A/C will not blow cold

Tiny
YELLOWGMC01
  • MEMBER
  • 2001 GMC SIERRA
  • 4.8L
  • V8
  • 2WD
  • AUTOMATIC
  • 112,000 MILES
So I rebuilt this truck from the ground up. I go to recharge the A/C and the pressure gauge that come with the AC pro kit shows in the red almost 100 PSI (while the compressor isn't engaged ) but when it kicks on it goes to 13-15 psi. And goes back to you the red. Even after filling it gauge says 13 psi while the compressor is on. And blows hot air. Any ideas anything would help.
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Saturday, May 25th, 2019 AT 2:38 PM

3 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Hi YellowGMC,

The pressures indicate a compressor issue. However, did you evacuate and pull the system down into a vacuum prior to charging it? There are a couple reasons for this. First, you need to check for leaks and if it holds a vacuum then it is not leaking. Second, is you need to be sure all air and moisture are out of the system prior to adding the oil and Freon.

Let me know if you did this and we can go from there. If you don't have a vacuum pump, you can rent one from most parts stores. You will want to go ahead and get the gauge set as well (two gauges with lines) so that you can monitor your high side and low side to ensure they are operating.

I attached a chart to show normal operating pressures based on the ambient temperature.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system
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Saturday, May 25th, 2019 AT 4:51 PM
Tiny
YELLOWGMC01
  • MEMBER
  • 2 POSTS
No, I didn't do any of that. I just bolted and plugged everything back up. I wasn't aware there was more to it.
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Saturday, May 25th, 2019 AT 9:59 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. Then most likely the compressor is not able to build the proper pressure because air and water compress differently then Freon and oil.

I would suggest not running the compressor again until you get the air and moisture out of the system. Also, if the compressor did not come pre-filled with PAG oil, then you will need to add the proper amount of oil as well. If not, the compressor will eventually fail.

The previous post had a link on it that explained how to do this. Please let me know if you need more info and I will be happy to assist. Thanks.
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Sunday, May 26th, 2019 AT 8:57 AM

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