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The AC compressor on my 99' Silverado won't engage, when i click the AC button. I checked the fuses (Don't know how to check the relay(s)) and all are good. If it wasn't such a pain to get to i'd try and just jump a power wire just to make sure. It spins fine, no bearing noise or anything like the clutch just won't engage and I get ZERO cold air coming out of the system on resurc or any setting. Oh, and i pressure tested the system and it's full and good to go no leaks, also all the electrical connections seem good as well.

Thanks in advance for any ideas on where to look before i go have the System drained and a new AC compressor installed.
 

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Don't replace the compressor just yet. They seldom are the problem unless it was knocking the last time it ran. What were your pressures when you checked them?
A quick test to check if your system is full is to disconnect the low pressure switch & momentairly jump the wires in the plug with a piece of wire or a paper clip. If the compressor clutch engages, all your wiring & relay is good & you have low freon issue.

** passing this test could also indicate a faulty LP switch, but you will have to determine this with a pressure test.
 

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Check the switch on the accumilator. Mine was bad on my 99 burban and I didn't know it, so I replaced the compressor. I spent 200 dollars on a 10 dollar problem. To check the switch just remove the wiring harness from the side of the accumilator and use a piece of wire to jump the signal on the harness. Your clutch should kick on. If it does, replace the switch and you should be golden. If your refridgerant is low, your clutch will still kick on for a split second until the switch tells the compressor that the accumilator is low. I hope this helps you.
 

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Pull out the pig tails for the low temp switch and high pressure switch, and jump the terminals on the pig tail, and try it again, but do them one at a time. If it kicks on when you have one jumped, then you need to replace one of those sensors.

Otherwise, I'd suspect the electromagnet in the clutch has gone bad.
 

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Not to insult you! But have you checked the belt that goes to the A/C compressor. Quite common for them to break and you will not even be aware because of where the compressor pulley is hidden. Belt can be replaced easily.
If belt good. Then I would believe that the pressure plate to pulley clearance is out of spec due to wear between the two. Can be adjusted relatively easily with loaner tools from local auto parts store. The pressure plate must be pressed on further reducing the clearance back into spec.
One quick check to see if the clearance is the problem. With Engine and A/C on take something and safely tap the compressor this should make the clutch engage. If it does then you will know for sure it is the clearance. If it still doesn't engage then check Low pressure,relays, and such.
The clearance problem seems to be quite common on this age truck. I just adjusted mine last month. I am also replacing the fan clutch.
 
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