2004 Hoe.
My A/C has been having this issue for 2 years now. It blows cold when I'm moving, but gets weak when I stop. It's worse when the rear is on as well. Fan is working fine.
Right now, I can get a minimum 60* coming out of the front and 70* from the rear while sitting in the driveway. With the front only on, I can get ~47* out of the front center. Ambient temp is 75* and ~30% humidity.
According to the service manual, the pressure readings should be:
LOW: 27-41psi, HIGH: 136-193psi.

Mine read:
LOW:58psi, HIGH 220psi.

According to the service manual, that places me in "Zone B" which means refrigerant undercharge or faulty compressor.
If it is in fact an undercharge of refrigerant, how can I be sure how much needs to be added, and how would this affect the Low & High pressures?

 

Are you sure about that? Higher pressure would mean too much refrigerant or higher temperature. The most likely scenario is insufficient cooling at the condenser. Make sure nothing restricting the air flow through the condenser?

 

Here is a thread that talks a bit about AC and has some helpful links as well...

https://chevroletforum.com/forum/gen...g-101-a-34615/

 

That's exactly what I thought. It doesn't make sense to me at all. There is nothing blocking the condenser. I can see right though it when putting a light behind it. No clogs, and the fan is nice, tight, and blows well.

 

Maybe a restriction in the freon lines which would cause hi suction and high pressures.

 

Also, it could simply be overcharged, depending on the service history. The system efficiency drops off on either side of the ideal pressure ranges.

 

drops off or shuts off... on my '95 tahoe if the pressure would drop below X psi, the A/C compressor would run... then shut off... then run... then shut off... the... etc...

i'd fill it, and it would run / last about a week.

 

I thought of that as well, but you would think it would work better with the rear running at the same time then....
My last guess was as posted above, with line restrictions causing high pressures. Not sure what to do at this point.
Is there a way to tell actual over/undercharge and by how much?

 

The problem is there's no way to know how much of the pressure is attributable to the refrigerant vs. air or other contaminants introduced by some DIY job. I've always left that to the shop with the right equipment for the full recharge service.

 

Agreed. Wasn't sure if anyone knew of any other tricks to try. My first guess from all of this was a contaminated system with a bad dryer. Guess I'll have to bite the bullet, or just be warm...