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tristan, ASE MASTER
Category: GM
Satisfied Customers: 1610
Experience:  8yrs. exp. ASE MASTER
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2002 GMC Sierra 1500: AC..already replaced..defrost, to floor, vent

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I have a 2002 GMC Sierra 1500 HD, and my AC is stuck on defrost. I have already replaced the mode door actuator, and gone through the re-calibration process, but this did not fix the problem.

In addition, the air will occasionally change by itself (from defrost, to floor, vent, and back to defrost) without any input from me.

I'm also getting two DTC's (B3761 and P0300). I know the B3761 has to do with HVAC actuator circuit, but it does not tell me what the problem is.

My next step is to replace the HVAC control unit, but it's $250 at the dealer - is there anything else I can to do confirm this part is bad?

unplug the connector at the mode actuator, check for 12 volts on the brown wire, ground/0 volts on the gray/black wire, 5 volts on the red wire. if any are missing reply back. next check the control circuit which is the gray wire, now with the connector plugged back in, check the voltage on the gray wire while playing with the a/c control switch switching it from defrost to panel (2.5 volts mean the actuator is stopped, 0 volt means its moving in one direction, 5 volts mean its moving in the other direction, if you are changing the switch and it stay at 2.5 volts, replace the control switch, if the voltage is changing but the actuator not moving, replace the actuator, everything is good and the actuator is moving, make sure the actuator is lined up properly or you may have a problem with the door sticking in the a/c door housing. good luck. and please accept my answer so i get credit. thanks T-MAN
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Customer: replied 5 years ago.
One more question - I assume I want the vehcile running and the AC turned on when I check the voltages at the mode actuator connector?
Customer: replied 5 years ago.

I just checked the voltages with the vehcile running. I used the gray/black wire for my ground. With the connector unplugged, I have 12volts on the brown wire, 5 volts on the red wire. I also had 5 volts on the grey wire.


Once I plugged the connector back in, I still had 5 volts on the grey wire, even when changing the switch from vent to defost - the voltage on the grey wire never changed from 5 volts. (your reply said if it stayed at 2.5 volts, replace the control switch - does this still apply if it stays at 5 volts?


Thanks, Scott