1998 GMC Sierra 1500 AC question

[BigPapaDune]

Hello, i'm new to this site, looks great so far, cudos to whoever mods it!

I'm not sure if I posted this in the right section, I figured transport was the closest to the right category.

I have a 1998 GMC Sierra 1500 5.0L 2wd.

Question is : what is the low and High pressure cut out on these trucks? I can't get my compressor to engage unless I short out the relay to force the compressor to engage. I get a low side reading of 20-22psi... That is with 24*C or 75*F ambient temp, A/C on Max with compressor relay shorted to engage.

I know the low side should be between 40-50 psi for optimal function. Is this below low pressure cut out, is this why my compressor won't engage. I read somewhere that it might be too low and it won't start. to save my equipment.

Any thoughts?

[The Viking]

20-22psi will give a saturated temperature well below freezing.
Keeping the A/C running at those conditions will cause the evaporator (the bit cooling the airstream) to become a block of ice, that is why the low pressure switch has cut the system off.

Most likely your system has lost part of it's charge and needs recharging but there might also be other problems.

:cool:

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[BigPapaDune]

Thanks a lot, seems like I can't get a high side reading at all. Gauge at zero... Weird! While suction side reading 25psi. Does that sound like my compressor shot?

[The Viking]

Nah,
Sounds like coupler not properly on valve or gauge is shot.

:cool:

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[BigPapaDune]

still having trouble.

I pumped a vac to 800 microns. charged to manufactures specs of 2 lbs. My high side coupler is defective so I only get a low side reading of 38psi when I force the compressor to run. ( jumping the relay) But now with a proper charge and with the relay in, 87 - 30 doesn't close to run compressor. . I checked all fuses. test my relay. I've jumped lowside switch, and high side switch and still nothing. Blower motor and controls seem to be working with the light on the button lighting up.

Any Ideas on this.