2008 Yukon Denali AC fluctuating between Cold and hot

[Joaquin72]

Just bought the truck. So don't have anything like a tech2 to diagnose. Low pressure switch and orifice tube replaced. AC system professionally evacuated and recharged to 1.6 lbs. The rear AC line is capped off.

At idle ac consistently stays cold. When accelerating I notice the AC goes warm. Then fluctuates from cold to hot while driving. Sometimes it starts to go warm and goes back cold right away. Sometimes it stays warm for long length of time.

So far the common denominator is the clutch is off when ac stops being cold

Any thoughts?

 

[Just Fishing]

If you have rear AC, then it should hold something like 3lbs.
I'm guessing that the compressor is cycling, thus you're seeing the cold and hot switching when the compressor stops and starts up again.

You will need an AC gauge to see what the high side and low side are doing, and add refrigerant as required.

I also hope you replaced the drier while it was apart?
moisture can get into the system if it's open, and the general rule is to replace the drier if it's left open for any amount of time.

 

[Just Fishing]

I'll add that my AC in my wife's Buick suv has been doing that off and on.
the gauge showed a weird pressure spike right before the compressor clicked off.
I removed refrigerant thinking I must have over charged it.
It got better at first, but it still did it.
Then the pressures were too low?!

I also noticed I could hear the compressor a bit more than I expected to.
In the end I added more Pag (46) oil (1/2 oz) using an injector tool that uses refrigerant to push it in.
noise stopped, and the pressures stabilized.

weird stuff sometimes.

 

[Joaquin72]

If you have rear AC, then it should hold something like 3lbs.
I'm guessing that the compressor is cycling, thus you're seeing the cold and hot switching when the compressor stops and starts up again.

You will need an AC gauge to see what the high side and low side are doing, and add refrigerant as required.

I also hope you replaced the drier while it was apart?
moisture can get into the system if it's open, and the general rule is to replace the drier if it's left open for any amount of time.

Since the rear line for the AC is capped off. They filled the system to 1.6lbs based on the required fill amount for front AC. Should they fill it to 3lbs even with the line capped off to the rear?

No the drier wasn't replaced but I'm going to have them replace it now that I know.

 

[Just Fishing]

vac the system if you do it your self.
you will want a ac gauge set, a cheap one will work.
I would start there if it's working as is.

HF seems to have a nice one.
they also sell the vacuum pump.

I like to get it on the vacuum as soon as possible, increases the lifespan of the drier.
be sure to balance the Pag 46 when swapping the drier.

 

[Joaquin72]

vac the system if you do it your self.
you will want a ac gauge set, a cheap one will work.
I would start there if it's working as is.

HF seems to have a nice one.
they also sell the vacuum pump.

I like to get it on the vacuum as soon as possible, increases the lifespan of the drier.
be sure to balance the Pag 46 when swapping the drier.

I had buddy at a grease monkey (quick service oil change business) do it. They have the AC machine that evacuates and vacuums system.

What should the AC be charged to if the rear line is capped off? It must of had a leak and the previous owner didn't want to fix it.

 

[Just Fishing]

if it was an unknown, I would say use the gauges and use the recommended pressures for the outside temperature.
big thing: you need to watch the gauges as it clicks on and off to get an idea of what's happening.

Like is the pressure spiking when the compressor kicks off?
or is the pressure dipping when it clicks off?

Note that I am not an AC guy, but I get away with it.

Lots of

 

[Joaquin72]

if it was an unknown, I would say use the gauges and use the recommended pressures for the outside temperature.
big thing: you need to watch the gauges as it clicks on and off to get an idea of what's happening.

Like is the pressure spiking when the compressor kicks off?
or is the pressure dipping when it clicks off?

Note that I am not an AC guy, but I get away with it.

Lots of
View attachment 404851

Lol I understand. I appreciate the input

 

[Just Fishing]

Googling, I found a full size truck w/o rear ac and the 5.3
Lists about 2lbs.
Accurate?
maybe!

https://www.4s.com/media/5421/four-seasons-capacity-guide.pdf

 

[swathdiver]

I think it's wild that we're seeing so many posts this week on this exact same symptom across multiple forums and different trucks and generations and even models. They are all low on refrigerant. At least, this is the most common reason for that symptom.

The truck will run a lot cooler with less stress on the system if you get the rear AC working again.