A/C Compressor oil

bad_assr

New member
I'm sure this has been discussed before, but I couldn't find the answer I needed. I'm replacing an A/C compressor due to a bearing failure. I am replacing it with a used, low mile compressor. I have all new o-rings and a new drier. How much oil and what specific oil needs to be put in the compressor and drier before I reinstall them. I plan on evacuating the system once the parts are replaced. Thanks.
 

jmoller99

Own a DAD ODB2 Unit.
The system was designed for R134a. Many auto parts stored sell the oil for it (PAG). Don't mix anything for R12 in your system.

From the Service manual...
 

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Bob of QF

New member
I know that most of the procedures say "never mix oils" but from actual field experience this is rarely a problem, in that PAG and the more traditional R12 lubricants do mix together well enough-- too many DIY conversions from R12 to R134a without any issues to believe the hype that you must flush out the old R12 with solvent before introducing R134a and PAG.

That being said?

A vacuum is really required, as you cannot permit any moisture in the system-- it'll (moisture) ruin the compressor, and drawing a vacuum for at least 30 minutes is not only a good idea, but is a must-- go longer, if you're working on a high-humid day. There is no such thing as "too much vacuum" but there certainly is too little!

The vacuum's purpose is to boil off any moisture in the system-- water boils at low pressure, remember, and 30 inches of vacuum (the theoretical maximum) will boil water at room temperature.

As for how much oil? That's a variable that depends on a lot of things, but if you cannot locate the exact value anywhere? Plug (with blue tape) the outlet on the compressor, and pour in PAG until it's full, slowly turning the shaft by hand. At the very least, you'll have filled up the compressor, ensuring it has plenty of lubrication.

Some factory or rebuilt compressors come pre-charged with oil-- it never hurts to ask your supplier if this is so-- and he may well have the spec on how much to use, too-- I've seen as low as 2 ounces, which is not much at all.

I also prefer to use the PAG with the lovely UV die already mixed in-- why not? It makes spotting a leak much easier than otherwise.

If you have the spec, and it calls for 7 ounces (say), and you can only get 1 or 2 ounces into the compressor before it overflows? Not to worry-- you can suck it in with your vaccum pump-- connect one of the two hoses to the pump, through your manifold guage set, and leave the other hose not connected to the manifold (be sure to close that valve). Start your vacuum pump, open the valve with the hose, and take the free hose (attached at one end to the vehicle) and dip it into the oil, letting the vaccum suck it up. I like to suck from the high (blue) side, and draw the oil into the low (red) side-- this means the oil will flow directly into the pump. When you've drawn in all the oil, close the valve (let the pump run, it won't hurt) then re-attach the free hose to the other side of the manifold-- wipe it on a rag first! Leave that side closed for now, though. Re-open the other side valve, and let the pump work a bit, drawing the oil through the pump. After few minutes, open both valves fully to the stop-- they tend to leak if you don't open them all the way.

Let it run for at least 30 minutes or even an hour if it's humid.

Obviously, you're going to need to weigh the charge in, once you've completed the vacuum-- there should be a charge sticker underneath the hood--- go by that, in either ounces or grams/kg.

Automotive systems are carefully balanced to work with variable speed compressors (engine driven, remember?) and you really cannot get an accurate charge just going by pressure/temperature differentials. Which is why weighing in the charge is a must.

However, if you find the system does not seem to want to "take" the full R134a charge? (as specified by the label).

Simply revving the engine will increase the suction action on the low side, drawing in more charge-- too much, if you let it.

Okay, I just checked-- my 2004 North American van had the sticker at the right (as you face the engine) up on the rail-- it reads:

1.9 LB R134a
0.890 kg R134a
Lubricant/OEL 001 989 08 03

This last is likely a part number for official MB compressor oil-- a good parts house should be able to cross reference that with a suitable duplicate, and likely also give you the proper amount too.

Good luck!
 

flman

Roadrunner, Genius of Birds ALWAYS WINS! NO FAILS!
I am a reefer man myself, and a little of the wrong oil, or the wrong refrigerant is alot better then a little air or moisture. We have been converting systems from CFC to HFCs for 20 years with no flushes or ill affects.
 
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Bob of QF

New member
Yes-- I agree. Alghough the bulk of my experience is in home A/C's (it's what I do for money these days... :2cents: :rolleyes:) I have to agree-- moisture is a killer. Next to that, is acid-- but since automobile AC's don't use high voltage electric compressors, there's never any worry about arc-faulting and burning of the refrigerant gas (which makes mustard gas... nasty).
 

Dougflas

DAD OWNER
With the system using a full charge of 1 lb 14 oz of r134, you do not want to over fill the system with too much oil. I have been doing mobile AC work since the 70's and this is what most manufacturer's recommend. Take your old compressor and drain the oil. Measure this amount. This amount is what needs to be placed in the new compressor. If changing the drier which SHOULD be done also, add 1 to 2 oz of oil to the new drier. If the system is completely and properly flushed, then use the oil amount specs the manufacturer says.

evacuating the system.... If you are not using a micron gauge, you need to run your vacuum pump a lot longer than 30 minutes. You better run the pump for a few hours at least. Run the pump for 30 minutes with both handwheels open then close of the high side so you don't draw the oil out of the system. Moisture will turn to acid after a while if the proper vacuum level is not reached which is under 500 microns. The problem with PAG oil is its absorbing moisture. Once moisture is in the oil, it's a done deal. Only the drier will absorb this moisture. You must work quickly while the system is open.
 

Bob of QF

New member
Re: Dougflas

I was not aware of that problem with PAG oil (as used in cars) -- in the home systems, the compressor oils are generally hydrophobic, but we also install new filter/dryers with a unit change-out anyway.

Changing a compressor in a home unit is not really worth the cost-- it's more labor than a compete unit, and the new compressor cost 3/4 as much as a whole new unit anyway (or worse!), so total cost is a wash unless you can get the compressor for free (under warrenty). Home unit compressors always ship with oil-in-- they are a sealed can, with oil in the bottom. I usually add in an acid neutralizer just in case, if I must swap a compressor-- it's a $15 bottle of insurance.

I'll remember your tricks in the future-- I typically don't do car A/C as there are too many variables for this aging A/C tech to keep up with, but I do fix my own occasionally.

And I agree: change out the dryer in automotove systems, and if your old compressor was blowing chunks? (internal debris blowing downstream) Change out the expansion valve too-- if you can get at it, that is. Not always easy with automotive systems. A clogged expansion valve (which is just a metered orfice/hole, with a pre-filter screen, which can become clogged) will prevent proper operation.
 

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