A/C clutch not engaging

Also I can only find 2 pressure switches on my AC circuit. One on the low side of the orifice and the other on the high side of the orifice. Which high pressure switch in the diagram am I missing and where is it supposed to be?

I see now the relay can be installed 2 ways and it works both ways.

What about the third pressure switch I can't find?

I have accounted for that one. Where is the other one?

In the picture it shows 3 switches. I have only found 2 of them. I circled the 3 pressure switches. Please confirm.

Okay.

I checked ECM. When I back probed pin 43 to ground the compressor relay closed and the compressor clutched pulled in. I let it run for 5 minutes at 2,500 rpm's and the temperature at the vents was 43 degrees F. But before I decide to get a ECM, I would like to test the request for AC input into the ECM. Can you tell what pin that is please.

Why can't I back probe that pin also?

Okay, but I should be able to test the input some how.

I would like to check all input devices to the ECM. I reference the diagram you provided.

1. The red X is the pressure switch you said my Sierra truck doesn’t have. Have I crossed out the correct switch?

2A. The A/C request / High Pressure switch is a series grounded circuit that comes from the HVAC control module, through the High Pressure Cutout Switch, and to the ECM Red Pin 55. The switch will open if the pressure is too high. Is this correct?

2B. The A/C Low Pressure switch is a grounded circuit that grounds the ECM Red Pin (Please provide Pin ID). The switch will close when the Freon pressures is too Low. Is this correct?

3. Once the above statements 2A and 2B are proven Grounded the ECM will ground Red Pin 43 which will activate the A/C Compressor Clutch Relay coil. Is this correct?

4. The AC Relay closes providing +12 volts DC to the A/C Compressor clutch, thus engaging the compressor. Is this correct?

List of A/C devices:

-HVAC Control module (only provides a ground via through the High Pressure Cutout Switch)

-A/C High pressure cutout switch (opens when the Freon pressure is too high) circled in green

-A/C Low pressure switch(Closes when the pressure is too low) circled in Green

-A/C Relay

-A/C Compressor clutch

-TWO 10AMP fuses

-ECM

-Wires and connectors

What other devices, if any, can be the cause of the A/C relay not to close?

Thanks

Okay, the AC request goes to red Pin 55.

The A/C High Pressure switch goes to which Pin?
The A/C Low Pressure switch goes to which Pin?

Thanks

Okay, but when the pressure at the High Pressure switch is below 375 PSI the switch is in the closed position. I can test this by checking ground at the Pin. What is the Pin ID?

Is the AC Request a grounded input from the HVAC Control Module when A/C is requested? What is the Pin ID?

Thanks