How much oil to add to a/c
 

I checked my low side pressure, and it was too low. So I added 12 oz of es-12. The reading was 50 lbs when it should have been 70 lbs, so I ordered another can and will install it when it comes. I have ac, and always have had some, although here a little ac goes a long way.:D

My question is "how much oil to add?" I have a 3 oz can available. I finally figured out from reading all these ac threads that the constant smear on the drivers side of my engine is from the ac system, and not the ps reservoir.

The system holds some pressure so there is not knowing how long it's been since any oil was added. Should I add any?

Only way to know how much to add is to remove all oil by flushing out all components, and filling the correct amount. Too much oil is as bad as not enough...

I thought you might say that... The problem is that I have no history so no idea when last oil was added. Besides which, the car was pressure washed before I bought it so no residue.

If someone added an ounce too much, would that ruin the system. I'm thinking adding a small amount might be in order, like maybe an ounce. I know this not how you operate in general; I'd just hate to see my compressor lock up for lack of maintenance. It's obviously throwing oil. A full flush is not called for since the system works, and besides, we hardly use ac here anyway!

Or will the compressor warn me when it's low on oil by squeaking before it locks up?

Disclaimer - I am not an A/C expert. This is what I do normally and it always works. If I replace the receiver/drier then I always add 8 oz. If I don't then I add 4oz if I 'feel/think' oil is low. A lot of the time I don't. I use the pressurized can and add it thru the low pressure port. I don't think an oz here and there will harm anything. This is not rocket science.

It will happen all at about the same time.

On my 98 E300 I had a leaking o-ring at the hose coming off the top of the compressor. Replaced the o-ring and added approximately 1-1/2 oz of oil and have not had anymore issues since. I had charged it a couple times and could see the oily residue on the outside of the hose. You can hang meat in the car if you wanted to:).

I have experience with adding too much oil, and it is not good. However, if there is any leak detector dye in the oil that was/is in your system, you should be able to see it with UV glasses and light. If the oil you see by your PS pump is in fact refrigerant oil coming from the compressor, you have a major refrigerant leak as well. Make sure what is leaking and where before assuming you can/should add oil to the system. You could have a small leak in one of the hoses (tiniest pin holes) that leak refrigerant, but not oil (oil molecules larger than refrigerant). That was exactly the problem I had:
http://www.peachparts.com/shopforum/diesel-discussion/369674-i-should-have-known-better-than-try-bring-gm-r4-compressor-back-life.html
Every time refrigerant was added, a little oil was added as well. It was assumed that oil leaked out with the refrigerant, but that was not the case. A cluster of small pinholes was found on the large hose going into the cabin (soapy water), but there was no oil present. The hose was replaced, and more oil added with the refrigerant charge. Don't do what I did and assume you can keep on adding oil each time you add refrigerant! The results can be disastrous!

You also said you are using ES12. I don't know much about these refrigerants or the specific oil they require, but you may have a problem wit oil compatibility as well. Again, it is always best to start from scratch, and flush the system. It also goes without saying, but you should fix any leaks first!

Es12 is compatible with any oil.

I do not think 1 extra oz of oil in the system will hurt anything.

Changing just a dryer, and using a full charge of 8oz oil is a very bad idea...

OK. I know there is at least one leak on my system where the metal tubes mount to the compressor. If I want to start right when I take this off to get it fixed should I just flush everything out and start over since this is a low point in the system and oil leaked from here?

I would be a good idea to flush everything but the compressor before hooking everything back up after fixing the leak. You should replace the drier, and check for leaks after doing the repairs.

Any brand of drier preferred?

pretty sure even the behr is made in china now, so probably no preference anymore

70 psig on the low side, w/ compressor running sounds way too high to me. I usually see 15-40 psig, depending on the temperature and if fan is running. If the engine is off, after pressures equalize 70 psig is about right on a 70 F day.

I have been frustrated at the inability to determine if there is oil in an old system. I have converted two old cars from R-12 to 134a, and in both cases I was not able top find any oil in the driers or compressors. Both were still working so there must have been a bit of oil but most had apparently leaked out with the various freon leaks over the years. I have since discovered that I can get a feel for the existence of oil in a system if I open the low pressure valve briefly while the system is running. Since the valve works just like a tire valve, pressing down on the stem opens the valve. The refrigerant and any oil circulating with it will squirt out on your fingers. Once the refrigerant evaporates there should be some oil on your fingers. It is my assumption that having a noticeable amount of oil is all that is needed. I am not an expert, and I am sure an expert will find this subjective oil measurement ridiculous. However, I now have an idea as to how to decide when to add one of those oil charge cans.

Here is a recent article on AC system oil charging:
A/C Oil Service: Avoid Compressor Failure With Proper Maintenance
It's from one of the trade mags I get at the shop. (nope, its not MB specific - just auto AC in general)
Hope this sheds some light on the subject...

Pelican still has the OEM for $190.

Is there a good way to know how much oil is left in the Compressor?