XJ 4.0 cannot get auxiliary fan to run.

[Four2score]

Having a problem with my electric auxiliary fan. 95 XJ with 4.0 for reference. The only way I can get the fan to turn on is when testing the fuse box with a paperclip jumping pins 87 & 30. And the paperclip does get very hot after a few minutes. I’ve tried replacing the fan with new, trying about a dozen different relays from known working jeeps, turned the A/C on and off, and unplugged the front water temp sensor all while the engine is running. While driving the Jeep the water temp sensor goes almost to the red but not quite. When I open the hood you can tell it is running hot, but the fan never comes on. Ever! I can only get it to come on when I jump the fuse box terminal where the relay is. Anyone have this experience and resolve before or does it look like I’m dealing with gremlins in the electrical wire harness somewhere so need to take it to an electrician? Sorry if this was discussed somewhere before but I couldn't find it.

 

[clean96XJ]

Overheating (while idling).. horrible gas mileage.. ?

Check/replace water temp sensor (for 1996 xj back of engine water sensor).

Swap with known working one to test?

Good luck.

 

[Jeepedo129]

The fan is controlled from the PCM. The PCM gets coolant temp reading from the coolant temp sensor and it gets a discrete(Open/Closed) signal from the high pressure A/C switch. If the A/C is turned on, the fan should turn on when the A/C high side pressure is in the operating range (Not sure on exact pressure) or the coolant temp is above 215ish (218 on the 01 but i think older years are lower)

If the fan comes on when you jump the fan relay that rules out the fan motor, the wiring to the fan, and the fuse. If the fan doesn't come on when the coolant temp is above 215 or when the A/C is turned on, then you really only have 2 maybe 3 possible culprits.
1. PCM is bad and not sending a ground to the relay.
2. Wiring issue between the PCM and the Fan relay.
3. Fan relay is shot

Couple quick checks you can do:

Change the relay with a new one.
Check for 12v and ground coming to pins 85 and 86 on the relay. With the engine on and A/C turned on or coolant temp above 215, check for a ground on pin 85. Pin 86 should have 12v any time the ignition is turned to the Run position.
-The PCM provides the ground to activate the relay, so if all the prerequisites are there (A/C on or coolant temp high) then its likely the PCM is the issue.
Also, you can jump the High side A/C pressure switch just to rule out a bad switch or low refrigerant charge.

Less likely -
You could have 2 problems - a bad coolant temp sensor and low on refrigerant (or bad A/C switch) But that is unlikely.

I'd start with the relay and some wiring checks and go from there.
Hope this helps