Same '91 5.2 auto 4x4 we were talking about a while ago.

The AC in this thing has never worked, but now is the time to sort it out because I had to pull out the HVAC assembly to take care of a stuck heater blender door and sort out some ducting screwed up by the PO.

The problem is that the compressor doesn't engage. A few years ago I took a stab at figuring out why, and found that if I force the AC clutch relay (in the driver's side shock tower relay/fuse box) on it runs and blows cold, just as it should. But for the life of me I haven't been able to figure out what drives that relay. The dash controls (other than the AC switch) are cables and vacuum, and all that stuff seems fine. But what sits between that dash switch and the clutch relay? The Haynes manual gives no hints whatsoever, and I really don't know where to start.

 

Wiring diagram. Not much to that circuit..... maybe the low pressure switch has failed?

 

Wow - thanks very much. Lemme stare this for a while and I'll probably be back with more questions...

 

Yeah, a couple. First, that low-pressure switch: I assume that's the one on the firewall where the AC lines pass through, and that it's meant to keep the compressor from running when there isn't enough of a refrigerant charge. Fair enough.

Second, what and where is the "electronic cycling clutch switch (in-fin sensor)" that's between the panel AC switch and the low-pressure switch?

Finally, the ECU is what actually drives the relay for delay and WOT control. That's pretty black-box, so it'd make sense to verify the other two before looking at this.

 

Low pressure switch: Yep, that's be the feller.

Clutch cycling switch: No clue..... not familiar with that system.

Yup. Check the easy stuff first.

 

Oooo... I don't like the look of that. From what I can google the cycling switch is a thermostat, and "in-fin" suggests that it's mounted on the evaporator coil. I just put that whole miserable HVAC assembly back in, and I'm not thrilled about having to wrestle it back out to get at this switch (if it looks like it's the problem).

Can anyone confirm/deny that this is where it is?

 

(Later...)

Working from HeyYou's diagram (the colours are wrong but the circuit is right), I've found the pressure switch and ECU I/O (WOT control) to be working fine. That is, I can ground out that signal and the clutch engages (whee!). And when I turn on the A/C dash switch its output goes to within about 20 ohms of ground, which makes sense. That leaves the A/C cycling switch (which is indeed stabbed into the evaporator coil fins and comes out with the correct three-pin connector) as the remaining culprit.

However, nobody (including the local christler dealer) seems to be able to find this part. The listing for it everywhere I've looked shows the two-terminal pressure switch instead.

Does anyone have a clue on this one?

 

If you go here, and look at the diagram, I do believe the part you are looking for is 13a..... which does NOT show up in the listing........ It *might* come as part of the evaporator core, as I see two different cores listed, and they specifically state "WITHOUT cycling switch..."

Maybe check some of the other mopar sites, and see if anyone lists it. I don't hold out a lot of hope though, as I suspect they ALL use the same databases for diagrams, and part numbers and such.

Edit: Oops. the first one does indeed show WITH cycling switch.... of course, it's NLA........

 

I've got the full ac system from my 90 I ripped out. Don't know if any parts cross to 91 but you're welcome to them.

 

Ah, yes. That's the right answer. It's not a good answer, but it's the right one. Completely explains why there's no reference to it anywhere. Only available (when it was available) as part of the evaporator.