How do you test a weak AC compressor?

[mnw365]

I need some advice. My wife has a 98 SL2 187,000 miles and up until October of last year the 13 month old rebuilt AC compressor had been blowing cold air. I tried to convince the auto shop to fix the problem but for some reason they preferred to stick by the 12 month warranty instead of trying to work with a 12 year faithful customer who had brought a lot of customers his way. Needless to say I have stopped using them and I am back to doing my own repairs. Here is my question. I took the SL2 in to have the oil changed last month and while it was in there I ask the mechanic to check the AC. He told me that the system was fully charged and that the problem appears to be the AC compressor is weak. Is there some way I can check if a compressor is weak or maybe the expansion valve is not working. I do not mind changing them out but I do not like to blindly start changing things until I find the broken part.

[saturncrzy]

i would start with the Comp. If it is weak then just replace. The best way is to turn it on and stick a therm. in the right center vent. the temp should read anyware between 43 and 50 i blieve.

[ssicarman]

Welcome to the Board.

Assumption is that the system ois not presently giving you cold air?

Only way to tell if the system is fully charged is to pull everything out and then recharge it to the proper weight of R134.
Do you have the pressure numbers from the shop when they checked the system?

[ahimsa1]

You'll want to put a manifold gauge on and look at the high-side and low-side readings - this will tell you what your compressor is really doing, and may also help reveal other problems in the system. For example, if the low side doesn't seem to be low enough and the high side doesn't seem to be high enough, the compressor is probably not doing a very good job. If the low side is low and the high side is higher than it should be, there could be a blockage in the tubing before the evaporator that is keeping the freon from circulating. Both of these are potential causes for a lack of cold air despite a full charge of freon.

These two Richpin videos will help:

youtube.com/watch?v=PMM80xlon3U
youtube.com/watch?v=T3o1l4r_ERE

If you do decide to replace the compressor, you'll need a vacuum pump in addition to the manifold gauge, and you'll probably also want to replace the drier at the same time. Harbor Freight has a manifold gauge for about $40 and a vacuum pump for about $15 which will do the job - these tools could be paid for the first time you use them.

[fdryer]

Originally Posted by mnw365

I need some advice... Here is my question. I took the SL2 in to have the oil changed last month and while it was in there I ask the mechanic to check the AC. He told me that the system was fully charged and that the problem appears to be the AC compressor is weak. Is there some way I can check if a compressor is weak or maybe the expansion valve is not working. I do not mind changing them out but I do not like to blindly start changing things until I find the broken part.

You're right, you're blind about airconditioning (you're not alone btw) until you learn either the correct way or from poor advice (the hard way). If you never meddled with your refrigerator or window airconditioner don't meddle with car a/c systems. They're all alike - they're sealed and never to be serviced unless they're BROKE. They're broke when they don't cool any longer. If you cannot repair your airconditioner or refrigerator you cannot repair your car a/c system, period. Every store bought R134a refill charge kit means the owner has a LEAKING system that's broken somehere but refuses to acknowledge that even a slow leak means a broken system. Our cars came from the factory fully assembled and in operating condition with absolutely no problems with the car's a/c system - a sealed system just like the refrigerator. So when it fails for whatever reason, its either repaired the proper way or (as most try) use the band aid approach of pumping in canned R134a from kits in hopes this will work. Just like using heavier weight oil to fix an engine burning excessive amounts of oil when blowing oil out the tail pipe. Band aids to avoiding the problem.

If you didn't know this, anytime a car's a/c system is 'opened', as in removing and replacing a broken part (compressor, hose, expansion valve, etc.) the full complement of refrigeration equipment must be used to restore this system back to operation. Any foolish attempts otherwise always results in expenses far greater than expected due to ignorance. Refrigeration leaves absolutely no short cuts and rewards the few with a fully operating system that even fewer achieve the first time out due to lack of knowledge and experience. Consider it on par to adding turbo charging or super charging to your engine. Not so easy. Simply removing any part of this sealed system, literally and figuratively, opens a Pandora's box of issues that requires more than casual knowledge of car airconditioning.

To isolate either a compressor or expansion valve issue would require more knowledge than expressed here on these forums because of the very complex operating environment for a/c systems. I'm certainly not an expert and still refer to manuals that do not clearly spell out when an expansion valve or compressor fails because refrigeration gauges are required to monitor pressures at specfic engine rpm in order to determine failures somewhere. Anyone give you this much information yet? Probably not, to hide the lack of knowledge. By the time you realize what you're getting into, most will be in over their head because of the lack of knowledge to interpreting gauge readings - basic a/c principles understanding temperature/pressure relationships in a sealed volume. Remember, once you open a normally sealed system, all the equipment is needed to restore this system back. Either learn now before doing so or leave this alone and do some simple tests before deciding how much time and expense will be needed to repair a problem that may not be.

Buy a small dial type thermometer, available at most auto stores, that can be inserted into the center vents and run the a/c system while driving. Monitor the coldest temperatures at whatever setting you like. Normal cold temperatures will range between 35F to 45F. Then read this; http://www.saturnfans.com/forums/showthread.php?t=80107.

[mnw365]

Thanks for all of the replies. I think I will borrow some gauges this week and check the pressures. I considered doing this before but I was not sure what I should be looking for.