Underdrive Pulley & Now A/C Does Not Work at Idle

[SOMhaveit]

Guys, I have a 2004 Z06 that the A/C was working fine on. I had to take the headers off to get them repaired so I figured, may as well put an underdrive pulley on while it's down.

Here's the issue: since the install, the A/C is all confused at idle. It can't decide to run or not, I can hear the compressor cycling on and off and the light indiciating the A/C is on/off flickers as the compressor cycles rapidly. When the engine is off idle the A/C is fine.

I'm seeing differing info as to whether the pulley underdrives the A/C, but the most recent info is that it does underdrive the A/C 10%. Clearly, something changed. I did hook up the A/C gauges and pressure is in the range.

I need a remedy. Anyone seen this before?

I did do several searches and didn't see a solution in the threads I found.

Thanks.

[Chevy366]

Must be low on freon ?
If not pressure switch .
Just a couple of thoughts .

[ChipsByAl]

Any codes? Including the HVAC module. What does the PCM say about commanded A/C?
Al

[SOMhaveit]

No Codes & not low on refrigerant.

I just used the scan tool and if I bump spark advance to 32 degrees at idle using BIDI, it works fine. A/C stays on & doesn't cycle frantically. Either the PCM is looking for more torque loss at idle and it's screwing things up or something like that maybe? I hate to just start screwing with parameters without out a clue what I'm doing. I usually know just enough to be dangerous, but not I'm disarmed.

[SOMhaveit]

Any codes? Including the HVAC module. What does the PCM say about commanded A/C?
Al

No codes. Do you mean A/C compensation Spark or one of the IAC PID's? Haven't logged any of those.

[SOMhaveit]

OK. I called SLP & the pulley is underdriving the a/c by 5% or so according to them. That shouldn't be an issue though, because my idle is well above 5% more than oem parameters. However, It has changed the relationship of the a/c compressor relative to the stock crank pulley, so the question is, what effect does spinning the pulley 5% less have that would make some sensor or the PCM send/receive a signal that makes it think the compressor needs to shut down?

[StuntHeavy]

What happens if you go back to OEM idle? Does the issue get worse or better?

[SOMhaveit]

No change. When idle drops below 1000, the A/C compressor starts cycling on and off rapidly. If it is above 1000, no problem.

[joecar]

...
Either the PCM is looking for more torque loss at idle and it's screwing things up or something like that maybe?
...That sounds like what is happening...

There are some AC torque loss tables in the calibration (under Idle), maybe try playing with those.

[SOMhaveit]

Thanks Joe. For the time being I just set A/C idle to 1050 & doubled the value in B4356 in the 0 to 689 kPa area & it's working fine. Also made sure low side pressure on the A/C is maxed at 35 psi. It's 91 degrees here now & better than 50% humidity.