Air Conditioner Won't Engage at idle RPM

[Boostedpete]

Hey, I have a 2003 suburban, and had the whole motor rebuilt and got it professionally tuned. The truck runs great, but the air conditioning compressor won't engage at idle, and sometimes doesn't seem to engage at all. Usually above 1k rpm it will engage. It's still a daily driven vehicle, so I'd really like to have AC working lol. It's not an issue with the air conditioning system, all the sensors, compressor, etc have been replaced/checked. The computer is calling for AC, but something in the ECU is blocking it from engaging. I'm not very familiar with tuning. So if you could help me to figure it out I'd be so appreciative.

[PREDATR]

Have you checked the AC pressures?

[blindsquirrel]

Auto or manual HVAC?

[Boostedpete]

Yes the system is filled and has good pressure

[Boostedpete]

Automatic AC tri-zone

[blindsquirrel]

Have you done the obvious like checking proper compressor clutch airgap? When you think it should be engaged but it's not, have you tried (carefully!) whacking the drive plate to rule out a weak magnet/excessive airgap?

There are lots of codes and specific temporary conditions that will inhibit clutch engagement:

A/C compressor clutch will not engage under the following conditions:
? The A/C high side line pressure is over 2957 kPa (429 psi).
? The A/C low side line pressure is under 138-172 kPa (20-25 psi).
? Throttle angle is at 100 percent.
? Engine speed is more than 5500 RPM.
? Engine coolant temperature (ECT) is more than 121?C (250?F).
? Ambient air temperature is less than 5?C (40?F).
? Engine is idling at a low unstable RPM.

Does the relay click when you think the clutch should be on, but isn't?

Can you jump the load side of the relay with engine off and make the clutch engage like it should every time with no weirdness? To check the relay and all related wiring at once, can you jump the dk green/white wire, PCM C2 pin 43, to ground and make the relay click and the clutch engage?

screenshot.23-04-2022 11.06.31.png

There is a page in the service manual called, believe it or not, 'HVAC Compressor Clutch Does Not Engage'. No, really. It has a flow chart. The flow chart has lots of links to other documents and diagrams that get lost when copy/pasting or saving as .pdf, if you really want to tackle this yourself I'd highly recommend getting access to some kind of service manual.

[Boostedpete]

Thank you for the detailed response. The compressor is brand new and it does engage perfectly fine, there's something in the ECU that blocks it below around 1k rpm though. Because I hooked it to a snap on computer and all the pressures, everything was great, and I checked manually with gauges too. If I try to force the clutch on through the snap on, it gets blocked until I hit the gas higher.

[blindsquirrel]

Scan data should show the reason for clutch inhibit.

[RDF1]

Ive had several cammed trucks come in that have been tuned with AC Issues and they had "Stall Saver" Settings all wrong. Havent figure out which "tuner" keeps doing this.

They would do something similar where no AC at idle. But a little off idle it works great.

[kevin87turbot]

Ive had several cammed trucks come in that have been tuned with AC Issues and they had "Stall Saver" Settings all wrong. Havent figure out which "tuner" keeps doing this.

They would do something similar where no AC at idle. But a little off idle it works great.

I've seen the exact same thing. It'll keep the AC from engaging at idle and make for a very unhappy customer.

[RDF1]

Prob the same tuner from south east MS.....

[Boostedpete]

I think you're correct. The more I read about stall saver it seems like that's the same issue I'm having. Any idea how to fix it?

[TheMechanic]

I think you're correct. The more I read about stall saver it seems like that's the same issue I'm having. Any idea how to fix it?

Compare with stock vehicle and copy over. As stated by blindsquirrel you need to check reason for inhibit

[Boostedpete]

How do I do that lol