housew

Jeep is throwing a code p1491 radiator fan control relay circuit.

I found wiring diagram for an older Cherokee and diagnostic procedure on this link (very useful by the way):

http://www.lunghd.com/Tech_Articles/Cooling/Electric_Fan_Diagnosis.htm

My fan works if I give it power manually and I have power going to the relay. The relay isn’t getting signal power that it gets from the temp sensor when the coolant is hot or from the ecu when the ac is on.

I chased the wire back and I have continuity all the way back to the pin hole in the ecu plug. I can’t test the ecu - plug connection cause the jeep wont run without the other 35 or whatever wires attached. I’m assuming its good cause all the other connections in that plug are.

So the ecu isn’t telling the fan to turn on, do I need a new ecu or is it possible that ecu doesn’t know that the A.C is on and hence isn’t sending a signal and I have to check this circuit as well?

Fan doesn’t come on when the coolant tempature gets too hot either. I don’t know if this goes through the ecu as well on a 99 or if it’s the temperature sensor sends current direct to the relay like the older ones.

I know I really need an fsm but im poor, I just need to know what would be causing this and how to test them.

Ecu is bad (everything else works except fan)?
Ac circuit not sending signal AND tempature not sending signal?

Anybody got ideas?

SeriousOffroad

First off, great job troubleshooting your problem the right way. Too many people just start replacing parts that they think might be the problem.

Yes the PCM (ECU) does complete the ground path for the cooling fan circuit.

Have you tried unplugging the the coolant temp sensor when the engine is running? The Aux. Fan should come on right away and you'll also get a CEL.

Double check the ground wires right next to the PCM while your at it. Disconnect and clean the terminal and the ground location as well.


Also, sent ya a PM.

mike37

I would check or replace the temp sensor first

SeriousOffroad

Why not swap relay's in the PDC first? It's an easy way to tell if the Aux. Fan Relay is bad.

housew

i did swap the realay, the relay is fine. the fan works if I put power to it directly.

ill try unpluging the coolant temp sensor when I get home. I'll do the ground wires as well. Good chance its a ground, my jeep has spent a fair amount of time swimming as of late.

frankz: thanks for the PM. That should help me a lot

housew

alright, so i have since checked all the plugs, removed the grounds, sanded down to clean metal, regrounded, and cleared the code.

I have just now noticed that my AC doesn't turn the compressor on. I didn't notice because I don't use AC. but I was useing it to test the fan, the fan won't come on unless the ac is on. I'm going to go for a drive tonight to see if it comes on when it gets hot. If it does the fan is fixed and I just have an ac problem (which is no problem in my case) and I can install my on board air from my old jeep.

the CEL is off, but my jeep hasn't been run long enough to heat up enough for the fan to come on I don't think. If it gets hot enough to trigger the fan, and the fan doesn't go, then ill get the CEL back correct?

Billfo

HouseW - Did you ever get to the point of determining if the ECU output driving the fan relay coil was bad?

I have a 2000 Cherokee and the AUX fan will never come on when the A/C is operated. A/C works great until inadequate cooling of the condenser cool results in a high pressure shut down of the compressor.

I have traced the problem to the ECU signal at the PCM fan relay socket - it is always pulled high so the relay is never closed. I cannot comment yet if the fan operates when coolant temp is above the trip point - weather hasn't been that hot yet.

So I am asking the same good question you had - Do I have an ECU problem, are there other dependencies prevent the ECU from signaling the fan relay?

Thanks,
-Bill

housew

this was a long time ago, but if i recall correctly I followed the wireing diagrams and discovered that it shared a path with some other (less important to me) things. One of those things was the horn, I had a horn that was shorted. Took me a while to figgure that out. but i unpluged the horn and double checked all the fuses and the fan came back to life