Re: Temp Gauge and Overheat Warning light

From: Mike
email: mike@secret-scotland.com
Date: 27 Apr 2006
Time: 03:54

Message

Guys, You aren't going to believe how easy this was to fix! Once again the symptoms. 1) Initially temp gauge rose to 170F and then leapt to 210F and the leap was synchronised with the overheat warning light on. 2) The Thermo-switch did not trigger the radiator fan to come on, despite the gauge readings 3) The top radiator pipe and the radiator were cold. 4) The expansion tank was not under pressure 5) After removing in line Thermostat from top radiator hose, the temp gauge didn't leave its rest and would then leap to 180-190F and the leap was synchronised with the overheat warning light on. So I incorrectly diagnosed the thermostat as the fault, but then realized that it was a sensor problem and assumed that it was the overheat sensor switch as it seemed to be cutting in too soon. After confirming that earthing out the overheat circuit gave a reading of 190F at the gauge I then checked the temp gauge and found that there was no female connector at the end of the red/green wire so the only contact was made by the exposed wires touching on the terminal of the coolant temp sensor. My theory is that the Coolant Temp Sensor was in a situation where it either showed too high a resistance or an open circuit. As the overheat sensor warmed up it eventually reached a point (around 170F) when its resistance was less than the temp sensors and (by the principle of Ohm's Law) this then caused enough current to take the route to earth of passing through the overheat sensing circuit. This explains the sudden leap in the temp gauge that coincided with the overheat light coming on. After changing the terminal on the temp sensor everything appears to be working fine. The temp gauge gives a more reasonable reading and the Radiator fan cuts in at about 200-210F. Sorry for the long winded explanation. You can tell that I've spent a lot of sleepless nights looking at wiring diagrams and thinking this one through.