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Message: <blockquote data-quote="INW-Iron-Steel" data-source="post: 23968" data-attributes="member: 11733">For what it's worth, I find it helpful to troubleshoot at the relay in the fuse box under the hood. Locate the AC compressor clutch relay and remove it. It should be a 4 pin relay. I can't recall if the 2000 is the same as the later trucks, so I would verify the wiring. But on my 04 they are labeled 87, 86, 85, and 30. 30 is constant power, 86 is switched power (ignition), 85 is ground (from PCM and triggered by AC button on dash), and 87 is output to the AC clutch.Jumping 30 and 87 you can then check the wiring from the fuse box to the clutch. You should get 12v at the plug. Grab the frame with a test light first and verify 12v on the positive side of the plug. Then verify the ground in the plug is working. If that checks out, it is in the triggering somewhere.Since the AC button/PCM trigger the ground on the relay, you can check that too. I jump the low-side sensor, start the vehicle, and switch the AC on. You should get a ground on pin 85 of the relay. I use a test light and pin 30 for my 12v source. Again, verify the early gmt800 wiring is the same as the later trucks before trying this. Hope this can help you get some cool air!</blockquote>