1999 Tahoe Low Side Cutting

[phillipm]

So I have a 99 Tahoe with C69 front and rear air. I've owned a few of these trucks and never fought the ac like this one. I'm running the 2.75lb charge as called out in my GM manual (I've also tried other volumes but this does seem to be the best with the newer replacement GM condensers)

What's happening is at 1500-1800 rpm it seems to work pretty well. However above that (2000 rpm) and the compressor cycling switch cuts the compressor at 22-24 psi like it should, however I'm only getting 50-55* air from the duct still. It will then head upwards of mid 60's duct temp and then turn back on. If I drive long enough it will start to get the duct temps down to the mid 40s but it takes a long while (30-40 minutes on a non heat soaked cab). And all of that is only if I have the rear air on. If I turn the rear air off, the system seems to cut the compressor even earlier meaning that I'm seeing my low side drop even more though and thus my duct temps may only average in the high 50' to low 60's at 2000 rpm. I've recovered and confirmed I do not have a leak in the system.

I can't figure out if I've got something going on with the rear air TXV causing my pressures to be too low when it's not running, like maybe it's hanging open some? Idling with both systems on with the charge at 2.75 isn't great either. If I just run the front only it does a little better. However if I drop the charge to about 2.5lb I can get better idling temps but it makes my cruising problem even worse. I just feel like I'm missing something.

I also tried going to a .072 OT from an .062 and that seemed to help my cruising problem a little but obviously gave up a little more on my idling temps.

Let me know if you think you need any more info, but I know these trucks can do better than this, I just think something is a miss yet.

Also, is there a way to test the TXV and if it's actually closing as far as it should with the rear air off?

[tbirdtbird]

Cabin air filter been changed?
Airflow over condenser reduced? Due to old fluid drive coupling on mechanical fan? All the electric fans on as soon as the comp is switched on?
Run a garden hose over the condenser and report what that does to your vent temps
Suction line sweating?

You seem to have a manifold gauge set, can you give us hi and lo readings at 1500-1800 RPM with max evap fan, max cool setting, recirc on, doors open, and also ambient temp

[JohnHere]

The specifications that I have for your truck with rear A/C call for 64 ounces (4.0 pounds) of R-134a. Presuming that my specs are accurate, and you have only 2.75 pounds (44 ounces) in the system now, then it's undercharged by quite a bit.

Your truck without rear A/C calls for 36 ounces (2.25 pounds) of R-134a according to my specs.

[phillipm]

Yeah I've seen those 4lb specs as well. I can tell you even getting to 3lbs and performance starts suffering quite a bit (especially at idle). I don't know where some of those companies got that from. When I called my local dealership they shared it usually lands between 2.75 and 3lbs depending on which condenser is in the truck. The .625" thick or the 1" thick version of the parallel flow. The GM manual for my truck outlines the following:

Refrigerant R-134a, with C60-0.91 kg (2 lb)
Refrigerant R-134a, Four Door Utility with C69-1.25 kg (2.75 lb)
Refrigerant R-134a, Suburban with C69-1.59 kg (3.5 lb)

NOTE:
C60 = Front Air Conditioning
C69 = Rear Air Conditioning


I mean, I'm with you, it seems undercharged but when I put more in it, it doesn't get better is what I'm saying I guess.

[phillipm]

Cabin air filter been changed?
Airflow over condenser reduced? Due to old fluid drive coupling on mechanical fan? All the electric fans on as soon as the comp is switched on?
Run a garden hose over the condenser and report what that does to your vent temps
Suction line sweating?

You seem to have a manifold gauge set, can you give us hi and lo readings at 1500-1800 RPM with max evap fan, max cool setting, recirc on, doors open, and also ambient temp

Yes I have all the tools you might want to do a/c work. I do this stuff all the time, it's just this vehicle is really killing me. I've never done it with the doors open but I've had the windows down.

1500-1800 for about 10min or so (windows open), high side stabilizes around 275-300 at 95-100*, low side in those conditions is 45-50. Honestly those RPMs it works great. Windows up it would settle in around 26-30psi on the low side with a duct temp of 43*. But when you get on the highway and start running 2-2200 at 70-80mph, the low side is dropping even more and running into the cut off switch. I know this because I jumped it and in that case the compressor stayed on and I was seeing 38* out of the ducts. Was great, but obviously not controllable and would likely start icing up if humidity started rising in my area.

Additionally if I do not run the rear air and I've only got the front going, the low side drops further and helps out my idle temps (I'm assuming the TXV isn't robbing the orifice tube of refrigerant). However, at highway speeds it makes the problem even worse and the compressor is cutting when he duct temps ares till in the mid 50's.

[JohnHere]

Interesting about the refrigerant-amount discrepancies. I don't know where the differing specs originated, but the files I have list the higher amount.

I'm beginning to wonder about the cycling switch being out of whack. The cut-off point looks about right, but the cut-on point looks higher than it should be. The switch in your truck isn't adjustable as pointed out in another recent thread, so the only thing you can do is replace it. Have you tried that?

[phillipm]

Thanks yeah I completely agree thats what it seems like. I swapped it with 2 other gm ones and no change. Ive landed on something is allowing the pressure to drop even though the front system is not fully cold. Thats where i was wondering if anyone has experience with the rear txv hanging open some. Would that cause the pressure to lower on the low side or no? I just dont know enough about OT & TXV in one system to diagnose what’s happening.

[JohnHere]

If you want to troubleshoot further, put back the stock OT and install a rear A/C block-off kit and see how the front A/C performs.