Hi: The radiator fans on my 2001 xk8 with 77000 miles are not running. The fans will not run when the a/c is switched on. The compressor operates as it should and the air is cold. When I put a 100 ohm resistor across the plug for the fan temperature sensor the fans will run on high speed. Since the compressor runs, does this indicate the a/c pressure switch is ok? I really do not know what else to check. Any help would be appreciated.

 

The AC pressure switch has 3 contacts for different pressure levels that reflect the demand on the AC system and the demand for 1 or 2 condenser cooling fans . Common thing to these 3 contacts is the pin 2 and 4 going to ground so that would be the thing to check first . In the connector pin 1 to 4 I think is the both fans to run contacts if you jump the wires in the connector assuming you have a good ground on pin 4 . Pin 5 to 2 is a over pressure switch that it's wire goes back to the AC control module to shut the AC system down as all cars have them .

By fan temp sensor are you referring to the engine coolent temp sensor ?

 

The a/c sounds like it is working properly. The fans are controlled by the ECM. If the fans come on to high with the resistor the coolant temp sensor is probably bad. You need to have the codes read for the ECM. Any problem would be stored in the ECM memory.

 

The thermister in the engine coolant sensor works opposite the old school variable resistor as low resistance jumper mimics high temp thereby turning on the radiator fans . This is not the positive contacts of the AC pressure switch so to keep those separate in troubleshooting .