A/C issue with 05 GMC with Dual Climate Control
 

As some of you may know, I sold my 02 rcsb a few months back and replaced it with a 2005 GMC Sierra crew cab. When I first got the truck, the air-conditioning didn't seem as cold as I thought it should. The A/C in my 02 used to freeze me out of the truck on high. For that reason, I took the 05 in to get gassed up. IIRC, it was slightly low, so after vacuuming it out, they filled it back up per GM spec. Since I bought the truck in Feb, I haven't really had many days where I use AC. It's now getting in the 90s in here SC and time to start turning on the AC.

I've noticed in the few months that I've had it, the air blows warm intermittently (with the A/C on). The truck has the digitial dual-climate control. Sometimes, when I turn the AC on, it will blow cold, other times it will blow warm. The kicker is ALL of the vents do it, not just one side like the issue with the blend doors. I can turn the truck off and start it back up, and SOMETIMES this will fix the issue. Other times, it will just keep blowing warm air. I haven't really paid much attention to it until today, but I did notice that when the air was warm, the compressor wasn't on. As soon as it did kick on, the truck started to cool down. I was going to jump the low-pressure switch on it tonight to see if it would work, but of course, I couldn't get it to blow hot air. :guns:

Also, I'm not sure if this is related but I'm starting to think that it may be. It seems like the efans on this truck run more often at idle than the 05 efan setup that I had on my 02 (yes, even using the same tables in EFILive). I'm wondering if one of the pressure switches is bad and is causing the A/C compressor to kick-off (and also activating the 2nd efan via a/c pressure).

Any help is appreciated! I need cold air. :cry:

theres a relearn/reprogram on hvac module that usually takes care of this problem.if not then its control head

I've done the relearn for the doors once before and it didn't seem to correct the issue. I guess I can always try it again. With that said, keep the ideas coming.

I had a similar problem with my Avalanche, it turned out to be a intermittent short in the low pressure sensor connector. With a high end scanner your can view the history codes for HVAC and see if there is a code for the pressure sensor circuit.

This is what I was trying to avoid. Hate to go to the dealership but I guess I just may have to do so. After driving for 15 minutes this AM with the A/C off, i turned it on to see if it would blow hot/cold. It was blowing hot and the compressor wouldn't engage. I had a piece of wire in the truck so I tried to jump the two pins on the connector for the low pressure sensor but the compressor didn't kick back on at all. I did do the relearn this after for the HVAC and while it did increase the capacity of air coming out of the vents, it didn't seem to make much difference regarding the temperature of the air.

Also, I was looking in EFILive earlier, apparently fan inputs from the A/C are for high pressure. I'm wondering if I have an issue with the high pressure sensor of the system.

Does everyone's AC get ice-cold in their crewcab/suv/avy? Mine is cool but it definitely isn't cold. This in itself makes me want to sell this truck and get a cts-v like i wanted all along. :(

My compressor would not engage when jumping the connector. Made a mistake in my previous post, the short was on the high pressure sensor which is by the compressor. You don't need to take it to the dealer, any decent shop can view the data. When I checked mine all the pressure readings showed --- instead of numbers. Drove me crazy...but it does get ice cold now that the compressor works all the time.

After spending a good part of the day trying to find the short I bought a power probe tracer and found the problem like in 2 seconds! It was a pretty good investment.

Check the voltage at the sensor connectors, the low side has two wires, one is ground and the other should read 4-5volts. The high side sensor has 3 wires, one is ground, one is 4-5 volts, and the other is a reference. My high side connector was broken internally about an inch back from the connector. New connector was like $12

Thanks for that! I misread your first post as well. I overlooked that you were talking about the actual connector instead of the sensor. Guess I'll see what I can figure out this weekend. I've gotta do something..its 95* here today and its not even June yet.

how do you do the relearn? sounds like that would be my problem. i have a 05 with digital daul sides.

when i turn ac on it doesnt blow anything out. compressor kicks on though and if i do over 40 it starts blwoing on the vents cold. i think its blower module but id like to try this relearn as i dont think its opening anything

Supposedly either A) unplug HVAC module or B) unhook negative cable from the battery. Hook it back up.

Start the truck.
The air should (automatically) begin blowing through the defrost for approx 15 seconds. DON'T TOUCH IT!
The air should then start blowing down by your feet for approximately 5 seconds.
Next, it should start blowing out of the vents. While the air is blowing out of the vents towards you, turn the HVAC off and restart the truck. Failure to turn HVAC off at that point will have the blend doors defaulted to blowing through the defrost.

Unplugging the HVAC controller works best.