jrddawgyo

I have a 1999 Jeep cherokee sport, the aux fan used to come on, now It has been silent. A/C blows cold, but aux fan stays off when A/C is on, also havent noticed it on even after idling for about 30 minutes, no obvious signs of overheating... These fans should come on when A/C is on, or am I wrong? Any past fixes appreciated. I replaced the fan relay, and checked all fuses. I know the fan motor is good. Any suggestions? Would temp sending unit also cause the fan to not come on if it is bad, even if the A/C is on?

SeriousOffroad

Check the High Pressure switch and associated wiring.

Does the fan come on when you start the engine and unplug the temp.sensor at the water neck? (This will cause your CEL to come on, but it will go out once you reconnect the harness.)

jrddawgyo

So there are also pressure switches to turn the fan on? Does this happen only when the A/C hits a certain pressure? I will unplug the sensor @ the thermostat housing and try that. Should that activate the fan even if the engine is not @ proper operating temp? The jeep sat a week, and I know it worked last week, almost chopped my fingers off b/c I left the key on while tinkering to listen to music...

CCKen

Actually, if you are getting cold air when AC is turned on your Low Pressure Cycling Switch and High Pressure Switch are working. The fan should cycle on and off when the Low Pressure Switch is cycling the clutch on and off.

By disconnecting the ECT sensor the fan should run if the fan, relay, its fuses, and related wiring are good. If the fan does not turn on check the fan itself: With the key out of the ignition, see if the fan turns freely. If its frozen you know what to do. If it turns freely, try jumping the fan to the battery direct and see if the fan runs (See below).

Before doing the following make sure there is no raw fuel in the engine bay.

See pics below.

Obtain a safety pin, length of 12 gauge wire, and an alligator clip (Lowes and Home Depot has them). Back probe the fan connector Blue wire. Touch the battery positive post momentarily (there will be a small arc) and see if the fan runs. If it does, there may be problems elsewhere which would require further troubleshooting.

Try these things and get back.

Pics:

CCKen

I see you checked all fuses, but make sure the 40 Amp fuse in the PDC is good. Replace it if in doubt (see pic below).

I don't know what method you used to know the fan is good, but the method I describe in post #4 also checks the fan ground circuit. The ground for the fan is on the left inner fender, under the wire harnesses going to the PCM.

If the ECT sensor was bad your engine wouldn't run very well and you wouldn't get a temp readout on your instrument cluster. The ECT sensor is not wired into the AC/Fan circuit.

jrddawgyo

I will not be able to do a hot test on the fan. I will send voltage to it after work to see if it runs straight off the battery. It does not come on when I unplug the temp sensor in the thermo housing. Also still does not come on with the A/C (it did last week). Typically I haven't EVER had bad electrical aux fans....but I will test it nonetheless. I doubled checked all fuses are in working order, I put a brand new spare relay in place of the old fan relay, so that is ruled out. What else is there that really controls that fan other than the above items?

jrddawgyo

SO to get it straight, the temp sensor on the thermostat does not trip that fan on? Is there another sending unit, or are you just saying that it is seperate from the A/C circuit? Like you said, my temp guage works, and is accurate based of my laser thermometer within a few degrees. It runs fine. I also noticed my upper hose for my rad is collapsed, but I am looking into the radiator cap/ catch tank/ thermostat issue for that...These things just randomly popped up yesterday, but they will get sorted out.

I will try out sending voltage to the fan if I get a spare minute at work shortly. Assuming I am correct, and my fan is functional by sending voltage to it, what else could possibly be wrong, especially since it didnt have issues last week and it was just sitting in my garage during that time....

jrddawgyo

I just tried this again, still nothing. Check engine light did not come on though, oddly, until I put load between the 2 leads simulating overly high operating temps. Still no fan...

CCKen

If disconnecting the temp sensor (ECT Sensor) doesn't turn the fan on and you replaced the relay, and the 40 Amp fuse is good, then either the PCM driver for the fan relay control coil is bad, the fan is bad, or the wiring to control the fan relay is bad.

FYI: Jumping the ECT connector like you did represents over 300*F coolant temp.

Perform that fan test as I described in post #4 and we'll go from there.

jrddawgyo

Ok, checking back in with some results. As for the test I did on the temp sending unit, I didnt just jump it, I used a resistor I had laying around...couldnt see all the markings, but I believe it was 480ohm or near that..Anyway, I triple checked all relays, fuses, and connections. Double checked the AC clutch was kicking in (obviously, my AC is ice cold), checked for voltage coming to the fan plug, checked voltage after plug, did an "ear" test to see if the relay was clicking on when the AC clutch kicked in, and then finally direct wired the fan to the battery, completely seperate from the wiring harness to eliminate any possible wiring issues while testing the fan. The fan did not turn, or even try to turn. Tomorrow I will be at the junkyard looking for a new aux fan. I cannot believe it went bad after simply sitting, last week it almost cut my fingers off..

As for the upper rad hose being collapsed, the overflow line and catch tank inlet/outlet were completely closed off, clogged. After clearing the clog, reflushing the whole system, my cooling system is operating properly again, save for the aux fan.

Another nice note: I just got a new set of BFG KM2's (33x10.5) Gotta love the jeep life. You buy one nice thing just to find another thing broken on your jeep

jrddawgyo

Bought a 15 dollar fan at a JY and threw it in. Works great now.