Engine fan issues

Have you checked the fan solenoid?

 

Could be the high pressure switch or just a bad connection to it

 

CK. back side of switch for power,then move down to sol. use a jumper wire to see if power gets it to turn off?

 

I had this problem with a 98 pete 377 with a detroit engine. Turned out that the fuel temp sensor wire insulation had chafed against the engine block, and the wire had grounded itself. The ECU saw that as high fuel temp and kept the fan on as a result. The CAT ECU may do something similar? Nothing that I would have ever suspected.

 

I believe they are on the firewall under the windshield on driver side. Has a two or three prong wire with an air line coming out of it leading to the hub.

 

I had the same problem on an '02 9400i with C15. Turned out to be A/C high pressure switch. There was a broken wire on the switch where the wires enter the epoxy seal on switch. New switch was about $60, and about 5 minutes to change out. (You will need the red switch)

 

You need to find out if its a pressure engage or a pressure disengage. Then Just put shop air to the line with engine off. If it does as it should, then build pressure in air system, remove the "out let" side of the solenoid, and see if air flow changes with key on or off. That will tell you if the solenoid is working. The next step is to see if power to solenoid when key on or off. It could be the relay for your engine fan also, you will need to find where that is. Then test volts at that to see if that is switching on and off. If that goes on and off, it could be the A/C system telling it to turn on. If you are running the defrosters the A/C comes on, that will also trigger the fan under the right conditions.