My 2000 Blazer's AC doesn't work after it took a lightning strike sitting beside a tree.
After replacing the computer, 4wd control unit, 4wd actuator, it is running fine, but the AC will not engage. I can jump the relay, going to the AC clutch, and it cools great. I have checked the charge, and pressure switch, and both are good. The pins on the computer plug (labeled white) are labeled
1. A/C compressor clutch relay control
25. A/C request signal
30. A/C cycling switch signal
If I want to check these pins, what should I see, voltage or ground, when the AC control is turned on?
The radio was also hit, so I am questioning if the temperature control unit is sending a signal to the ECM

 

Is your HVAC system manual or automatic? I will assume manual because you did not mention a class 2 serial data connection to the PCM from the HVAC control module.

The AC compressor clutch relay control grounds the relay coil with a call for cooling to energize the compressor relay and supply power to the compressor. The other side of the relay coil is run or start fused power. The relay contacts are supplied by fused always on power.

The call for cooling from the HVAC control module on the AC request signal wire is a positive voltage supplied through a closed high pressure switch on the back of the compressor and then into the PCM.

The PCM is also looking for a ground on the AC cycling switch wire provided by a grounded and closed low pressure switch at the accumulator.


George

 

Sorry to leave out the control type. It is automatic temperature control.

 

OK. Everything I told you is still true but now there is a class 2 serial communication signal and other sensors involved so its more complicated. You can still look for a 8.5V or higher call for cooling on the:

hvac control module>lt blue wire>compressor hi pressure switch>dk green/wht wire>PCM path

and ground on the PCM>dk green/wht wire>relay coil path.


George

 

and here is your circuit diagram:




George

 

and here is your circuit diagram:




George

 

I have confirmed the ground through the low pressure switch, but I am not getting any voltage on the LT BLU / DK GRN WHT wires running through the high pressure switch, when the AC control switch is turned on.
The button lights up, but no voltage is on the LT BLU wire feeding the circuit.
Can I jump 12v to the DRK GRN on the high pressure plug to test the clutch circuit?

 

Yes but just to be safe, disconnect the lt bl wire back to the HVAC control module and use a fused 12V source.


George