Northern Banana

Thanks James and Jonathan, Some progress made - found a 'stretched' black and yellow earth wire drivers side of the battery (ecu ground I suspect). Not getting voltage drops now, for reference the constant voltage to the rad fan is still there, so maybe it is a variable voltage/speed type. Still got a negative air temp sensor reading. The sensor is functioning as when I blow a hair drier in to the air intake it shifts. Don't think I can recalibrate it in easimaps and wiring looks ok. Job for tomorrow.

That looks like the correct one. Thanks. The cooling fan isn't run via a relay and the loom doesn't have a take off for a separate fan switch.

broken image link

This is possibly different to the road car of the same age, as for example, the race cars fuel pump primes on the masterswitch, even with the ignition switched off. Seems to have only 4 relays rather than the 6 on the standard wiring diagram. TIA

OK I'm going to pop up a new post to see if anyone has a wiring diagram for the vtti race cars.

JK, Have you come across this wiring diagram before? https://andrewrevill.co.uk/Downloads/Wiring%20Diagram%20-%20Main%20Vehicle%20Loom%20-%20Corrected.png

12.5v at the battery. "I don't think that the difference between 11.6 V and 13.2 V is turning the fan motor on. I wonder if that 11.6 V is coming from another circuit that won't supply much current." I would agree. One line of thought is that if there is a parasitic power supply, when the switch properly opens at 97 degrees and delivers a full 12v+ it is causing the issues with current heading back up the source of the parasitic supply. However if I let the temps rise beyond 97 degrees with the fan T connector disconnected I don't see the voltage drop at the ECU.

"That's with the connector separated? And the voltage between the two terminals in the connector?" Yes, also double checked with earth to chassis.

This mornings findings With master switch off and ignition off - zero voltage to cooling fan (reading taken from T plug connector) When I turn on master switch (ignition still off) I get 11.16v at the fan T connector If I connect the fan up in this state it doesn't run so assuming fan needs minimum 12v to operate. If I unplug all relays (I have 4 under the dash) I still get 11.16v at the fan T connector. Same 11.16v with ignition on, engine not running Same 11.16v with engine running 13.2v when coolant reaches 97 degrees (when T connector plugged in fan runs, this is when voltage drops at ECU occur) I suspect we have to assume that the race cars wiring does vary for the road/standard car. As soon as you switch on the master switch (ignition off) the fuel pump primes. When the variable valve timing variant was introduced Caterham had bespoke looms produced to removed all the unnecessary ancillary stuff. When I look at the relay pic below it doesn't correlate to the wiring colours referred to in the wiring loom.

Sorry for the delay replying. Thank you for all your advice, its really helpful for someone with only a basic knowledge and 4hrs minimum drive from a knowledgeable caterham specialist. I'm hoping to get into the workshop later on today and will work through the above. <Many thanks

Positive to the fan is black/green.

Car Sigma 1.6 vtt 270r (race car) Could anyone enlighten me as to how the cooling fan is wired/powered in these cars. I'm having an issue with voltage drop at the ECU when the fan kick in 97 degrees. When cold and the fan not running I get 12v at the fan plug, when it reaches 97 and gets the signal from the ECU it reads 12.5-13.5v is that slight increase in voltage what switches on the fan? This doesn't seem right. Obviously being a race car it may have been modified but can't get my head around how its configured. Thanks

Thanks James spot on logging it the issue occurs when the voltage drops the ecu seems to shut down and easimap pauses. need to work my way through the electrics but only seems to occur when warmed up so possibly a cracked main cable possibly a duff master switch. will try a replacement air temp sensor but it seems to run the same connected reading minus 8 or disconnected.

Thanks useful stuff. The voltage drops were when the engine was hunting for revs/close to stall. It is normal voltage when idling properly and when further up the rev range. I've ordered a new crank sensor and need to get the scuttle off to allow a proper check of earths and connectors. When the racecars move up the ladder and windscreens are removed, the wiper mechanism simply gets pushed down under the scuttle and the wiper head holes plugged. Not great having that wobbling around with a good chunk of the loom cable tied to it! The minus temp value and pause in the live data is as you suggest concerning, will unplug the air temp sensor to see what default value it throws in or if it shows an error. From speaking to others the live data values should be reporting seamlessly.

Link to YouTube clip of easimap screen highlighting issues.