NewJerseyXJ609

Hey everyone,

Alittle backstory. I have a 00 xj sport 165k miles. No oveaheating issues to speak of. All around just a solid jeep. Ive owned it for about 4 months now.

In those 4 months i have not once seen or heard the aux fan kick in. Whether the a/c is on.. nothing.. Maybe its not supposed to come on with the a/c..maybe the motor hasnt got hot enough to trigger it..

So today i set out to troubleshoot the fan since summer is around the corner.. I first used my power probe to shoot power directly to the fan. The fan worked like a charm. First time ive ever seen or heard it run! Second thing i checked was the relay. I tested it with the power probe and by my test it was good. After that i again used the power probe to shoot power from the PDC to the fan and the fan engaged.

So we know
A:The fan works
B:The relay is good
C:The wiring from PDC to fan is in good shape.

What am i missing here? The jeep likes to hang right under 210. Ive seen it go over 210 for a breif moment here and there but only while sitting in traffic and it never was hot enough to engage the fan i suppose... But when i have the a/c switched on the fan should be coming on intermittently shouldnt it? What tells the fan its time to come on?

Thanks guys. Yours answers will help me fix my jeep and a freind of mine is having similar issues. ( He has a CEL for the fan circuit and the fan is in-op)

Something else i want to add: When i plug the relay into the PDC the relay 'clicks' once. Nothing after that. I tried swapping the relay out with another one for piece of mind.. still nothing. Im using the assumption that when the a/c is switched on the fan should be on to test it btw.

98REDXJ2

could your coolant sensor be bad and just not sending the signal to engage it?

I noticed mine had now been working for awhile too but turned our my aux fan was toast.

NewJerseyXJ609

Yeah I was thinking that I forgot to add that to my list of questions.

Is there two separate coolant temp sensors? One for the ecu/fan and one just for guage? Or does the sensor on the thermostat housing do it all?

The guage seems pretty accurate.

1976Mustang

I can't remember for the post-97's, but pre-97's have to sensors. One on the tstat housing for the computer and one on the back of the block for the gauge.

NewJerseyXJ609

That's what I thought. This is my first lat model jeep. So i'm not sure on these.

If someone could clarify for me: If there are two sensors? Which controls the fan which controls the guage?

And I still need someone to go outside turn their jeep on and turn the a/c on.... Does your aux fan come on intermittently?

SeriousOffroad

your XJ has 1 sensor, it's in the t-stat housing. unplug the connector and see if the fan comes on.

During normal operation the fan comes on when coolant temperature exceeds 213 deg. F.

For A/C or Defrost the Aux. Fan comes on when only if there is adequate pressure in the A/C system. low pressure as a result of a refrigerant leak means no Aux. Fan.

CCKen

Here's a method of testing the electric radiator fan coolant over-temp setting:

NewJerseyXJ609

Awesome thanks alot guys. I checked my a/c level and it's low on freon so since the system has low pressure that's probably why the fan isn't engaging then.

As soon as I get my hands on a proper resistor I'm going to try the other test and I'm saving that in my bag of tricks

And so I have this correctly, if I perform the test on the temp sensor and the fan still doesn't turn on we can safely assume the coolant temperature sensor is the culprit?

CCKen

No. The ECT sensor also provides the PCM with temp data used to drive the coolant temp indicator in the instrument cluster through the CCD Data Bus. If the ECT sensor was bad you wouldn't have coolant temp indication.

If the fan doesn't turn on doing the test there could be many other things wrong with the system Test it out and we'll go from there.

Here's an article about 2000/2001 AC system. It's written as a school thing but it does explain how your AC System works. Stash this away in your trick bag as well.

http://www.askamechanic.info/askamec...nt/view/73/47/

NewJerseyXJ609

Awesome, thanks. I figured the sensor ran the guage also. And judging from the guage it seems pretty on point.

I will try the test and get back to the thread to share the results. An I'm going to read that article. Thanks again

NewJerseyXJ609

Hey CCKen... Couldn't I just use a paper clip to test the temp sensor in place of the resistor? So if the resistor makes the guage think it's at 235 the paperclip would just spike the guage all the way hot

CCKen

Yes.

A paperclip would simply short it out and indicate the highest possible temp. The ECT is a thermister, a resistor, that changes value as it's temperature changes. In the case of the ECT, the lower the temp it sees the higher the resistance it creates. And the higher the temp it sees the lower the resistance it creates. Shorting the connector out (Zero resistance) would trick the PCM into thinking the temp was very high.

At -40*F coolant temp the resistance would be between 291,490 and 381,710 Ohms, and at 248*F the resistance would be between 370 and 410 Ohms.

The ECT resistance modifies a voltage supplied to it by the PCM. The modified voltage is what the PCM interprets as the coolant temperature.

Using a 470 Ohm resistor simulates the ECT resistance equivalent of 230*F - just above the value that would turn the fan on.

If you're that anxious to see if your fan runs you could try a paperclip and see if it would put the fan on, but, if nothing else, it will put a CEL on. An additional problem of shoving a paperclip in the connector pin sockets is that the sockets are very small and a paperclip may expand the sockets to the point that the pins of the ECT would no longer make good contact.

NewJerseyXJ609

Good info! I just really wanted to test it today but I was at work and for some reason we had no resistors on hand. Tomorrow morning on my way in ill stop at radioshack and hopefully they will have em. I don't want to chance anything. When I was at work I was asking around to see if anyone had one and one response I got was "use a paperclip" so I figured I'd ask.

freegdr

Is it overheating ,fans only run when ac pressures reach a certain point on newer model xj they do not run like older models .

NewJerseyXJ609

Nope like I said no overheating. I just thought it was strange never seeing/hearing it come on in the months I've owned it. And either summer around the corner I want to make sure the cooling system is in proper order.