mounted against the bulkhead with the rearward wires facing towards the outside of the car. Fuses are also mounted adjacent to the relays. Given the fuses and their wiring are present between you and the relays it's not easy to determine what wires go into what relay. To work around this limitation I decided to remove the whole board from the bulkhead, lower it down and gain better visibility.


It wasn't immediately obvious how to remove the fuseboard from the bulkhead, however after some investigation, it became apparent you need to remove the top and bottom relay, behind which are two nyloc nuts at the end of bolts holding everything in place. The screws come through from the engine bay just next to the airbox. Removing is easy if a little fiddly on your own.


Once I'd removed the nuts and bolts and carefully lowered the fuse board I made an immediate and very interesting discovery. One wire terminal used on the fan relay was not sat home, it was just sitting inside the plastic body of the housing, and wasn't sitting in the retaining mechanism which mates with the inserted relay. On closer inspection I believe the terminal to be the control terminal on the lower current side of the relay, so most likely the signal from the ECU. At this point I was confident and slightly relieved I'd found the problem. To move things forward I inserted the terminal into the block and made doubly sure it held both from the movement of the board and also reinsertion of the relay, the terminal remained secure and fixed into place.

Relay terminal not seated for the fan.

Before remounting the assembly back onto the bulkhead I decided to perform some testing to at least validate the theory this was the problem.

Unfortunately I needed to reassemble the PCM connector, battery and battery cables which I'd removed for access to the PCM connector. This caused a bit of a sidetrack as I'm never entirely happy with the mounting of the PCM connector and battery cables, I can spend hours messing with their layout. Once finished though I could start testing.


I was going to perform two tests, short circuit the heavy current side, and then the low current side. In truth I couldn't decide how to go about the low current side, and well, chickened out, I feared I might damage the ECU. I settled for testing the heavy current side. The ignition was turned on, and a small jumper wire inserted between the two heavy current terminals in the block, not relay. Sure enough the fan immediately fired, great. I think I'm on the right track, yes there could be a fault with the ECU, but highly unlikely.


At this point I finished tidying up a few loose ends, left the fuse board hanging, and rolled the car onto the driveway. I was going to start it yet again with the aim of achieving a running fan. The car was started left for about 5 minutes so that the temperature gauge started to move. At this point I decided to test the theory of pulling the ECU sender plug, causing it to go open circuit. I'd had mixed comments on whether this would work, the theory is that as the temperature of the car increases so does the resistance of the sensor which is read by the ECU. If you disconnect the sensor, you get open circuit or infinite resistance and hence max temperature, a condition which should bring on the fan. Lotus 7 members had said this would work, although Oakmere said not. As mentioned I left the car for 5 minutes to move the car off its cold start fueling and pulled the plug. The fan kicked in! I've never been so happy to hear a car fan!

ECU Temp Sender Plug, when the engine is warm can but unplugged to trigger fan

I'd opened this blog mentioning my frustration, well I am very annoyed about this.


This quality issue from Caterham could have been a costly one if I'd have let the car overheat. I'd received comms from Caterham suggesting I let the car warm up further etc, but at no point did anyone suggest the wiring at the relay might be at fault, I had to discover this myself with zero experience. This went way beyond the expectations of the build. If you are a UK mainland resident it may be worth noting that the diagnostic work I carried out should be covered under warranty, and so technically you don't need to do it. As I'm resident on the Isle of Man, I didn't really have any other choice. That leads me to the flip side of this, being forced to solve such quality issues is forcing me to learn more about the Caterham, but also topics such as automotive electrics, I appreciate I'm only scratching the surface, but everything helps and ultimately building a Caterham has to be treated as a learning experience.................


I think ......


Let me know what you think, should I be more tolerant of this sort of build issue? Don't forget a 420R of this specification is currently around 40K GBP!


Footnote, Whilst I don't make this point in the body of the blog, the troublesome terminal is positioned at the highest and furthest point under the bulkhead. There is simply no reason for the situation to have arisen through my actions. Just in case you were wondering :-)

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Meet Mark
Loves cars, light weight specials, jDM, Lotus, Caterham, Rallying and Trackdays