Rear A/C failure. Sounds coming from engine compartment.

[Greg408]

I could use some advice on my 2003 Yukon. It has a stock 4.8L with 140K miles. Last year I replaced my water pump/belts and just last month I replaced most of the water hoses in the engine bay. A week later, I noticed the rear A/C was not cooling the truck. Secondly, now I am starting to hear random noises coming from the engine bay. It's not continuous, so i can't listen with stethoscope. The sound is in short 3-4 second bursts, sounding like possible bearing going out somewhere. It only happens when A/C is on. I don't loose power when I hear the noise and typically happens when engine is above 2K RPM.

The front A/C is still working fine. I checked the temperature of the high and low pressure lines at the back of my truck. The cold line was not very cold. I went up to the engine compartment and found where the refrigerant lines split off from the compressor. The one going to the rear was not ice cold, but the one going up to the dash of the truck was ice cold.

Does this mean something in the line is clogged (expansion valve/orifice tube)? I don't think this would be a result of the system refrigerant being low, but I could be wrong.

 

[mizzouguy]

If I'm not mistaken, the rear unit uses an expansion valve setup while the front uses the orifice tube setup. I would probably start by taking off the rear passenger side panel where the evap housing is and verify the blend doors are all working correctly. I'm not sure what the external line temp is supposed to be on an expansion valve setup. Bursts sound like ac clutch bearing since it only happens with ac on.

 

[Greg408]

Thanks, mizzouguy, for your comment. I've seen multiple videos of troubleshooting/testing the rear side panel area, including the blend box. It seems like the gears in that box crack pretty easily. I hope it's just the gears...

I went to NAPA and had them print me a parts list of typical a/c replacement part list. Here's what was listed:

3638376, A/C Compressor w/Clutch, $261.49
408001, A/C Accumulator, $48.59
217011, A/C Expansion valve, $27.99 (this is the one you mentioned)
207301, A/C Orifice tube, $2.69

I'll have to research replacing the rear expansion tube procedure...
I'll update after I pull the side panel cover and test the mix box.

 

[mizzouguy]

I'm interested to see if it is the blend doors in the rear or some other issue.

 

[Greg408]

Ok, so the temperature control flap gearing is broken, so that's a culprit of possible warm air. I unplugged the unit (below) from the wiring, and I held the flap in the cold position while running the front and rear a/c system. It should blow cold air, right?? It didn't though. So, I still have a problem...



When I buttoned-up the blower assembly, this is how I left it: unplugged in the "cold" position. I honestly don't care about this control flap as long as it's permanently set to cold. As long as the computer doesn't disable the rear a/c because I unplugged the temp-control flap motor. It never gets cold enough in So Cal to turn on rear heat - just A/C. Do you think unplugging it (below) is a problem?

 

[mizzouguy]

Wow, usually the blend door itself breaks before you break the gear in the actuator. Thats weird that it still did not work. Were you feeling the air coming out the ceiling vents or the "floor" vent in the rear with the truck running when you held the blend door in the cold position? Or did you take off the ductwork and feel the air coming right out of the box leading to the ceiling vents?

 

[Greg408]

I'm sure the door is good, because I can feel the blend door when it hits both ends of the duct work. When I tested it I did it using the ceiling Air ducts. I waited a while for it to cool down, but it never did. It probably would have been more reliable to remove the duct work at the blower.

 

[PatDTN]

Can I tuck a question about rear AC into your thread? My 2009 LTZ keeps turning on the rear AC when I start it on hot days. Is that normal? Could there be a rear seat sensor picking up the weight of my turnout gear on the back seat?

Sorry to borrow your thread but I thought someone reading might know.

 

[mizzouguy]

GTFO^ lol not even the same year range. No clue...
@Greg408 My truck is black and the roof/ceiling vents get hot. Takes a while to get cold air out of those vents if it's been in the sun. Even once I'm running down the road with the rear unit on full blast it never really gets cold because it's battling the roof temp. I'd try putting it on the floor and feel the air temp there.

 

[Greg408]

OK, thanks I'll try that. I drive home 4 miles from work, so maybe I'll see a difference when I get closer to home.