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George H.
George H., ASE Certified Technician
Category: Chevy
Satisfied Customers: 18665
Experience:  GM Tech ASE Master Tech 15+ years
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Chevrolet C1500 4x2: 1990 C1500 454 ss 7.4l. A/C blowing hot

Customer Question

1990 C1500 454 ss 7.4l. A/C blowing hot air. Compressor clutch not engaging. Charged system--low-side pressure is good. Pressure switch is good (tested w/voltmeter). Clutch engages when I bypass pressure switch. Near as I can tell, this all indicates bad A/C relay, but I can hear relay clicking when I turn A/C on and off at dashboard. Can a relay that audibly cycles still be bad?
Submitted: 1 year ago.
Category: Chevy
Expert:  George H. replied 1 year ago.
Hello I will help you with your question,
If the clutch comes on when you bypass the pressure switch then the problem is at the switch. Either low pressure or a faulty switch.
A relay can click and still fail to close the contacts but that is unusual, you can swap out relays to test but I think you need to do some circuit checking.
Do you have a voltmeter or 12 volt testlight to use?
Customer: replied 1 year ago.
Voltmeter--yes. 12v light--negative.
Customer: replied 1 year ago.
Also, Hi, George, I'm Mike.
Customer: replied 1 year ago.
Pressure switch tested good--zeroed out when tested w/voltmeter.
Expert:  George H. replied 1 year ago.
Good, see if you have continuity through the switch, if you have power to the switch just check the other wire, it should have voltage as well if the switch is closed. If the switch is open and you are using a can gauge I think you are undercharged. How much did you add and what is the gauge reading with the compressor running?
The only thing you have to do to get the system running and cool is to jump the pressure switch?
Customer: replied 1 year ago.
"The only thing you have to do to get the system running and cool is to jump the pressure switch?"Funny you ask that--when I jumped the pressure switch, the clutch engaged, but it still didn't blow cool, even after a minute or so of idling and low revs. I figured it should have been blowing cool, but thought maybe having the lead pulled off the pressure switch would somehow prevent it from cooling.
Expert:  George H. replied 1 year ago.
It really sounds like the system is undercharged.
How much did you add and what does the gauge read when the compressor is running?
Customer: replied 1 year ago.
Hmm...yes, using a can gauge. Was running right in the middle of the ambient temp range, added a few more pounds just to be sure. Will double check.
Expert:  George H. replied 1 year ago.
This system came from the factory with 2.8 lbs of R12 in it.
Somewhere along the way it was retrofitted to R134a. I don't know if the compressor was changed but I doubt it, I hope a high pressure switch was added.
At this point I would recover what is in there, vacuum the system and check for leaks then refill with 35 ounces by weight and see how it cools. The charge has to be adjusted from there but if the compressor valves are broken it can run all day with a full charge and it won't cool a lick.
Let me know what you have in the system
Thank you
Customer: replied 1 year ago.
Yes, it's R-134a. Saying I added "a few pounds" betrays my ignorance w/the a/c system. Pressure went from ~45 psi to ~47 psi. Cooled like a champ since I bought it in 2012. Recently sat a while (~6 months) as I tried to hash out some ignition issues. Can't say for sure the compressor was changed, but I figure it probably was since it has a high-speed holographic sticker on it.Naturally, the battery is discharged after spending all day starting it to check the compressor.
Customer: replied 1 year ago.
Where would I check for a high-pressure switch?
Expert:  George H. replied 1 year ago.
Ok, you cannot fill by pressure.
If it was working after the conversion it should still work.
The cans come in different sizes, you don't want any oil or sealer or "super freeze" or other garbage just R134a.
Usually they are 12 ounce cans, if the system is empty put 3 cans in and see if the compressor comes on.
I don't know how much you put in but I am guessing it is far from full.
Is there a pressure switch in the back of the compressor? If this had a high pressure switch added it would be there.
Let me know what you find
Customer: replied 1 year ago.
Couldn't find anything that looks like a pressure switch (though, obviously, I'm no expert). Now less confident that the compressor has been replaced, as there are obviously screw-on R-134a adapters on both the high- and low-side ports. But it sure looks too new to be 20-some odd years old.
Customer: replied 1 year ago.
George, I have to break off to work on some other stuff. Is there a time limit on how long we can continue diagnosing this issue?You've been a huge help so far--muchas thanks.
Expert:  George H. replied 1 year ago.
Just post back to this thread and I get notified. I am usually online, if I am not it may be a couple of hours til I get back to you.
We work at your pace, review what I posted above about what needs to be done to get the system full and I will watch for your post.
Thank you