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HVAC Discuss Heating, Ventilation & Air Conditioning related issues on Duramax equipped trucks.

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Old 04-11-2019, 05:00 PM   #1 (permalink)
GreenDuramax04
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LMM AC Problem

This is on my 2008 Suburban with a LMM. I bought this a couple weeks ago so I don't know all the information from the swap.

For some reason my compressor wont kick on. Had a mechanic look at it and he said the system has plenty of Freon, hi and low switch are working correct. He said he commanded the compressor on with his power probe and it came on fine and the ac was cold. But he says that the PCM isn't commanding the compressor to come on.

He doesn't know why and he said its a little more difficult to trouble shoot because he isn't sure what is from the Diesel donor and what is Suburban when it comes to wiring, etc. (looks like the fuse box is Suburban) He wasn't able to spend a whole lot of time on it but said I can bring it back and he can dig into it more.


It does have a manual climate control.

What else would cause the pcm not to send the signal to the compressor? I talked to the previous owner and he said that the AC worked last summer.

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Old 04-13-2019, 06:39 PM   #2 (permalink)
Ron Nielson
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Start with basics, fuses and relays. It could be the control unit, then again maybe not. Very hard to tell from here. There are Youtube videos by a guy named Richpin06 (I think) that has a couple of videos on troubleshooting a/c units. Very good information.

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Old 04-14-2019, 01:45 PM   #3 (permalink)
GreenDuramax04
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Thanks Ron. He does have good information.

I've ruled out sensors, control panel, Freon. So far I haven't seen any bad fuses. Going to check a little more into relays.

Is there another control unit other than the dash controls?
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Old 04-15-2019, 01:35 AM   #4 (permalink)
Ron Nielson
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Air Conditioning Compressor Malfunction

Diagnostic Instructions


Diagnostic Fault Information







Circuit/System Description

When the A/C switch is pressed, the HVAC control module grounds the A/C request signal circuit. This input will request the ECM to ground the A/C compressor clutch relay control circuit, which will switch the A/C CMPRSR CLUTCH relay on. With the relay contacts closed, battery voltage is supplied to the A/C compressor clutch assembly.

Diagnostic Aids

The following conditions must be met in order for the ECM to turn on the compressor clutch:

  • Battery voltage is between 9-18 volts
  • Engine coolant temperature (ECT) is less than 123C (253F)
  • Engine speed is greater than 600 RPM
  • Engine speed is less than 4,760 RPM
  • A/C high side pressure is between 2929-269 kPa (39-425 psi)
  • Throttle position is less than 100 percent
  • Evaporator temperature is greater than 0C (32F)
  • ECM does not detect excessive torque load
  • ECM does not detect insufficient idle quality
  • The ambient temperature is above 1C (34F)
Circuit/System Verification


  1. Engine running, press the A/C request switch. The A/C compressor clutch should engage.
  2. Place the mode switch in the defrost position. The A/C compressor clutch should engage.
If the clutch does not engage refer to A/C compressor clutch Circuit/System Testing


  1. Ignition OFF, disconnect the harness connector at the A/C low pressure switch.
  2. Test for less than 5 ohms between the ground circuit terminal B and ground.
If greater than the specified range, test the ground circuit for an open/high resistance.
  1. Ignition ON, verify the scan tool Low Pressure Switch parameter is Open.
If not the specified value, test the signal circuit terminal A for a short to ground. If the circuit tests normal, replace the HVAC control module
  1. Connect a 3-amp fused jumper wire between the signal circuit terminal A and ground. Verify the scan tool Low Pressure Switch parameter is Closed.
If not the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the HVAC control module
  1. Connect the harness connector at the A/C low pressure switch. Verify the scan tool Low Pressure Switch parameter is Closed.
If not the specified value, test or replace the A/C low pressure switch.
  1. Ignition OFF, disconnect the A/C relay.
  2. Ignition ON, verify that a test lamp does not illuminate between the control circuit terminal 86 and ground.
If the test lamp illuminates, test the control circuit for a short to voltage.
  1. Verify that a test lamp illuminates between the ignition circuit terminal 85 and ground.
If the test lamp does not illuminate, test the ignition circuit for a short to ground or a open/high resistance. If the circuit tests normal and ignition circuit fuse is open, test all components connected to the ignition circuit and replace as necessary.
  1. Verify that a test lamp illuminates between the relay B+ circuit terminal 30 and ground.
If the test lamp does not illuminate, test the B+ circuit for a short to ground or a open/high resistance. If the circuit tests normal and B+ circuit fuse is open, replace the A/C clutch relay.
  1. Disconnect the harness connector at the A/C clutch coil.
  2. Test for less than 1 ohm between the ground circuit terminal 2 and ground.
If greater than the specified range, test the ground circuit for an open/high resistance.
  1. Connect the harness connector at the A/C clutch coil.
  2. Connect a 10-amp fused jumper wire between the B+ circuit terminal 30 and control circuit terminal 87. Verify the A/C clutch is activated.
If the A/C clutch does not activate, test the control circuit for an open/high resistance. If the circuit tests normal, test or replace the A/C clutch.
  1. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
  2. Command the A/C relay output function ON and OFF with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.
If the test lamp is always ON, test the control circuit for a short to ground. If the circuit tests normal, replace the ECM. If the test lamp is always OFF, test the control circuit for a short to voltage, or an open/high resistance. If the circuit tests normal, replace the ECM.
  1. If all circuits test normal, test or replace the A/C clutch relay.
Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. See: A L L Diagnostic Trouble Codes ( DTC )\Testing and Inspection\Verification Tests


  • Air Conditioning Compressor Replacement (LY2, LY5, LH6, L76, L92 and LMG) Air Conditioning Compressor Replacement (LLY/LMM) Air Conditioning Compressor Replacement (LU3)
  • Compressor Clutch Plate/Hub Assembly Replacement (4.3L, 6.6L) Compressor Clutch Plate/Hub Assembly Replacement (V8 - Gas)
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2009 Arctic Fox 5er, Silver Fox Edition 29-5T, 3.6 Onan LPG, SteadyFast Stabilizers
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Old 04-17-2019, 09:14 PM   #5 (permalink)
GreenDuramax04
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Thats some great info Ron. Thank you!

Found the problem to be a broken wire to the low pressure switch. Pain in the A$$. But I have AC now!
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