Why doesn't my Aux fan come on w/o the A/C?

My Auxilary fan comes on and off with the A/C like its supposed to, but it doesn't come on when my XJ gets hot in traffic (without the A/C on) like it should at 218*. What is it that goes bad in the wiring. I don't know much about wiring but I took a look at mine the other day and all the I saw was a wire coming off the wiring harness by the computer into the plug that goes into the fan. Everything looked good there, but maybe there is something that I'm not seeing. I keep on hearing about people replacing a relay and it fixes the problem? What relay? Where is it?

I pretty good mechanically but a real dumbass when it comes to
wiring. :dunno:

Edit: I've recently replaced the temperature sensor on the thermostat housing that the computer reads off of.

If your fan cycles with the ac then the relay is ok. Maybe it's not getting hot enough. I'm assuming the temp sensor in the tstat housing is ok since you mentioned replacing it. Are the connections to the sensor okay? You haven't noticed lower than usual gas mileage have you? I'm not sure about this, but if your sensor is faulty (or the connections), maybe the pcm thinks it hasn't warmed up and would continue to run in open loop mode causing poor mileage. On the other hand, maybe I don't know what I'm talking about here. :gag: Just an educated guess.

You say it's not coming on above 218* like it should which leads me to believe that your temp gauge shows higher temps than that. Have you checked your temp sending unit that is on the drivers side rear of the head. That's the one that goes to the gauge. I had a bad one on my old 93 YJ and that gave me high readings.

K

Thanks for the reply. I get really good gas mileage. In my driving, which is mainly hwy, I get 18-19 mph granted I can keep my foot out of the gas. . I also forgot to add that I replaced my rear temperature sensor, the one that the gauge reads off of about a year ago. I replaced pretty much everything I could think of as preventative maintenance on my XJ about a year ago when I had some extra cash.

I'm starting to think that maybe the gauge is off. I know its rare but everything in my cooling system is new. New modine 2 row radiator, new Napa water pump, all new hoses, new carquest clutch fan, new Mopar thermostat, and a recent flush in which I burped the system forever to be sure all the air was out. I'm stumped here.

Oh, and new properly adjusted serp. belt

If you go to a dealership or specialist with a DRBIII or equivalent scan tool they will be able to read what the engine controller thinks the engine temperature is. You can compare this with your temp gauge reading and see if they tally. The engine controller uses info from the sensor on the thermostat housing to control the electric cooling fan as well as fuelling and ignition advance so if the motor is running ok, this sensor is probably ok.