A/C clutch not engaging

Tiny
FRO250
  • MEMBER
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?
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Thursday, October 29th, 2020 AT 9:34 AM
Tiny
ASEMASTER6371
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Okay, they are backwards

Pin 30 is constant voltage. It is where you have 87 marked.

Pin 87 goes to the compressor.

Pin 86 is ignition power or is where you marked 85.

Pin 85 is the ground for the control side of the relay.

Roy
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Thursday, October 29th, 2020 AT 9:59 AM
Tiny
FRO250
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I see now the relay can be installed 2 ways and it works both ways.
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Thursday, October 29th, 2020 AT 12:16 PM
Tiny
FRO250
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What about the third pressure switch I can't find?
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Thursday, October 29th, 2020 AT 12:16 PM
Tiny
ASEMASTER6371
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I attached a picture of the switch for you to view.

Roy
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Thursday, October 29th, 2020 AT 12:24 PM
Tiny
FRO250
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I have accounted for that one. Where is the other one?
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Thursday, October 29th, 2020 AT 1:16 PM
Tiny
ASEMASTER6371
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There are no other switches.

Roy
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Thursday, October 29th, 2020 AT 1:28 PM
Tiny
FRO250
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In the picture it shows 3 switches. I have only found 2 of them. I circled the 3 pressure switches. Please confirm.
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Monday, November 2nd, 2020 AT 8:44 AM
Tiny
ASEMASTER6371
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I see that. The only thing is that the wiring diagrams are generic for most models.

You only have 2 switches.

Roy
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Monday, November 2nd, 2020 AT 8:59 AM
Tiny
FRO250
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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.
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Monday, November 2nd, 2020 AT 10:34 AM
Tiny
ASEMASTER6371
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You will need an advanced scan tool to monitor that signal to be sure the switches are sending a signal.

Roy
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Monday, November 2nd, 2020 AT 10:42 AM
Tiny
FRO250
  • MEMBER
Why can't I back probe that pin also?
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Monday, November 2nd, 2020 AT 11:34 AM
Tiny
ASEMASTER6371
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You cannot tell if the ECM is processing the signal by voltage. That is based on programming inside the ECM.

Roy
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Monday, November 2nd, 2020 AT 11:41 AM
Tiny
FRO250
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Okay, but I should be able to test the input some how.
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Monday, November 2nd, 2020 AT 1:37 PM
Tiny
ASEMASTER6371
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Okay, red connector pin 55 in the diagram I attached.

Check for power with it disconnected. You should have voltage. Then hook it back up and there should be 0 volts if the ECM grounds the signal.

Roy
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Monday, November 2nd, 2020 AT 1:51 PM
Tiny
FRO250
  • MEMBER
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
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Tuesday, November 3rd, 2020 AT 10:07 AM
Tiny
ASEMASTER6371
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2. Correct. It opens at 375 pounds.

2B. I did in the last response. Pin 55 is the ground for the low pressure switch.

3. Correct. Once the ECM sees this signal, it should ground that pin.

4. Correct again.

Your X is incorrect. That is the one you have. The other sensor is the one that does not apply.

Roy
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Tuesday, November 3rd, 2020 AT 10:14 AM
Tiny
FRO250
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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
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Tuesday, November 3rd, 2020 AT 11:18 AM
Tiny
ASEMASTER6371
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Low pressure switch to Pin 55.

High pressure is an external ground not controlled by the ECM. The ECM provides power to the switch as in the diagram I posted.

Roy
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Tuesday, November 3rd, 2020 AT 11:41 AM
Tiny
FRO250
  • MEMBER
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
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Tuesday, November 3rd, 2020 AT 12:19 PM

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