81 DeVille AC comp question

[Aaronz28]

So i finalyl tracked down and got voltage to my compressor - turns out a series of bad grounds and corroded connections... anyhow - the system is very low -    as such - the compressor simply does not come on at all....

is this correct?    I hear an AC relay firing somewhere and a test light shows that voltage changes to the compressor everytime it clicks - 

my question -  should the compressor still engage the clutch periodically?  or will it simply stay off all the time until it has sufficient pressure....

This car has sat for nearly 20 years -  no evidence of oil or leakage on any of the hoses - just no usage in 20-25 years.

i ask because i have about 5 cans of R12 - which i'd like to charge - but if in fact the compressor or the clutch mechanism has failed - i don't want to waste all that liquid gold....   

There are two leads going to the compressor - one from the harness - and the other going to the back of the compressor which i suspect is lifting the ground if the pressure is too low?

can anyone confirm or help me diagnose to determine if the comp or clutch had failed - or if i just need to fill it up..

I am waiting on my manual - so i can't really reference it

Thanks

Aaron

[TJ Hopland]

I don't have a manual from that era in front of me at the moment but your theory sounds correct on the electrical part.   I do remember something about a ground being switched in the clutch circuit.   Cadillac did it differently than the others with manual controls so that is what is messing with my memory on the subject. 

Chances of a system from this era sitting that long and still having a good charge or simply just being low are very slim.   Do you know what sort of ambient conditions this spent the last 20 years in?   Big temp swings like well below freezing in winter and 100+ summer are really hard on em.   When the system leaks its original charge it can actually suck air in in the cold but then actually sort of build (air) pressure when its hot.  Its the air that screws things up.

If you really don't care about long term success you can try charging it or vac it down and charge it and it would likely work for a while but if you really want it to work you should properly discharge it then take everything apart for a good flushing and new O rings,  receiver dryer, orifice tube, then do a proper lube, vac, and charge with R12 if you have it.   Compressor apparently being good is a good sign so a flush should do the trick.   IF the compressor was bad that would be more crap in the system and depending on the style of condenser replacement may be a more reliable option.    Even if the compressor is good its seals especially the front one could be a little tired.   They tended to leak when they were new so 30+ years isn't likely to have helped.  Long term success would likely be best to replace the compressor or have it rebuilt.

[Aaronz28]

so - i just went out - and there is a diode between the positive and the ground on the AC comp -  is that stock?

2nd - when i did manually ground the compressor on that terminal -  the clutch engaged and even stayed engaged for a few seconds once I released the ground -    shut the AC off - then turned it back on -  and it required manually grounding again -   so i'm gonna guess its simply low pressure -

I'll sacrifice one can of R12 and see if that makes any difference in its cycling... iknow of no other way to test.

Thanks

Aaron

[TJ Hopland]

Put a proper dual gauge set on it and see what the standing pressures are then manually engage the clutch if you have to and see what happens to the pressures?  That would be the right way to start out.   

It does sound like it may be low pressure.  The compressor running may be enough to temporary bump the low pressure enough to satisfy the low pressure switch but as soon as it builds a pressure differential it drops again.    Big question is what is the pressure building on?  Air?  134?  R12?   

Other warning that sounds less likely with yours but could apply to those reading along is are you sure its R12?  Lots of reasons people didn't leave adapters on the systems when they charged them with 134 or other stuff.

In this case sitting for 20 years increases the chances no one put anything else into it.  Mid 90's everything was expensive when it came to AC systems because it was the 'changeover' period.    Its when you have a car that has had some driving in the last 20 years chances increase that someone did something with it. 

Not knowing for sure what is in a system is another good reason to just start over if you want long term success.

Some older and better shops will have a refrigerant tester.   Some will say what it likely is other will just say if its pure r12 or not.   I have one of the R12 or not machines.   Problem is you need enough pressure in the system to run the tester so if its really low you can't get a good sample.

[TJ Hopland]

Oh ya, the diode was stock.   It was originally hidden by what really looked to be duct tape. 

[76eldo]

If the pressure is low the compressor will not engage.
I'd be tempted to replace the compressor even if it seems to work.
They are not too expensive and you may want to convert to 134A since its a newer type compressor.

I'd think if it's been sitting this long the clutch will fail in a short time.

Brian

[Scot Minesinger]

I worked on a 1982 Cadillac which is probably similar and the r134 iced you out of the car.  The electronic board kept compressor from turning on.

If car has sat so long, probably best to flush system and convert to r134 minimum.  Then you may need to replace hoses with barrier type and probably compressor will not last.