Cars turn on at 210 degrees and off at 198 degrees. I suspect trucks are similar.
Fan should turn on when you unplug the two-wire coolant temperature sensor. It is fed 5 volts through an internal resistor from the engine computer. The voltage should be between .5 and 4.5 volts. The higher the temperature, the lower the voltage. Anything outside the normal voltage range will trigger a diagnostic fault code and turn on the Check Engine light. This will happen when you unplug the sensor. The circuit voltage will go up to 5.0 volts, the light will turn on, the code "Coolant voltage too high" will be set, and since the computer knows it can't trust the reading, it turns the fan on just in case the engine is overheating.
If the fan turns on when you unplug the sensor, the system is working properly. If it doesn't turn on, find the fan relay and bypass it by jumping the terminals or by removing the cover, reinstalling it and squeezing the contacts together. If the fan runs now, you know the motor, wiring and fuse are ok.
caradiodoc
Aug 7, 2009 at 10:28 PM
