Getting power to the relay but nothing to the compressor

[Notnilc20]

I have a 1999 GMC Sonoma 2.2. I've attached the wiring diagram for your reference.
Before I delve into it let me first state that my compressor clutch kicks in when jumping terminal and when jumping the relay terminal. I've also switched relays with the horn relay (it's good) and nothing
My pressure is good and the pressure sensor (3wire) is sending 1.9 volts iirc from the 5v reference to the ecu (I think that's how it works). There is no low pressure switch is just the one sensor on the high side line.
When I switch the ac on on the controls inside I hear and see the idle up go and then settle back.
I have back probed the PCM at the proper terminals and it all seems to check out.
On the relay terminal the 12v all time and 12v when key on its good as well as the fuses.
Also the switch terminal on relay is getting 12v. But it only shows 12v when I plug the relay in. But it does not send the 12v to the terminal for the electromagnetic clutch. I am at a loss of what the issue could be. It all seems to be pointing to the relay but the relay tests good. I'm just a DIY'er so I am not too experienced with this stuff. The truck blows ice cold air when I hot wire or jump it. Please help.

[JohnHere]

The system might be low on refrigerant, not unusual for a vehicle of that age. When you say the pressures are good, have you actually connected a Manifold Gauge Set to check them? Is the system all original, and has it ever been serviced?

[Notnilc20]

Hi John. Thanks for the reply. I did hook up a manifold and had about 85 psi IIRC. When I start the engine and press the ac button and the clutch doesn't engage I jumped it and it was working fine with ice cold air. Low pressure was 35-40 psi and high was I think around 130-150psi but it's been a couple weeks so those psi might be off from what I remember. But it was in the neighborhood. I did add a can of r134.
The compressor was replaced a few years ago but I had the ac running great last summer and I replaced the pcm because of a bad fuel injector ground control and now the ac is not working. but I dont know if it's because of the new pcm or what? I back probed the wires on the diagram on the pcm and the pressure sensor and was getting the proper readings (I think). I-m at a loss and don't know what to do. Any help is appreciated

[JohnHere]

My first inclination is to recover/evacuate/recharge to get the correct amount of refrigerant into the system, and then see if the compressor runs normally. Even though the compressor engages when you jump it and the system blows cold, it could still be low on charge.

The specifications that I have for your truck are as follows:
—32 ounces net weight of R-134a, and 8 fluid ounces of PAG-150 — Standard Cab, with OE Harrison Compressor;
—32 ounces net weight of R-134a, and 8 fluid ounces of PAG-46 — Standard Cab, with Denso Compressor.

If you find an underhood decal with specs different from these, the decal always takes precedence.

[Notnilc20]

I will try that John. Thank you very much.

[Notnilc20]

My first inclination is to recover/evacuate/recharge to get the correct amount of refrigerant into the system, and then see if the compressor runs normally. Even though the compressor engages when you jump it and the system blows cold, it could still be low on charge.

The specifications that I have for your truck are as follows:
—32 ounces net weight of R-134a, and 8 fluid ounces of PAG-150 — Standard Cab, with OE Harrison Compressor;
—32 ounces net weight of R-134a, and 8 fluid ounces of PAG-46 — Standard Cab, with Denso Compressor.

If you find an underhood decal with specs different from these, the decal always takes precedence.

A little update. I was able to get some time to take it to a quick ac service shop and they evacuated the system, pulled a vacuum and filled up with the proper amount of r134. My hopes were high but soon dashed as the clutch still didn't want to kick on. At least now I know what it isn't. Any other ideas? thank you.

[JohnHere]

Okay...we've got the system charged to specs.

It could also be the wiring from the relay to the clutch, the diode that prevents stray voltage induced from the clutch coil from traveling back to sensitive electronics, or the PCM itself.

Try the easy test first. Put a VOM's test leads across the wire between the relay and the clutch. If you get no continuity, then the wire is open someplace.

Let's hold off on the rest for now.

[Cusser]

I was able to get some time to take it to a quick ac service shop and they evacuated the system, pulled a vacuum and filled up with the proper amount of r134. My hopes were high but soon dashed as the clutch still didn't want to kick on. At least now I know what it isn't. Any other ideas? thank you.

Wow - a "real" AC shop would've/should've gone the next step to diagnose why the compressor clutch wasn't getting power. I hope JohnHere can help you with the electronics.

[Notnilc20]

Okay...we've got the system charged to specs.

It could also be the wiring from the relay to the clutch, the diode that prevents stray voltage induced from the clutch coil from traveling back to sensitive electronics, or the PCM itself.

Try the easy test first. Put a VOM's test leads across the wire between the relay and the clutch. If you get no continuity, then the wire is open someplace.

Let's hold off on the rest for now.

When I jump the relay the compressor kicks on so the wiring should be fine between the relay and clutch. But I checked anyway for continuity on the wire between the relay terminal that feeds the compressor clutch and it read 1.4 ohms.

[JohnHere]

When I jump the relay the compressor kicks on so the wiring should be fine between the relay and clutch. But I checked anyway for continuity on the wire between the relay terminal that feeds the compressor clutch and it read 1.4 ohms.

Then something has to be amiss in the area of the relay...maybe the relay itself or a loose female plug?