A/C clutch not engaging

Tiny
FRO250
  • MEMBER
  • 2001 GMC SIERRA
  • 6.0L
  • V8
  • 4WD
  • AUTOMATIC
  • 18,700 MILES
Truck listed above is an 1500 HD.
About 8 weeks ago I started having trouble with my truck starting. The starter wouldn't crank intermittently. I thought it was the key ignition switch, because it is tricky to turn back into axillary. But it turns out it was just the starter. I replaced the starter about 6 weeks ago and around that same time I noticed my A/C wasn't working. The A/C light on the dash is illuminated showing A/C on. If the Freon is low it will just flash a few times and then stay off. But it isn't doing that now it just stays on. But I tried to put more in, but it didn't take very much. I may have over pressured it. When I depress the A/C button I hear what sounds like a damper changing position, but the blowing air isn't being cooled. I bridged the low pressure switch, but the clutch didn't engage. I checked the coil and it was open. Clutch coil failed. I couldn't find a new coil so I got one from the auto wrecker. I bench tested the clutch and it works. I tested the clutch again after install by applying 12 volts. A/C clutch works great. I bridge the terminals at the A/C relay and clutch engages. Bad thing is I tested the clutch with the engine running and had the clutch engaged for a minute or 2. It still didn't blow cold. I swapped the AC relay with the horn. Both relays good. 10 amp fuse is good and has 12 volts there. I can't find any fuses under the hood nor under the dash in the cab that are blown. I also bridge the high pressure switch, but and I think it is reverse acting. I ordered a scanner of Ebay that is supposed to do all diagnosing for GMC Sierras. Foxwell NT510 Elite Obd2 Car Code Reader Vehicle Diagnostic Scan Tool All Function Scanner for Buick/Chevrolet/Cadillac/GMC Reset with EPB SAS ABS Oil EPB. It is supposed to arrive today. October 8 2020. I can't find a wiring diagram for my vehicle, which makes it difficult to check point to point/ end to end of wires. I unplugged the computer climate control in the dash and plugged it back in again, but no change. My fear is maybe I broke a wire or connection when doing the starter or damaged the A/C system when trying to add Freon.

Please help.

Jeff
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Thursday, October 8th, 2020 AT 9:13 AM

91 Replies

Tiny
ASEMASTER6371
  • EXPERT
Good afternoon,

I attached the wiring diagram for you to view of the compressor circuit.

Check the fuses I circled for power on both sides. It is not a visual test, it is a power test.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check the voltage at the low-pressure switch and high-pressure switches as well.

Let me know when you want to do some testing and I will help guide you through.

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak

Roy
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Thursday, October 8th, 2020 AT 10:18 AM
Tiny
FRO250
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There is power on both side of the fuses. 11.8 volts with the engine not running and 14.0 volts with engine running.

Low Pressure and High Pressure switches have 10.4 volts with engine not running and 12.5 volts with engine running.
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Saturday, October 10th, 2020 AT 1:43 PM
Tiny
ASEMASTER6371
  • EXPERT
Okay, the next step is to check the Freon pressures. Can you give me the high and low side readings of the Freon?

https://www.2carpros.com/articles/car-air-conditioner-not-working-or-is-weak

Roy
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Saturday, October 10th, 2020 AT 3:42 PM
Tiny
FRO250
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Low side 67 psi
High side 67 psi

See attached image.
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Wednesday, October 14th, 2020 AT 5:02 PM
Tiny
ASEMASTER6371
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What is the outside temperature?

Roy
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Wednesday, October 14th, 2020 AT 5:04 PM
Tiny
FRO250
  • MEMBER
42 degrees Fahrenheit.
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Wednesday, October 14th, 2020 AT 5:08 PM
Tiny
ASEMASTER6371
  • EXPERT
Okay, I posted the diagram of the relay previously. Can you verify power to the clutch relay with the key in the on position?

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Roy
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Wednesday, October 14th, 2020 AT 5:20 PM
Tiny
FRO250
  • MEMBER
I failed to mention that my GMC dealer says there is 2 A/C Compressors listed for my truck and mine is the least used. GMC part # 89024907. I only mention it, because I can't the third A/C Switch in the diagram. I don't have any wires going to my A/C Compressor.

I made a Relay Buddy to make it easier to verify power to the relay.
Pin 85 has power and goes to one side of the relay coil.
Pin 86 doesn't have power and goes to the other side of the relay coil. When pin 86 is grounded the relay closes and the A/C clutch is activated.
Pin 87 has power.
Pin 30 doesn't have power and goes to the A/C Clutch. When pin 30 is jumpered to positive on the battery the A/C clutch is activated.
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Thursday, October 15th, 2020 AT 5:31 PM
Tiny
ASEMASTER6371
  • EXPERT
Okay, pin 30 should have power all the time.

Pin 87 goes to the clutch itself.

Pin 86 is ignition power.

Pin 85 is the ground. That is controlled by the ECM based on inputs from the pressure sensors.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

When you ground 85 and the compressor runs, what happens to the pressures? Can you tell me those pressures?

Roy
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Friday, October 16th, 2020 AT 1:51 AM
Tiny
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It's been below freezing for the last week. When it warms up I will try. I think the A/C is disabled if it is too cold.
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Monday, October 19th, 2020 AT 2:09 PM
Tiny
ASEMASTER6371
  • EXPERT
Okay, keep me updated with the results.

Roy
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Monday, October 19th, 2020 AT 4:28 PM
Tiny
FRO250
  • MEMBER
Low side was 25 PSI
High side was 140 PSI
That was after one minute.

The high and low side were both 52 PSI before the compressor was started. The outside temperature 50 degrees Fahrenheit.
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Tuesday, October 27th, 2020 AT 5:05 PM
Tiny
ASEMASTER6371
  • EXPERT
Okay, the pressures look good when it is running.

Based on your reading both at the relay and the switches, you may have a bad ECM.

Roy
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Wednesday, October 28th, 2020 AT 1:13 AM
Tiny
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How do I test for that?
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Wednesday, October 28th, 2020 AT 1:26 PM
Tiny
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Also, when I ground Pin 86 the compressor started, but after waiting 2 minutes it still didn't blow cold air.
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Wednesday, October 28th, 2020 AT 1:29 PM
Tiny
ASEMASTER6371
  • EXPERT
I attached a diagram of the red connector on the ECM. You need to back probe the wire I circled. There should be 12 volts there. If there is, the ECM is not grounding the relay and the compressor will not work.

Roy
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Wednesday, October 28th, 2020 AT 1:36 PM
Tiny
FRO250
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I don't understand your statement. You said there should be 12 volts there, and if there is the ECM is not grounding the relay.
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Wednesday, October 28th, 2020 AT 8:20 PM
Tiny
ASEMASTER6371
  • EXPERT
That is correct. The ground will have voltage from the clutch relay until the ECM grounds that wire. Then there will be 0 voltage and the relay will engage.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Roy
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Thursday, October 29th, 2020 AT 2:13 AM
Tiny
FRO250
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Oh, so I check this with the electrical connector still connected to the ECM. Make sense now.
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Thursday, October 29th, 2020 AT 6:43 AM
Tiny
FRO250
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It's probably me, but out information is lining up the same in your statement a copied and pasted below.

You said
Okay, pin 30 should have power all the time.

Pin 87 goes to the clutch itself.

Pin 86 is ignition power.

Pin 85 is the ground. That is controlled by the ECM based on inputs from the pressure sensors.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

When you ground 85 and the compressor runs, what happens to the pressures? Can you tell me those pressures?

I said
Pin 30 goes to AC Clutch coil and when jumpered to +12vdc the clutch pulls in and the compressor runs.
Pin 87 has +12vdc all the time
Pin 86 doesn't have power and goes to the other side of the relay coil. When pin 86 is grounded the relay closes and the A/C clutch is activated.
Pin 85 has power and goes to one side of the relay coil.

My 86 and 85 is the opposite of yours
My 30 and 87 is the opposite of yours

I have marked the Relay Pins on your diagram according to my findings. Please see attached image.
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Thursday, October 29th, 2020 AT 9:30 AM

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