Ive had an '01 XJ for coming up on 2 years. But the aux fan always comes on as soon as I turn the key to on. It will also stay on until the key is turned to off.. There's this little black box that runs into the Power Distribution center (very obviously a home job).. and I've tried replacing the relay and fuse for the fan.. any suggestions?

 

Is this regardless if the a/c or defrost is engaged at the time?
Otherwise I'd be willing to bet that the PO had cooling issues and wired the aux fan to come on immediately
On I believe 98ish on up, (could be wrong on the start year)...but on up... the aux fan is tied to two things. One, if the a/c compressor is engaged (both for a/c and the defrost option) uses drier air to help with defrosting.
OR... when the temp sensor reaches I believe somewhere around 220 degrees F to assist with cooling.
So if it's wired on... I'd bet they had an issue with cooling and just rednecked it instead of fixing it the right way by finding out what was causing the root of the issue.

 

On the 01 the fan is only tied to coolant temp or the ac high pressure switch...if the high pressure trips it unlatches the compressor and turns on the fan...just turning on the ac does not automatically turn on the fan. What does your coolant gauge read...an open in the sensor reads low and the fan turns on but that should trip a cel

 

Interesting... mine works the correct way....but on my 99, it automatically comes on when the a/c is engaged for a/c or defrost function. I hope to never have the temp cause it to come on... lol

 

Thats the way a 99 is supposed to work..can't imagine that the aux fan doesn't come on in traffic...the clutch fan doesn't pull enough air to keep engine cool. If you get a pencil point or two past 210 you have tripped on aux fan, you just don't know it

 

AC could be overcharged.

 

at that low of a temp? I suppose it's possible, but thought it wasn't supposed to kick in until like 215-220 range.
Will have to check it now that you said it....but fortunately mine only wants to creep up after sitting for several minutes sitting in warm temps in stopped traffic.

 

The numbers on the gauge get squished together on the up side of 210, so 220 is just a hair or two over the 210 mark. If you have an obd2 reader you can look at the parameters...and see where the fan trips on. Sadly since the gauge is driven by the computer and the computer only gets temp signals from the sensor in the t-stat.....you have no external or secondary verification of sender accuracy. The scanner and gauge will almost exactly match EVEN if the temp is actually off because your single source sender might be off.. On mine 200 is barely a needle width before the 210 mark so 220 is only about a needle above 210

 

Most of the time she sits just under the 210 mark and will rarely move.... only in extended stop and go when it's warm out during the day or even an evening down here where it's still 75-80 at 9 at night.
But that being said... I prefer to be proactive instead of reactive.... I've tested the mechanical fan/clutch when cold and seems right what it should be for resistance...but I've honestly never tested it after having the system fully up to op temp and then shut off and test.
Think I'm gonna do that soon just for peace of mind. Whether it shows I need a new one or not, I'll know.

 

Wondering...If his temp sensor is unplugged or grounded out would this also cause it to come on as soon as the switch is turned on? He says nothing about his temp gauge.... just a thought.