Put a vacuum to the a/c then charged it. It took 1.5 of the 2.5lbs it calls for and so I fired it up and turned on the a/c.

The pump engaged but the air was still warm and the pump got real hot, too hot to touch almost. The low side was at 350 and I couldn’t tell what the high side was because the adapter wasn’t right.

What would cause this?

  • Was the pump running when you measured the low side?
    – HandyHowie
    Commented May 16, 2020 at 10:59
  • 1
    The expansion valve may be broken, so the refrigerant can’t go anywhere.
    – HandyHowie
    Commented May 16, 2020 at 11:00
  • 1
    Did you install oil in the new pump?
    – Moab
    Commented May 16, 2020 at 12:26
  • 1
    350psi on the low side? You've got a blockage in the system somewhere. The low side should top out at ~45psi maximum while running. Most vehicles today have an over-pressure switch which prevents destroying the compressor if there's a blockage. If this is a new compressor, did you put the correct oil in it (different problem from over pressure blockage). Commented May 16, 2020 at 12:35
  • It’s a was a used pump but yes I did put the correct amount and type of oil in it.
    – Jon Eddy
    Commented May 16, 2020 at 19:26

1 Answer 1


The only way you could possibly have 350psi on the low pressure side is if you have fitted the pipes to the incorrect ports on the compressor. This is definitely possible on some compressors.

You must have the low pressure pipe connected to the high pressure port on the compressor.

Can you add a details of the type of compressor fitted? A photo would also help.

  • The truck is a 92 GMC Sierra 1500 with the 5.7. The way the lines hook up to the pump it’s not possible to get it backwards. They are connected to each other. I am out of town until tonight but when I get home I will take a pic.
    – Jon Eddy
    Commented May 16, 2020 at 22:05
  • Do you know the model of the compressor? It should be written on it somewhere.
    – HandyHowie
    Commented May 17, 2020 at 7:33
  • It says model number 338 or 838 part of the first number is kinda wore off
    – Jon Eddy
    Commented May 17, 2020 at 20:13

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