Coolant temp sensor duff? Emerald K3 signal sensor. Zetec Black top, Raceline water rail. Drilled thermostat.

[Nigel Riches]

I have been having a problem with the cooling fan on my Caterham not reliably switching on and off, so I brought home from my work a calibrated  "K" type thermo couple and a Fluke 51 II.

I taped the thermocouple to the coolant sensor reporting to the K3, not the temp. gauge sensor. I then watched on the  "live adjustment screen", and the Fluke display as the engine warmed up, both initially report at 16c, however as the engine warmed up the live adjustment display indicated ahead of the calibrated thermo couple, by an increasing margin, at 95c live adjustment screen the cooling fan cut in, but the true temp at the sensor position was varying 72 - 76 c, the fan did not switch off at any point after this.

I had a bit of a hands on feel around the pipes, the top hose was hot, surprise, but the return hose from the Radtec Ali radiator was a lot lower, some 20 c, measured. I can't recall the thermostat temp. but it isn't 70 c or 92 c, as those are on the bench, but i do recall drilling a 1/8" hole through the thermostat mounting plate, inline with the recommended thinking at the time I was installing the engine  

Now I am very much a techno dough brain numpty in this ecu stuff, I managed to find in the "events" page some temps set for the fan to cut in, out. Fan on at 96 c, off at 88 c, probably not unrealistic for a Zetec, 

The K3 drives a relay for the fan operation, so probably not a ECU problem, if it is seeing some skewed info from the sensor.

I will endeavour to hook up an external relay from the thermo switch on the thermostat housing, not sure of the temp. trip point for this, but if the fan cuts in and out then I will have more idea about the sensor serviceability  

So, my learned 7 brothers, is there some other "thing" I should be looking at?

This ECU business is the new version of the Weber / Dellorto carb stuff from years gone by.

A little bit of knowledge is a dangerous thing, I am definitely in that area.

Any info, as always, gratefully received, thanks.

Nigel.