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AC Dreadfully Warm When Driving, Cold When Idling

AC Dreadfully Warm When Driving, Cold When Idling

Postby Damage42x [OP] » Sep 03 2020, 11:28pm

Hey all. I have an '03 Sierra 1500 5.3, and simply the AC is too warm when I drive it, especially on the highway, and it gets so warm that sometimes I literally stop on the side of the road just to let cabin cool a bit.

Some hints that might help someone identify my problem.

*Heater core was leaking the smell coolant into the cabin years ago, so I had it bypassed and blocked since I rarely ever use it.
*I always smell the outside air and fumes when I'm driving with the AC on, as if the recirc is switched off, but...
*Re-circulation works, when the AC is off and the recirc button is switched I feel nothing coming out of the vents while driving +70mph, and when the AC is on it definitely gets warmer when I switch the recirc off, so I don't think it's the problem.
*Pushing the AC switch on or off feels like it makes absolutely no difference when I'm driving, I don't feel the air getting any colder or warmer.
*It gradually gets colder when I slow down, and becomes ice cold when idling, regardless of how hot or cold the weather is.

I personally think it might be the high pressure refrigerant sensor, but I'm hoping someone has an idea about what's going on before I start throwing money on unnecessary parts.
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Damage42x
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Re: AC Dreadfully Warm When Driving, Cold When Idling

Postby Damage42x [OP] » Sep 04 2020, 2:26am

Also, the climate controls I have are the dual zone manual controls. The 5th speed isn't working, so I probably need a new resistor.
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Re: AC Dreadfully Warm When Driving, Cold When Idling

Postby RivGuy » Sep 04 2020, 4:45am

Based on the title of your thread the first thing that came to mind was possibly your serpentine belt was slipping on the compressor pulley. But that would cause a noise that you would hear and could be caused by a bad belt tensioner or a compressor that was failing and causing enough resistance to make the belt slip at higher RPM's.

Have you vacuumed out the A/C system? I ask because that will do 2 things. If you can hold a vacuum with the system evacuated then you will know there are no leaks.
Second, by evacuating the system you will remove any moisture in the system that could be freezing up at higher rpm's and causing the system to go 'HOT, yet allow it to produce cold air when the moisture in the system thaws. This allows you to get Cold air at idle and lower RPM's.

I'm not sure though. You seem to have several other things not working correctly that could cause all kinds of problems. i.e. Heater core bypass and possible resistor failure. I would start at the beginning and work toward the end.

Do you have a set of A/C gauges to measure your compressor pressures.? If not, you need to have a shop help you with the diagnosis.

I can't help you with the proper diagnosis of your A/C system on line without reading low and high pressures while the engine and A/C system are running.

Also, i don't believe it would be a failure of your High Pressure Switch. If that switch fails you would experience NO A/C at any RPM.

There is thermal temp sensor in the HVAC plenum that the computer reads info from. If that sensor has failed along with your Heater Core being bypassed the HVAC system is not getting the readings it needs to control the cabin temp. this system also compares the inside cabin temp to the HVAC temp to be able to provide you the correct inside temp.

There are SO many things that could be wrong.

You need to start by using the GM Factory Manual for troubleshooting the problem. If you follow that you should be able to find the source of the problem.
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