I am putting on a used ac compressor and as of right now it does not have any oil in it. Should I put oil in it and if so can I use transmission oil or will the 134a refigerant do the job? I am putting the ac on a 2002 Dodge Grand Caravan.

2 Answers 2


You will need the correct PAG oil. Usually there is a sticker in the engine compartment telling you which oil and how much the compressor needs. Do not use any other oil.


Yes, but...

The Air Conditioning (A/C) compressor is a device designed to pump vapor/gas and NOT liquid. When your car was built at the factory, they add a very specific amount of PAG oil to the system. PAG is actually an acronym for Polyalkylene Glycol, and it is a synthetic oil used in A/C systems to lubricate the compressor. In use the oil migrates around the system as you use your air conditioning.

So with that said, too much oil is bad; remember I said the A/C compressor is designed to pump gas and not liquid. You can break a compressor by pumping too much liquid.

Too little oil is bad; the oil lubricates the moving parts within the pump to reduce wear and improve durability.

You've not told us about the conditions under which you are repairing this system. A trained A/C repair guy would remove the old A/C compressor, carefully drain the oil in the pump into a measuring cup, determine exactly how much oil was removed, and replace the exact same amount.

If on the other hand, you live in say a high humidity area, and the A/C compressor was removed years ago, with open lines just sitting there, you would be forced to total drain all the oil out of each and every component within the system. The oil is very hydroscopic (meaning it absorbs water) and when it does that it loses its functionality. Being open to the atmosphere for more than a few hours is bad, really bad for the oil.

so yes.

  • You have to use special PAG oil.
  • If open to the atmosphere for any length of time, the oil within your system is contaminated.
  • You need to have the correct amount of oil in the system. If you add the entire amount of oil used in the system, without regards to oil already in there, you are doomed to fail. When you remove the old compressor, you need to carefully measure the amount of oil contained in there, and just replace that.

With your comment about transmission oil, I strongly suggest that the replacement of refrigerant compressor oil in your vehicle should be handled by an A/C professional.

  • most definitely. Not knowing what you are doing with AC is a very risky operation. When the PAG is exposed to moisture for any length of time it becomes acidic.
    – Old_Fossil
    Commented Sep 2, 2016 at 19:14

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