Defrost requires the AC compressor ...?

Question

So I'm parking the XJ today after driving home, and as I get out I notice that the aux cooling fan is still spinning (Ignition off, key out). It's been raining, so it hasn't been whatcha might call "warm" out. Well, I have a motorcycle that does this sometimes, and I've only had the Jeep since October, so I decide to check it again later.

Twenty minutes later and the fan's still going ...!?! (The engine is very much on the cold side by this time.) I unplug it and go back inside and google this group and discover its not uncommon for this to happen.

I go back outside and fiddle (sorry for the tech speak) with the relay, and move the control OFF of the Defrost setting and lo and behold the fan doesn't spin after I plug it back in. Chalk up another repair to "hitting it with a bigger hammer". (Someone remind me to ask my mechanic about this later.Thanks.)

So, anyway, to get to the point (what was my point again?...oh yeah), while reading past posts on the issue, I read several posts that stated that the AC compressor is activated when you use the Defroster.

...?wtf?...

Please, someone explain. This doesn't seem to make sense.

TIA

(95 XJ Sport, 4.0L Auto)

...

billy ray · Accepted Answer

Passing the cabin air over cooling coils dehumidifies the air then it passes over the heating coils.

Will Honea · Answer

The a/c is turned on by selecting the defrost position.  The electric fan is turned on by the A/C relay.  That means the fan is indirectly powered up by the defroster (a/c dries the defrost air to make it workbetter).  BOTH the a/c and the high temp switch on the radiator trigger the fan thru a relay and both are switched power thru the ignition switch, so if the fan keeps running with the ignition off youhave some sort of electrical problem - possibly a stuck fan relay.  You don't give a model year, but on the 86-90 XJ, the fan relay is on the driver's side fender next to the air cleaner.  I'm not sure about the location on later models.  If you go back tomorrow and plug the fan back in and it starts right off, pull the relay.  That should shutit off and indicate a stuck relay.  Replacement relays are cheap - just get a new one and plug it in IF that seems to be the problem.

Danny · Answer

-snip-Usually the case.  The fan control on my Cherokee shows the defrost setting as being an A/C setting.  It's normal practice, since using the A/C in defrost mode will clear a windscreen in seconds, due to the difference in humidity of the incoming (conditioned) air.  Merely putting the heater on will usually result in a wet and warm windscreen that takes longer to clear.  A/C can be used in any heat position, I believe.It's a bit of a circular argument though, since when the A/C is off (on cars I'ved owned) the incoming air seems damper, so mists up more readily, requiring the A/C to quickly clear it...

Kate · Answer

I don't know a thing about your problem, but I got a good laugh out of your post!Kate: So I'm parking the XJ today after driving home, and as I get out I: notice that the aux cooling fan is still spinning (Ignition off, key: out). It's been raining, so it hasn't been whatcha might call "warm": out. Well, I have a motorcycle that does this sometimes, and I've only: had the Jeep since October, so I decide to check it again later.:: Twenty minutes later and the fan's still going ...!?!: (The engine is very much on the cold side by this time.): I unplug it and go back inside and google this group and discover its: not uncommon for this to happen.:: I go back outside and fiddle (sorry for the tech speak) with the: relay, and move the control OFF of the Defrost setting and lo and: behold the fan doesn't spin after I plug it back in. Chalk up another: repair to "hitting it with a bigger hammer". (Someone remind me to ask: my mechanic about this :: So, anyway, to get to the point (what was my...

noneyabusiness · Answer

AH!! NOW it makes sense!Thanks a bunch (to you AND the others who replied)!...

Jeff Strickland · Answer

Don't bother your mechanic with this one. The AC runs with the defroster because the AC is really a dehumidifier. The fog on the inside of the windshield is humidity, the AC is a dehumidifier. See the connection?

Defrost requires the AC compressor ...?

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