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  1. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    This entire time, the previous owner promised he replaced the system properly as he was a mechanic by trade. I finally got the service manual for the GMT400 today and I realized that the A/C HIGH PRESSURE CUT OUT SWITCH that is located on the very rear of the compressor simply wasn't there. I...
  2. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    I didn't know the service manuals were available on the web until now, thanks. I'll make sure to post the solution once I start working on the Truck again.
  3. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    A breakthough, I got a piece of long copper wire and I touched it to the negative terminal on the battery, I then brought the other end of the piece of wire and I pressed it into the relay socket on the pin that's supposed to be ground while the relay was installed into the socket and the A/C...
  4. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    I found this online from someone else with a GMT400 truck with working AC "With the truck running and AC button pressed, the resistance to ground was .555 mili ohms (basically closed or minimal resistance) With the truck running and AC off, the resistance to ground was 8.55 mili ohms" With the...
  5. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    From what I understand: Applying power to terminal #30 will engage the compressor clutch provided all the safety pressure switches are satisified. Check for power at #86 and #87, they both should have power in run. #86 is power for the relay coil, and #87 is the power for the clutch circuit...
  6. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    To confirm, I should be able to test that buy getting a test light like this? https://www.harborfreight.com/6-12-volt-circuit-tester-4288.html And then connect to clamp on the test light to battery positive and then stick the other end on the socket where the ground should be?
  7. M

    Tried So Many Things But No A/C in 1999 GMC Sierra K1500 Classic

    What I've tried so far. Verified all Fuses The Power connector to the low pressure switch gets 12v when the car is on. If I jump it, it doesn't cause the AC compressor clutch to engage The Power connector to the high pressure switch gets 12v when the car is on. If I jump it, it doesn't...

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