Engine Tips on how to troubleshoot and fix AC

AnthonyA1234

Active Member
Aug 17, 2020
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Hello I just purchased a 1992 mustang gt. The compressor clutch does not engage. I’m not sure if the compressor is broken or if I just need to add more refrigerant. Can someone tell me the steps for how I can engage the clutch to see if it works. I understand there’s some sort of low pressure switch you can jump to engage the clutch to see if it works. Can anyone tell me where that is? Thanks.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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On the firewall behind the passenger side head, there is a small door, knock on that door three times, if no answer, its broken :jester:
But seriously, you will see back there a tank looking thing with a switch and two wire plug, jump the wires in that plug, pump runs, possible low freon, pump don't run, other problems,
Things to consider, just adding freon does not fix the problem, how long since it last worked? There is hi and low pressure involved, it can get pretty involved.
 
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AnthonyA1234

Active Member
Aug 17, 2020
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On the firewall behind the passenger side head, there is a small door, knock on that door three times, if no answer, its broken :jester:
But seriously, you will see back there a tank looking thing with a switch and two wire plug, jump the wires in that plug, pump runs, possible low freon, pump don't run, other problems,
Things to consider, just adding freon does not fix the problem, how long since it last worked? There is hi and low pressure involved, it can get pretty involved.
I’m not sure, the car sat for 5 years before I bought it. I just got it running. Do I do this while the car is in auxiliary mode? Also to clarify, by jumping the wires you mean connecting the wires in the plug correct?
 

Bird76Mojo

Active Member
Jul 12, 2018
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What he's talking about is a pressure switch that won't allow the compressor to run if refrigerant pressures are low. So yes, you'll be using a jumper wire in the wiring plug itself. Don't let it run long if the compressor does kick on. Just long enough to see that the clutch on the compressor engages.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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AnthonyA1234

Active Member
Aug 17, 2020
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So I did it and the clutch engaged and began spinning. I’ll take it to get recharged whenever I get the car on the road. Also, if any of you could help me out, the car has a gauge connected to what I believe to be the fuel Schrader valve so I assume it’s a fuel pressure gauge. It reads 30 at idle and correct me if I’m wrong but shouldn’t it be 40? The car has what I believe to be a fuel pressure regulator on the fire wall but I turned it and the fuel pressure didn’t change. Any info would be much appreciated.
 

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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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With the engine running, the pressure would be around 32-33psi. You need to pull the vacuum line off the fuel pressure regulator (located behind the intake “neck”) plug the vac line and read the pressure then. It should be 38-39psi. At idle, with vacuum connected the pressure will be slightly lower.


That knob you were turning is not for fuel pressure, it’s for the rear disk brakes. It’s an adjustable knee-point valve. It regulates the flow of brake fluid to the rear calipers so that you don’t get too much initial pressure at the rears. Hopefully you didn’t play with it too much.
 
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manicmechanic007

5 Year Member
Sep 26, 2017
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See if the compressor turns by hand
Install gauge set see if pressure is above 28 lbs (should be about 80)
If low leak diagnosis
If gauge pressure is high / normal
Paper clip jump pressure cycling switch (most likely fault if freon is charged)