91 Park Ave. A/C troubles...

My wife's 91 Park Ave. A/C has quit again for the fourth time in as many years and fixing it is becoming a rite of summer.It is the standard A/C setup without climate control.All fans have always been functioning properly
It began with me (I am an A/C mechanic by trade, so I understand the components and the cycle well.) troubleshooting the problem to a bad clutch. Sent it to my mechanic for diagnosis, without tipping him off, and he concurred with my finding. Repair made and working fine. Later that summer it quit on us. Brought it back and he had it working in a few minutes by "resetting" the module Says it may eventually need replacement. Here's where my knowledge drops off...
It worked fine for rest of summer. Next summer the module burnt out. He replaced it, and worked fine for 2 weeks and quit again. He diagnosed it, same problem and replaced it no charge. Next Summer it quit again. Lost whole charge. Blew the fusible relief plug on compressor.Repair and re-charge. Quit again, lost charge. Plug OK, leak tested,& replaced Evap coil. I brought home the old coil and tested it myself. Held 60# of air in a bucket of water 24 hours. He made good on it and ate the cost of replacement.Working fine. Later that summer it was working intermittently, and the lines were frozen. Brought it to him and they realized the compressor was not shutting off. Made a call to a friend of theirs, crawled under the passenger side dash and said... Shorted module causing problem. replaced again no charge and worked fine for rest of summer.
Guess what happened first hot day this year? I briefly looked at it, compressor not pulling in, jumped LP switch, still not pulling in.
Now the question; Where is the "Module" and how might I recognize it? Any troubleshooting help would be greatly appreciated.
I have known this mechanic and his son for 25+ years and I am sure he is honest, although maybe not the best a/c guy.

I cut my teeth on those cars, for the life of me, I'm not aware as to what you guys are referring to as the module, especially since it is not a climate controlled car....

Those cars will set a low refrigerant code, and can be reset by unhooking battery for 10 seconds. The most common a/c failure mode is loss of refrigerant due to a leak..most common leak location is the compressor shaft seal and the compressor belly o-rings.....

So when it is inop, is it low on refrigerant? my guess is that it is. You can test for this by reseting the code, then run the system- if it rapid cycles and doesn't cool, then it is low on refrigerant, and will soon set the code again.

Your mechanic may just be resetting the code and saying it is a module---ask him for a part number, or a location of it-- point to it,, or give you one of the old ones.....

The code is set by the ECM, not any module...so is he replacing the ECM? That would be the main computer for the car.....

It is the standard A/C setup without climate control.Do you have digital display in the control head?

brcidd, if it were indeed low on refrigerant, wouldn't jumping the LP cutout trick it into thinking the charge is ok? Or is it not that simple.
HotZ28 - No it doesn't have a digital display.

As mentioned, try disconnecting the negative battery cable, then hook it back up and start the engine to see if the compressor will run momentarily. If it is indeed low on refrigerant, you can add some while the compressor is running. Has anyone changed the expansion valve (orifice screen) on your system? Are you using R134a, or R12?

My mechanic said he did replace the exp valve when they did the original compressor replacement.
I am fairly certain it is a drop-in replacement, but I don't believe it is 134-A because when I hooked up my gauges to it once, the pressures were out of correct range for R-12, I asked him why and he used a term like "Hot-Shot" or similar which I know to be drop-in. If it runs I'll reclaim the charge and put R-12 back in as I still have about 20# or so. ;-)>

Jumping the low pressure switch won't help if a code is set....been there done that several times.....your mechanic needs to bone up if he is to be an a/c expert.....

My first C-car, I spent 6 hours tracing problems as to why compressor won't come on.. until I finally hooked up the Tech I and found the low refrigerant code. Once reset- all was fine.... You just have to know what to look for...

If you were able to hook-up your R12 lines to the pressure lines, evidently the system has not been converted. When using Hot Shot (http://www.icorinternational.com/images/1_HSPS.pdf) for an R12 replacement, it only requires 80% (by weight) of what an R12 charge would require. If that procedure was not followed, you will have erratic pressures and line freezing. I would evacuate the system and hold a vacuum for 45-min to boil off any residual moisture & check for leaks, then recharge with R12. I hope they used mineral oil when replacing the compressor. :dunno: NAPA Capacity Guide (http://www.napabeltshose.com/downloads/download_common.cfm?file=NapaCapacityGuide2-8-05.pdf&folder=news&view=napa_hc)