Kaane

iTrader: (2)

I have a 1999 silverado. Just finished swapping a 6.0 LQ9 into it.

I am having a weird AC issue.

It has all new parts. New compressor, new Condenser, new accumulator, new low pressure and high pressure switch, new orifice tube, fresh recharge, no leaks.

The PCM will not cycle the compressor properly, it works on and off. When I connect tech 2 scanner, I see AC request signal to the PCM, I can see the compressor cycling switch closed yet, it says AC relay command - OFF

Here is a picture.



I have tried bypassing the low pressure switch, makes no difference plus the scanner correctly shows its operating properly.

I have tried bypassing the high pressure switch also makes no difference, when I unplug it it correctly shows AC request off, plug it back in I get AC request on.

It does cycle once in a while, and the AC is ice cold when it does, it just doesn't keep running for long. Since this is a 1999, that has a fixed compressor, is there some other crazy logic that makes the PCM not engage the compressor? I was under the impression that this is a simple analog system with low pressure switch that cycles the compressor and the high pressure switch to shut it off if pressure gets too high. Is there anything else I am missing that would make the PCM do this?

subeone

iTrader: (5)

Jump the switch by the dryer. What happens?

jayyk31

iTrader: (2)

Which compressor are you using? And it is a truck tune?

Kaane

iTrader: (2)

Nothing happens when I jump the switch by the dryer. If I jump the relay it works fine. When I see the AC Relay command on it works fine, so it's not a wiring issue.

Truck tune, 1999 original PCM, truck was 4.8, just swapped to a 6.0, had 2 compressors, both the same, HU6 Delphi style, fixed compressor.

Kaane

iTrader: (2)

I got it figured out. Its a tune issue due to less than perfect idle. I have a small cam and it makes the idle slightly unstable, seems the ECM shuts of the ac compressor if it detects idle issues. Once I held the throttle open even just a bit, the compressor kicks in and ice cold air.

G Atsma

What are the cam specs? Tuning might smooth out the idle.

Kaane

iTrader: (2)

I had it tuned, it runs fine most of the time, but it seems idle is just not to the PCM's liking.

216/220 .560 lift 114LSA

Tiny cam for a LQ9

G Atsma

You have -10 degrees overlap, which should be able to be tuned into a fairly smooth idle

Luke19901

do you have your stall saver RPM at a value that its clipping? if it activates stall saver it will play havoc with the A/C clutch. cuts it in and out as it sees fit

gametech

iTrader: (1)

Because of the small size of the cam, your tuner may have done nearly nothing at all to the idle tables. As you have figured out, any idle instability will cause the ac to cut in and out. I had this same issue when putting an A/A intercooler on a whipple blown 2002 Silverado, although my problems turned out to be mostly due to an IAC that went bad at exactly the same time as the modifications.