Aux fan not working, but working, but kinda not? It's a Jeep thing

[Sobek]

Hey guys

Was hoping to get your input on something. I did google this as best I could but there's surprisingly not THAT much detailed info on the matter beyond "put in a manual switch" (something I do actually want to to, are there any manual fan switch harnesses out there to buy quick and easy?).

Anyway, my issue;

A few weeks back I took out my blower motor to check it and clean it for the first time in 25ish years. Thankfully, it was totally clean and no debris anywhere in there whatsoever, so I dusted it off and put it back together. On the way in I snagged and broke the brittle old tiny vacuum line that goes into the firewall right beside it without realising. As soon as I did this I lost my aux fan and all vent air except for the window vents. Fixed the vacuum line and now I have my vent air back, but my aux still refuses to turn on when I switch on the A/C / vent etc (I have no working A/C, all the lines rotted away and it's essentially just a pulley, but it's set up to still trigger the aux fan, been using it this way for over a decade so I basically have a manual aux fan via the vent controls).

If I give it direct power, it works. If I disconnect the thermostat sensor, it works. I replaced both the A/C and AUX fan relays with brand new parts (after testing other working replacements) and also replaced the (perfectly fine) fuse in the fuse box in the engine bay labelled AUX fan. Still, no fan.

People online in the past seem to just default to "ECU's stuffed just go full manual". Is that it? Can you guys think of ANYTHING else I could be missing here? Any other bits of wiring, or vacuum related nonsense or SOMETHING I could look at?? Really appreciate it.

But yeah if there are any ready-made manual fan harness / switch setups one can buy for an XJ let me know. Happy to wire it up all myself but a kit is always nice. Cheers

[rainman]

turning on the ac switch is actually a Ac request signal to the pcm.
the pcm then decides if it "wants" to engage the clutch and the aux fan. one of the inputs it looks at is the ac pressure switch, before deciding to switch on the ac.
there is no harness that im aware of. go jeep has a write up on it on his site. if yours is 96 or later you will trigger an engine light unless u use a relay cause the pcm can see you fvkn with the circuit and doesn't like it.

good luck with it.

[awg]

the aux also triggers at 105C...does it still do that ?

allowing my vehicle to idle with no air flow for a few mins (once its warmed right up), the Aux will cycle on & off

[Sobek]

Originally Posted by rainman

turning on the ac switch is actually a Ac request signal to the pcm.
the pcm then decides if it "wants" to engage the clutch and the aux fan. one of the inputs it looks at is the ac pressure switch, before deciding to switch on the ac.
there is no harness that im aware of. go jeep has a write up on it on his site. if yours is 96 or later you will trigger an engine light unless u use a relay cause the pcm can see you fvkn with the circuit and doesn't like it.

good luck with it.

Oh I see, I wasn't aware the ac controls were PCM controlled, but that makes complete sense, and explains why so many people just go straight to "PCM is buggered, bypass it". Mine's a 95 so hopefully should be no check engine light. I think I'll just do the manual switch, bugger it. Thanks mate

Originally Posted by awg

the aux also triggers at 105C...does it still do that ?

allowing my vehicle to idle with no air flow for a few mins (once its warmed right up), the Aux will cycle on & off

To be honest I have no idea. I removed the a/c condenser years ago and it improved the cooling performance SO much that I don't think my XJ has even seen 100c ever since. In hot traffic stopped at lights behind cars for ages I think I got it to 90c once but that was it. It just cools so good now, and I did a new higher flowing water pump and the thermostat a couple of years ago now. I'll give it a go anyway.

Assume it doesn't come on at 105 like it should, what would that suggest. PCM again?

Thanks guys

[rainman]

If not coming on at 104 I would look at relay, fuses wires, and fan.
Pcm not so much- however anything is possible

[Sobek]

Tested and I can confirm it comes on as expected at 105c!

[HamishW96]

Hey Sobek, the aux fan will NOT come on if the AC isn't charged, as the AC compressor will not kick on. If you wanna test it, just pull the ol' temp sensor out of the thermostat housing. It'll come on right away haha.