evaluating an AC compressor when system is currently discharged - PeachParts Mercedes-Benz Forum

kuene · #1 04-25-2018, 11:30 AM

The compressor on my car (82 300D) is currently hooked up via the belt, not seized to my knowledge, and not original to the vehicle. My system is still r12.

I'm about to start an attempt to revive my AC. Before I pour incredible amounts of money into it or tearing apart the whole system and dash - which i'm unwilling to do - I want to start with receiver dryer, vacuum the system, attempt to check for major leaks, new oil and some r12 I have. Perhaps just swap in a new expansion valve too. But obviously all of this is pointless if my compressor isn't coming back to life. How can I evaluate the basic operational condition of the compressor? Since my system is not charge, its obviously not going to kick on.. or should it?

Also - is this service port in this photo my high side or my low side? https://photos.app.goo.gl/CihVmHgBXLgemLeW2

tangofox007 · #2 04-25-2018, 12:11 PM

Quote:

Originally Posted by kuene

Also - is this service port in this photo my high side or my low side?

Low.

ILUVMILS · #3 04-25-2018, 12:12 PM

You can jump the low pressure switch and the compressor should come on. Don't run it for more than a few seconds though.........

MCallahan · #4 04-25-2018, 12:57 PM

See if the center portion of the pulley can be turned by hand. If so, compressor is not locked.

97 SL320 · #5 04-25-2018, 01:02 PM

Does this have the R4 ( GM ) compressor? I'd pull the discharge hose and look for metal / gray coating ( aluminum wear ) in the compressor discharge port as both are a sign of compressor issues.

With the engine off, turn the compressor clutch by hand, it should rotate pretty smoothly. ( not the belt driven pulley, the face of the clutch )

A failed compressor is the least of your concerns if you are unwilling to pull the dash for a evaporator change. The next step is to pressurize the system with nitrogen or even R134 since it is handy and R12 is $$$ . See if it holds pressure overnight, of not, isolate components with plugs to narrow things down. With R134 you can use a freon sniffer to locate an actual leak.

When pressurizing with R134 you do not need to run the compressor or fully change the system. Just connect the hose, open the can and let the gas flow. When pressure has stabilized, disconnect the can.

kuene · #6 04-25-2018, 01:20 PM

Thanks for tips. I disagree about what is and isn’t the lest of my concerns. The compressor is one of the components I can easily get access to. If it’s crapped out, then my AC quest ends. If it works, then I move on to next step. I’m not assuming my evaporator is fine, but I’m not going to start this whole thing by tearing apart my dash. AC will be a nice to have.

Is it safe to put r134 in m system to check for leaks under pressure when I am definitely not converting it from r12? It won’t harm the seals or other components?

Diseasel300 · #7 04-25-2018, 03:14 PM

R134a won't harm the seals or anything else in the system. The W123 and W126 aren't known for having a lot of problems with the evaporator. The later 124's and 140s are much more common evaporator replacement candidates.

If you're going to have the system open already, do the job right and do it once. Check the discharge side of the compressor for metal as mentioned above and replace *EVERY* o-ring you can get to. They're your most common source of leaks.

Be aware that modern expansion valves are designed for R134a and have a smaller orifice size. Running R12 on an R134a expansion valve can result in poor cooling. Unless the system has been open to the air for a period of time or had the compressor grenade and send shrapnel through the hoses, the expansion valve is probably ok.

KrustyKustom · #8 04-25-2018, 10:28 PM

Do you have A.C. Tools?
I would pull a vacuume on the system and see how long it stays tight. Next if it has remained sealed and nothing is oily you could try giving it some 134a, jump compressor out and see how it sounds and works.

From there you can decide if you want to go ahead with full 134a conversion, changing O rings etc or dig deep into the savings account for some r12. I like r12 but would not charge it with such unless I knew all was tight.

Sometimes when older dormant systems are brought back online they will find any weak points in the system within a week or so of using them.

ROLLGUY · #9 04-25-2018, 11:10 PM

Guaranteed you have a leak, and that is why the system is discharged. Most likely it is an Oring or compressor seal. It could also be a hose. It would be best to find the leak before proceeding. Pulling a vacuum will NOT help in any way to find a leak. Pressurizing with nitrogen is the best way. Spray a mixture of soap and water on all the fittings and hoses. The problem with the R4 is that it has no oil sump. When the refrigerant leaks out, the oil goes with it. If you find a leak and then fix it, you won't know how much, or if any oil is still in the system. Guessing at how much oil is remaining is just that- a guess. Too little oil, and the compressor will eat itself up. Too much oil, and the cooling efficiency will suffer. The BEST thing to do is flush the entire system except the compressor, and start over. While flushing, if you see anything that looks like metal, or the oil is black or dirty, the compressor should be replaced. Once everything is back together (the drier is the last component connected), check for leaks again.
Of course I would suggest going with a Sanden retrofit. You will be money ahead in the long run. Even if your R4 has life left in it, my bet is that it is not a lot of life. A short lived R4 is a common theme in these cars........Rich

kuene · #10 04-26-2018, 09:31 AM

Thanks for all this great info.

I do have gauges, vacuum pump, new orings, and in fact cans of r12 and a can of r12/oil.

I’m not going to just throw r12 into the system and risk losing it and of course being a jerk with the atmosphere.

I don’t think the system has been fully open to air - in fact when I just touch the shrader valve a little pressure still release - perhaps that is meaningless.

I think I’ll evaluate the compressor and post some pics of it next.

barry12345 · #11 04-26-2018, 10:39 AM

The small amount of pressure remaining is a better situation then none at all. At least in my opinion. Defeat the low pressure cut out for a second or so to see if the compressor runs as well.

MCallahan · #12 04-27-2018, 12:12 AM

Guy told me the AC didn't work in my W124 when I bought it. Same as yours, it had a little pressure in the system. Read 20psi static when I put a gauge on it. Charged it up and it works perfectly. I'm sure it probably has a slow leak, but the compressor was free and it still had 134 in it. I vacced it down and recharged it in Feb. Freeze you out right now with temps in the 80s.

ROLLGUY · #13 04-27-2018, 10:30 PM

Quote:

Originally Posted by MCallahan

Guy told me the AC didn't work in my W124 when I bought it. Same as yours, it had a little pressure in the system. Read 20psi static when I put a gauge on it. Charged it up and it works perfectly. I'm sure it probably has a slow leak, but the compressor was free and it still had 134 in it. I vacced it down and recharged it in Feb. Freeze you out right now with temps in the 80s.

Your W124 probably has the Denso compressor, and is much better at keeping sealed than the R4. The Denso is similar design to a Sanden.Having 30+ year old hoses doesn't help either. I would not gamble with an R4 or old hoses, hoping it will keep the R12 in the system. It is just too costly.

MCallahan · #14 04-28-2018, 11:55 AM

Running 134 in this one, so not too worried. Hoses seem fine. Will take my chances until proved otherwise.

vstech · #15 04-28-2018, 12:08 PM

NEVER CHECK FOR LEAKS WITH VACUUM...

ALWAYS USE PRESSURE FOR 24 OR MORE HOURS.