Hello Cherokee Forum Community!,
I know this has been posted and talked about a billion times and I am not try to post another new thread about it... However, I cannot find any answer regarding this. At least for my situation... So here is goes.
1994 XJ
All stock
4.0
My aux fan is not coming on when the temperature is >218. I understand the temp should not be be hitting those temperatures, but it is currently and I am working on that. That being said, I still want this fan to function properly. ( It was functioning properly last week. )
The fan does come on when:
I turn on the AC and /or defrost. ( The AC is at operating pressure. )
Use a jumper on in place of the relay.
Use a jumper on the connector to the sensor in the tstat housing.

I have replaced the sensor in the tstat as well as the one on the back of the cylinder head.
I tested the relay and it is good. ( I also swapped it with the AC relay just in case. )
The mechanical fan stops quickly when I spin it by hand. ( I assume that means it is good? )
Ohm'ed the sensor to the relay and the relay to the fan. All was well.
Ohm'ed the sensor itself and it was about 5.5K which seems accurate at the time.
I think that covers everything I checked...

As far as the overheating issue. I have only replaced the radiator and the thermostat.
I did a compression test on all 6 cylinders and all were right under 150.
The gauge type I used was not able to do a leak down test.
I will be replacing the water pump next.

For now I can troubleshoot the heat issue, but I really need your help with the aux fan. I am at a loss on it.

Thank you.

PS: I forgot to add, when idling the heat will increase over about 10 minutes to 240-ish, however when driving down the road it is at about 212. ( Which still seems high. )
Additionally, I bought this XJ from a friend a couple weeks ago and when I got it the temperature gauge did not work. I bought a gauge cluster from a '93 and transplanted the temp gauge into mine. So I do not have much history on it.

 

Does your radiator have a temp sensor in it on the side that has the trans cooler lines ?

 

It does not.

 

Ok , then your computer controls the relay that turns on the fan . you will need a wiring diagram to find the wire that goes from the computer to the relay so you can test it to if the wiring is good . If it is then it mite be the computer . but 1st get a non contact thermometer to see if you motor is really running that hot .

 

I did temp the engine, it is indeed running that hot. Thanks for the idea of the wire to the computer. I suspect the computer itself is the problem, but I am a new owner and I don't really know how these things work.

 

All I can find is this... I don't know what/where the ECT Sensor Input is though.

 

Disconnect the tstat sensor connector, with the engine running... do not jumper anything...the fan should come on. I cant remember but is your tstat sensor 1 pin and it grounds thru sensor body or is it 2 pins. if 1 then check resistance from the pin to the brass case and pin to the block while hot, as close to 210 as you can get it. It should read ballpark 600 ohm....same reading if 2 pins..just read pin to pin...If its over 800 or higher then your sensor is defective if your coolant is really at 210

 

I have a two pin. I checked the resistance while the Jeep was hot, gauge reading above 212 and the resistance was 850-1000... so 199 on the high end. I tested with a IR temp gun last week and it seemed like the gauge was accurate.

A separate temp I completed:
Unplugged the sensor and obviously the fan came on.
Put a 10K resistor in and the fan turns off. ( Simulates 77 degrees F. )
Put a 400 resistor in and the fan comes on. ( Simulates 246 degrees F. )
I know these resistors are a bit on each extreme, but they are what I have.

I think this confirms the ECM is working... ?

 

Just as an update. I have ordered a mechanical water temperature gauge and I am going to hook this up at the tstat housing. ( Using the sensor's location. ) I think this will give me a better idea as to what the temp actually is compared to the IR temp gun.

 

850 will not turn on fan...you need to be below 700... do you have round pins or square/ rectangular pins on your sensor....might have an accurate old one from my old 92...because of the location of your gauge sensor compared to your tstat sensor the readings may be a good bit off. I put my tstat sensor in a pot of boiling water to test resistance...always know thats an accurate 212

 

Yeah, I understand that 850 wont turn it on but I am concerned that it is so far from the gauge. It is two square pins; I assume your 92 uses the same as my 94.

Thanks.

 

I hooked up a mechanical water temperature gauge and the temp looked good. I placed a 10k resistor into the temp plug let the Jeep run. The temp went up to 221 degrees which is kind of high, but I discovered that the 10k resistor was causing the Jeep to idle up to 1k RPM. So hopefully this is the reason for the higher temp. Of course since the resistor was in, the fan would not turn on. ( The resistor was simulating 77-ish degrees. ) I swapped the 10k resistor for a 450 to turn the fan on. The temp lowered to 195 and stayed there. Hopefully that is a good sign... So the conclusion at this point is that the cylinder head sensor is the problem. This is a new part, however I had read from other forums that people have not had luck with anything other than the MasterPro... Maybe if you can get an actual Mopar sensor. Anyway, I picked up the MasterPro today and I will try it out soon and report back.

 

This is the sheet I am using for resistance --> temperature conversion.

 

Unfortunately the result was the same with the MasterPro sensor. I will post again if I think of anything... At this point I am leaning towards installing a separate gauge for the temperature. It is possible in my case that the dashboard gauge was never correct. I bought a gauge cluster from a '93 and took the temp gauge out and placed it into mine.

 

Alright, this is my last update. I have gave up on the temp gauge in the cluster. I have installed an AutoMeter cylinder head gauge. The nice thing about this gauge is that it senses at the proper place... the cylinder head. All other sensors I have dealt with were sensing at the tstat body or others were in a coolant line.
The AutoMeter was a bit pricey, but knowing the temp at the cylinder head is very valuable.

https://www.autometer.com/2-cyl-head...port-comp.html

This gauge so far is very accurate for me... my fan kicked on right at 218 on the gauge. ( I thought the fan wasn't working because my old gauge was exceeding 230 degrees F.