The auxiliary fan on our 1990 Cherokee is not coming on. When I bypassed the coolant temp sensor on the radiator it ran. I put in a new sensor and it does not run. I tested the old sensor by boiling some water on the stove and it worked. Without the fan the Jeep over heats. When I bypass the coolant temp sensor, the fan comes on with the key and Jeep does not over heat. What is going on?

 

I am going to watch this thread as well, I believe I might have the same issue as you, I have replaced that sensor twice already on my '88 and the fan doesn't seem to be kicking on. I don't know if it could be my temperature sensor being bad and not giving me the correct reading but I am getting an IR gun at Sears this evening to see if the temperature is actually reading that high. Mine does not have the physical look of overheating. Might suggest the same for you, does it look like the car is overheating when the temp gauge says it is getting hot? Like you pressure bottle getting full or overspilling? Any other leaks anywhere? That fan is supposed to kick on at 218 *F. If you have access to an IR gun I would point it at the upper radiator hosue where it runs into the top of the thermostat housing to see what temp is there, should be 195*F as this is what thermostat is fully opened. I would also point it at the temperature sensor which is all the way in the back of the head, that might be a little warmer but see if it corresponds to the reading in the car. Hopefully someone else will chime in with more suggestions, that's my plan at least.

 

Doesn't it only turn on when the A/C is on?

 

Should kick on when the A/C is on and when the coolant temp reaches 218 or somewhere around there I don't know if that is the exact temp.

 

Yep easy way to see if its working to turn the ac on, it should turn on. Otherwise it turns on at 217. What are you using to bypass the sensor, just jumping the plug straight to the battery?

If nothing else, just wire it to a toggle switch in your cab, thats what i did for wheeling purposes.

 

Yeah mine will turn on when the A/C is on but its not coming on when the temp reaches 217 or whatever, that's why I think it might be the temperature being off on the gauge. I had a new sensor put on a month ago and the temperature consistently sits at like 215ish and I think this is really 195, its just weird.

 

I cut the wires on the harness from the old sensor and wired them together. With that set up the fan runs any time the key is on. When it gets hot (temperature gauge goes up and coolant tank blows) and I check the continuity on the sensor it just stays open. The only thing I can think of is that the radiator could be clogged and hot water is not circulating to the area where the sensor is.

 

A/C is not charged, so I don't think that will turn on the fan. I cut the wires on the harness from the old sensor and wired them together. With that set up the fan runs any time the key is on. When it gets hot (temperature gauge goes up and coolant tank blows) and I check the continuity on the sensor it just stays open. The only thing I can think of is that the radiator could be clogged and hot water is not circulating to the area where the sensor is.

 

It doesnt matter if the ac is charged, when the computer sees you turned on the ac, it turns on the fan.

 

What he said. No AC doesn't work but if i turn on switch it still turns on.

 

I wonder if it could be the radiator? How many miles on the jeep? How long is it sitting before it starts overheating, what is ambient (outside) temp when it starts overheating. When was last time the fan clutch was replaced? Have you flushed system to see what the coolant looks like?

 

Somebody correct me if I am wrong. But isn't the temp sensor/sending unit for the fan on the block not the radiator? I seem to remember there is one sending unit for the gauge (radiator unit) and one for the ECU (block unit).

 

It's the other way around the gauge is the one on the head and the one for the ecu is on the driver side of the radiator. The "newer" ('94 and later I think) are the ones with the sensor and sending unit all in one on the thermostat housing.

 

Thanks for setting me straight. I look briefly at the FSM. There's a trouble shooting guide for this issue. Sorry but my FSM is a Secured PDF and I can't copy anything from it. I can do screen shots if that will help.

 

I am thinking of a flush. I haven't done it yet. I thought overheating was because auxiliary fan was not running. It will heat up in about 5-10 minute at idle. Ambient temperature is in the 70's. When I have fan wired to run, it is fine. It is just the the temperature sensor seems not to be getting hot. I thought it was the sensor, and replaced it. When that did not work I tested the sensor in a pot of water on the stove, and it worked. That is why I think of flushing. I think it has about 150,000 miles. If the flush doesn't work, then maybe a radiator?