Chrysler Repair: A/C on switch at control panel burns out, compressor clutch, climate control

Question
Okay, I'll explain in sec why I'm asking on a Chrysler thread, but first I need to mention the vehicle is actually a '94 Coachmen Class A RV.  Coachmen used a Ford f53 chassis, and at least one of everything from every manufacturer who ever made a vehicle.  The problem is this.  Driving down the road late last summer, I got a serious amount of smoke out of the dash.  Some quick screwdriver activity, and I found that the A/C power switch on the climate control head was melted beyond repair.  No biggie, we ran the house A/C on generator to get by.  A couple of days ago, I pulled the unit, and found that it was a Chrysler part, commonly used for climate control in 91-93, in a lot of their cars (Acclaim, Charger, Spirit, etc).  It's an older unit, but I found one local salvage that had one, which I grabbed up.  No problem reinstalling, everything back together and looking good, A/C running cold.  You're getting ahead of me, right?  About an hour in, there's that burning plastic smell again.  Sure enough the switch is stuck, and the A/C is no longer on.  Although I've not yet dug deeply into the engine compartment, the exposed evaporator is also a Chrysler part, so I'm making the assumption that the entire chassis A/C is from that same family.

So, obviously in addition to two burned up climate control heads, I've got something making that switch overheat enough to toast it.  Nothing else seems to be adversely affected, but with electrical, I'd hate to be wrong.  

Any ideas?


Answer
Hi Dave,
I've looked at a '94 circuit diagram and it appears to me that the control switch in question is part of the control system of the compressor clutch, mediated by the pcm (engine computer). The pcm will turn on the clutch if either the A/C or defrost button is pushed, the blower in the cabin is set to one of the run positions, the a/c cycling switch (which is mounted in the evaporator fins and is closed so long as the fins are not below freezing temp which is a design to prevent ice up of those fins) and the refrigerant pressure in the system is in the correct range. When that pathway from the ecm thru those switches goes to ground the ecm will energize the clutch. None of those components should draw a lot of current through the control switch EXCEPT for some reason the cycling switch has a third pole with a 12V supply coming from a 5 amp fuse. I don't quite know what that is for except that temp sensor is probably a solid state device and needs that voltage for bias to determine the temp of the fins accurately.
So my thinking is there may be a short in the fin cycling switch that is allowing the bias voltage and hence the current to flow thru the control switch straight to ground which will overheat it. So see what kind of resistance to ground you see there, and if that was a dead short or not. The cycling switch has 3 wires: the white is a 12V supply (center pole), the other two wires at the switch (the side pins) I suspect then are the normal switch contacts which is what the pcm control is looking for to be either open (if the fins are iced) or closed to decide whether to activate the clutch. All that is low current stuff as best I can tell.  Check the resistance between the center pin and the side pin which was connected to the dark blue/yellow wire, at the cycling switch, to see if there is any measureable resistance. If it is shorted then I think that is a sign of a failed fin sensing cycling switch. I have to admit to a bit of guessing here but it makes sense to me. That is the only current source that could melt the switch.
You can use the "thank and rate" tab to get back to me quickly without having to wait for "availability".
Roland
PS You might be able to substitube the defrost switch from the original unit for the burned out AC switch and not have to but another one?