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AC compressor install, how much oil to add to system

greenvan on Sun July 15, 2012 1:15 PM User is offline

Year: 1994
Make: Dodge
Model: Grand Caravan ES
Engine Size: 3.8L
Refrigerant Type: R134a
Country of Origin: United States

1994 Dodge Grand Caravan ES 3.8L, I am replacing the original Denso AC compressor (18 years of use) and I am putting in another Denso OEM compressor and the instructions state to drain out the oil from the old compressor and compare to the oil amount of the new compressor (you are supposed to drain that also) and only add the same amount. This makes sense if just the compressor failed on a working system. The defective compressor was making a lot of noise before failing; I had fluorescent dye in the system to check for leaks. It was leaking at the top of the compressor at the low and high side fittings for the past few years, I would a vacuum and add a filter/drier with 1 ounce of fluorescent dye and I would be good for another year (looking back, I should have fixed the leak). My question is:
1) The old compressor has no oil, bone dry. Should I assume that all the oil is gone from just the compressor and just install the new compressor (pre-loaded with oil, 7.4 oz) as is?
2) Or, assume that all the oil has blown out of the system and add the Total system minus the new compressor oil (13.4 - 7.4 = 6 ounces) ?
(this has a dual air, service manual says: compressor 7.4 oz, filter/drier 1oz, condenser 1oz, evaporator 2 oz, rear evaporator 2 oz, TOTAL = 13.4oz) I will be adding a new filter/drier (with the 1 oz oil).
3) Should I also replace the expansion valve? I hate to do this as it does not leak; this AC has worked amazingly well for the past 18 years, always blowing very cold air.
Thanks for any advice you can provide.

ToyMR2 on Mon July 16, 2012 12:00 PM User is offline

I have almost the same situation with my 1985 Toyota. The compressor was not "bone dry", but had very little oil in it. Hopefully we will get a good answer here.

doctorbee on Mon July 16, 2012 12:07 PM User is offline

the best thing to do would be to flush the entire AC system and put it the OEM recommended amount of oil.

Cussboy on Tue July 17, 2012 11:12 AM User is offline

I wouldn't replace the rear expansion valve unless I found after the repair that the rear was not working, especially because in your case the old compressor did not seize.

As to how much oil to add - that's an edjamacated guess. When I replaced my truck evaporator in 2002 due to a leak, I added the 2 ounces of oil the book recommended. Well, a year later my compressor seized, so my theory is that I lost more than 2 ounces of oil with that leak. So in 2003 with the compressor removed, essentially zero oil was measured. I backflushed from drier back through condenser to the compressor fitting, and I put in the full amount stated for compressor plus about 2 ounces, and so far, so good. So for your system, my "guess", and only a guess, is 10-12 ounces.

ToyMR2 on Tue July 17, 2012 2:38 PM User is offline

I assume the new compressor for my Toyota has the recommended amount of oil in it. My old compressor leaked oil and when enough R-12 leaked out the drop in pressure caused two o-rings at the lowest points in the system to leak also. The car involved is a Toyota MR2 and that car has very long metal refrigerant pipes due to the fact that the engine and compressor are at the rear of the car and the condenser and other parts are at the front of the car.

There can't be much oil left except for a layer of coating. I'll see what comes out of the drier when I remove it. The Toyota service manual says the compressor oil capacity is 60 - 100 cc. And the receiver/drier is 20 cc and the condenser is 40 to 50 cc and evaporator is 40 to 50 cc. And it says to add oil to the compressor when replacing any of these items. It doesn't even mention anything about the hoses or long tubes under the car. So I assume there should be 160 to 200 cc (5.4 to 6.75 oz) in the compressor if every part of the system is replaced or flushed. And if only the compressor and receiver are replaced then 80 to 120 cc should be in the compressor. Does this make sense?

I'll see what's in the receiver when I pull it. If only a few drops come out then I will assume the system has the minimum amount of oil in it. If I have 160 cc in the compressor I don't think there will be too much oil in this system. Maybe 180 cc would be good? So I guess I need to drain the oil out of the new compressor and just make sure there's no more than about 180 cc in it. Is this a good guess???

Cussboy on Tue July 17, 2012 8:13 PM User is offline

Originally posted by: ToyMR2
I assume the new compressor for my Toyota has the recommended amount of oil in it......So I guess I need to drain the oil out of the new compressor and just make sure there's no more than about 180 cc in it....

Yes, I'd measure the amount of oil that's really in your new compressor. Also, is your 1985 staying R-12 or going to R134a, as the oils are different. I'd sure make certain what type of oil is really in there, or drain and add my own. I found that the compressor for my 1988 truck had PAG oil in it (used IR spectroscopy) and not the mineral-type oil that I needed as I was staying R-12.

ToyMR2 on Wed July 18, 2012 2:54 AM User is offline

I have owned my 85 Toyota for 17 years. The a/c didn't work when I bought and I replaced everything but the compressor and evaporator and it was trouble free all these years up to 257000 miles. It has always had R-12. I have a new Denso compressor that is identical to the original one and the label on it says R-12 and mineral oil. Toyota Parts sells the Denso compressors set up for R-12. I know R-12 will work very well in this car for years to come.

ToyMR2 on Sat July 21, 2012 11:08 PM User is offline

I just got around to working on it. I'm draining the oil from the new compressor. There is some oil in it, but too little to bother to measure. So I'll see if 180 cc goes in.

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