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Daniel Jones
Daniel Jones,
Category: Chevy
Satisfied Customers: 498
Experience:  GM A Tech at Straub Buick GMC
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Have a 2008 Tahoe that will not engage the ac compressor,

Customer Question

have a 2008 Tahoe that will not engage the ac compressor, have no power to # ***** fuse for the compressor, if I jump power to the right side of the fuse it will run, checked all fuses and relays, just installed a new fuse center under the hood, still no luck
Submitted: 1 year ago.
Category: Chevy
Expert:  Daniel Jones replied 1 year ago.

Hello. Welcome to the site. Thanks for coming.

I'm ***** *****.

I'm here to provide as much information and insight as I can, to best answer your question.

I will need your VIN# ***** get the proper wiring diagram.

Thank you for posting your question. Your business is very valuable to us.

Thank You

***** *****

Customer: replied 1 year ago.
1GNFK13088R122450
Expert:  Daniel Jones replied 1 year ago.

Here is the circuit description and the wiring diagram. Fuse only has power when PCM grounds the relay. Make sure there is power at two terminals of the A/C compressor relay.

The A/C system can be engaged by either pressing the A/C switch or during automatic operation. The HVAC control module sends a GMLAN message to the powertrain control module (PCM) for A/C compressor engagement. The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil. The A/C compressor diode will prevent a voltage spike, resulting from the collapse of the magnetic field of the coil, from entering the vehicle electrical system when the compressor is disengaged.

The following conditions must be met in order for the A/C compressor clutch to turn ON if they are not all met and an A/C request is made the LED on the A/C switch will flash:

  • Ambient air temperature above 7°C (45°F)
  • A/C low pressure switch signal circuit is grounded
  • A/C refrigerant pressure sensor parameter is less than 2957 kPa (429 psi)
  • PCM receives an A/C request from the HVAC control module
  • Engine coolant temperature (ECT) is less than 121°C (250°F)
  • The engine rpm is more than 550 RPM
  • The throttle position is less than 100 percent

If you need more help, please use the "Reply" tab to continue our conversation - I'm happy to answer follow-up questions.
If you have no follow-ups at this time, please remember to Rate My Service.

Thank You
***** *****

Customer: replied 1 year ago.
I believe that all of conditions are met, the ac switch light is steady when pressed, the unit works fine when power is applied to the right side of fuse # ***** running a fused wire from a 12 volt terminal in the fuse block, just disconnect after getting to where we are going, I will check power to relay this weekend, have a trip planned for tomorrow.
Customer: replied 1 year ago.
one other thing every once in a while it is jumped the air flow stops coming out the right vents and quits putting out any air but the blower is still blowing
Expert:  Daniel Jones replied 1 year ago.

Is it only the right vents?

Does it change if you go to floor and defroster?

Customer: replied 1 year ago.
no it all the vents ,not sure about the changing from floor to defroster, just got home ,will check tomorrow. thanks dan
Expert:  Daniel Jones replied 1 year ago.

If it is not switching Modes from defroster to vent to floor it may be a mode actuator.

Customer: replied 1 year ago.
I checked the ac relay has power to the right side two terminals, also the air will switch from floor to defroster like its supposed to. lumped the low pressure switch with paper clip, nothing was going to do high pressure switch but has three terminals, didn't want to mess any more up , still no power to # ***** fuse, cycled ac switch several times with no luck, also shut off rear air , same results
Customer: replied 1 year ago.
I checked the ac relay has power to the right side two terminals, also the air will switch from floor to defroster like its supposed to. l
jumped the low pressure switch with paper clip, nothing was going to do high pressure switch but has three terminals, didn't want to mess any more up , still no power to # ***** fuse, cycled ac switch several times with no luck, also shut off rear air , same results
Expert:  Daniel Jones replied 1 year ago.

Do you have access to a scan tool that can access the computer data?

Customer: replied 1 year ago.
I have my snap-op 2500 scan tool , with 2000 cartridges, could check through the obd 2 port if they haven't changed the plugs
Expert:  Daniel Jones replied 1 year ago.

You will probably not be able to access it with the outdated cartridge.

You need to see if there is a A/C request signal in the ECM.

Customer: replied 1 year ago.
just checked Tahoe with a update scanner , ac switch is showing to work through the ecm, the ac permission says that it is withheld , a/c request yes, will go on and off with the snowflake.
Expert:  Daniel Jones replied 1 year ago.

Circuit/System Verification

  1. Verify the A/C system has the correct refrigerant charge.
    • If the A/C system has the correct refrigerant charge
  2. Verify the scan tool A/C High Side Pressure Sensor parameter is between 2957–151 kPa (22–429 psi).
    • If the A/C High Side Pressure Sensor parameter is not between 2957–151 kPa (22–429 psi), refer to DTC P0532 or P0533
    • If the A/C High Side Pressure Sensor parameter is between 2957–151 kPa (22–429 psi).
  3. Verify the scan tool Evaporator Temperature Sensor parameter is greater than 2°C (34°F).
    • If the Evaporator Temperature Sensor parameter is not greater than 2°C (34°F), refer to DTC B3933
    • If the Evaporator Temperature Sensor parameter is greater than 2°C (34°F).
  4. Engine running, press the A/C request switch. The A/C compressor clutch should engage.
  5. Place the mode switch in the defrost position. The A/C compressor clutch should engage.
    • If the A/C compressor clutch does not engage refer to Circuit/System Testing.
    • If the A/C compressor clutch does engage.
  6. All OK.

Circuit/System Testing

Note: Before executing Circuit/System Testing, all conditions shown in Diagnostic Aids must be met.

  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.
  3. 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.
  4. 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.
  5. Disconnect the harness connector at the A/C clutch coil.
  6. 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.
  7. Connect the harness connector at the A/C clutch coil.
  8. 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.
  9. Connect a test lamp between the ignition circuit terminal 85 and the control circuit terminal 86.
  10. 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.
  11. If all circuits test normal, test or replace the A/C clutch relay.
Customer: replied 1 year ago.
do not have this problem fixed yet
Expert:  Daniel Jones replied 1 year ago.

Can your scanner access the HVAC module data?

Customer: replied 1 year ago.
it will I gave you the info on a previous response
Expert:  Daniel Jones replied 1 year ago.

You checked it through the ECM, can you check it in the HVAC module.

I haven't used the snap-op 2500 in a long time, so I don't know if you can get that data. Also see if there is a function test to turn the clutch on with the scanner.

Customer: replied 1 year ago.
I am using a otc genisys scanner from work
Expert:  Daniel Jones replied 1 year ago.

Did you do the Circuit/System Testing I posted in the previous response?

Customer: replied 1 year ago.
Not yet been working 14 hour days will check this weekend do now that the compressor will not run with the defroster. And the relay has power to the right side terminals. Ac switch goes on and off according to the scanner with ac request saying withheld
Expert:  Daniel Jones replied 1 year ago.

The circuit test should help you determine where the problem is.

If you need more help, please use the "Reply" tab to continue our conversation - I'm happy to answer follow-up questions.

Please remember to Rate My Service when you are completely satisfied with the info I have given you.

Thank you for posting your question.

Your business is very valuable to us.
Thank You
***** *****

Customer: replied 1 year ago.
checked the circuit test today, terminal 86 has power , terminal 30power, terminal 87 no power, terminal 85 no power, turn relay 180degrees and power on 85, power on 87, no power 30, no power 86, can turn relay back ,jump 30 to 87 and compressor will run, just cant shut off with ac switch.
Expert:  Daniel Jones replied 1 year ago.
  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.
Customer: replied 1 year ago.
what else controls the function of the ac besides the control head in the dash
Expert:  Daniel Jones replied 1 year ago.

The PCM will provide a ground for the A/C compressor relay enabling it to close its internal contacts to send battery voltage to the A/C compressor clutch coil.

If you need more help, please use the"Reply" tab to continue our conversation - I'm happy to answer follow-up questions.

Please remember to Rate My Service when you are completely satisfied with the info I have given you.

Thank you for posting your question.

Your business is very valuable to us.
Thank You
***** *****

Expert:  Daniel Jones replied 1 year ago.

Hi, Just wanted to get an update on how you made out with vehicle.
Did I provide you with enough info to fix the problem, it would help to close out this question in my database.

If you need more help, please use the "Reply" tab to continue our conversation - I'm happy to answer follow-up questions.

Thank you for posting your question.
Your business is very valuable to us.
Thank You
***** *****

Customer: replied 1 year ago.
Finally going to get home at a decent time tonight ,hope to check with scanner and light on the two terminals you said to check, sorry for taking so long just been real busy at work keeping all the equipment running for 5 crews. thanks dan
Customer: replied 1 year ago.
I checked with the scanner last night , it will not operate the a/c from the scanner. The only thing that is not showing as normal is it says the a/c request is withheld., all other functions say normal.

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