[Bob in St. Louis]

Ok guys, this problem has not only humbled me, but all my gearhead friends too. For about four years now, I've had an A/C issue with my truck, and it's kicked my ass.
The vehicle is a 2002 GMC Yukon XL 5.3 AWD, with front and rear A/C units. 170,000 miles.

Here's the (unfortunately long) story, best as I can remember.

Four summers ago it started with a very loud hammering sound with the A/C on, but only on acceleration. Sounded like fully automatic gun fire. It was thought that the A/C belt tensioner was weak. I replaced the tensioner, and belt. The noise was gone, but the next day the new belt got thrown off. So I thought the compressor was trying to seize up, so I installed a new compressor, belt, and orifice tube (the old tube was very clean).
It was fine for the rest of that season (but it never really got extremely hot again).

Three summers ago, the A/C compressor would shut off, but only when the ambient temperature was above about 90. After jacking with it for some time, I gave up and took it to one of my buddys that works at a Chevy dealer. He determined there was something "weird" going on with the high pressure cut off circuit. This vehicle happens to have two high pressure switches. One on the condenser, one on the back of the compressor. In order to avoid an ugly, long drawn out electrical diagnosis, we decided to bypass the high pressure circuit that goes to the condenser.
It was fine for the rest of that season. (but it never really got extremely hot again).

Now remember, it only acts up when it's over (about) 90.

Two summers ago, the first time it got above 90, the R-134a was blown out of the pop off valve on the compressor. It was then thought, that the expansion valve on the rear A/C unit was blocked and needed to be replaced. At this point, I'm tired of messing with it, so I put one in. The next time it got over 90, all the 134 came out of the pressure release valve.
I recharged it, as summer was about over. It was fine for the rest of that season. (but it never got hot enough that I thought it should have acted up).

For the planets to align enough to get a day over 90, and for me to have time to work on it are uncommon. To date, I've not been able to get a day when it's over 90, I've got time to mess with it, and had access to a gauge set. So I've never been able to see what the high side pressure reading is (when the system is fully charged).
Each and every time I've evacuated the system (after having 'tripped' the PSI release valve), there's only 0.5 lb remaining in it. The capacity is 2.7 lb.
Fortunately, when I can keep moving, keep the RPM up, and keep airflow through the condenser, it can actually maintain a reasonable cabin temp. The vent temp while on the highway can get as low as 50, which is pretty good for a system that's missing 4/5 of it's refrigerant. At an idle on a very hot day, it's useless.

This summer, I removed the high side aluminum line that comes out of the compressor and blew air through it (125psi from my air compressors blow gun). It was clear.
I then removed the aluminum tube from the outlet of the condenser, and blew air though the condenser. It was clear. Then, I blew air though the aluminum line that leads to the orifice tube. It was clear. I replaced the orifice tube even though it was clean. That's the entirety of the high side system, (except for what goes to the rear unit), so I recharged the system to capacity. It worked great until the next time it got hot, then it released the 134 again.

Today, it's 93 outside, so I figured I'd put the gauges on it. Now, at an idle, the vent temp is pretty high. About 80. The compressor is not short cycling. The high and low side at about 40 psi.
When I bring the RPM to about 1500, both sides go to about 25psi, and the vent temp is 55. (how's that even possible?). If I take the RPM much higher, the low PSI switch shuts it down. From past experience, I know there's about a half pound in it. I'd love to get three cans of 134, but to spend $45 just to see what the pressures are, and then to watch it all go up in a white cloud, doesn't thrill me much.

A few random thoughts and comments;
- In the past four years, the system has probably been recharged about a dozen times. I've added some of the BG dye/lube once or twice.
- I've shown the truck to some of the Volvo techs at work, but after recharging in a shaded shop, even on a hot day, the pressures are perfectly normal. It's only after it's sat in very hot temps that it "blows its top". None of them have any clues, nor do any of the Chevy techs I know.
- I've thought about finding what the guy did four years ago to bypass the one high side pressure circuit. THen I'd at least be able to keep the 134 in the system. But what are the chances he'd remember what he did to a truck four years ago? This still might be my best option, but then I'd still have to diagnose what the high side is shutting it off for?
- The high and low side service ports are right next to each other, just a few inches on either side of the orifice tube, right next to the dryer.
- I've thought about blowing compressed air though each component, one by one, on a day when it's over 90 (like today), but I just can't imagine any aluminum component would be affected by ambient temps enough to restrict the flow of 134 enough to make it blow off. Plus, normal underhood temps combined with the normal operating temps of high side components make me think that a few degrees higher ambient wouldn't do anything detrimental to an aluminum tube or condenser.
 
I hate to say it this way, but I'm running out of parts to replace. Any ideas would be greatly appreciated. This has become more than frustrating.
Sure, I could sell it, but I rather enjoy not having a vehicle payment.

Thank you in advance, and thanks for reading all of that. ha ha
Bob

[srb]

Sorry, Bob.  All I can think of is "This is George Washington's original axe.  The head and handle have both been replaced, but essentially it occupies the same space".

I guess we'll see if any AudioCircle members know as much about GM A/C as they do about audio.  Good Luck!

Steve

[Bizarroterl]

After all that I'd give up and take it to a automotive A/C shop and have them fix it (when it's above 90deg).

[thunderbrick]

This sucks!  I don't know nuttin' about A/C, but maybe you could sell the Yukon on AC as "great operating condition", if you know what I mean.   

Just remember, what is posted on AC stays posted on AC, so you can't play dumb(er) later on!   

[JerryM]

It sounds as if the condenser isn't gettting enough cooling, thus the problem only on hot days. Have you checked your fan clutch on hot days? Do you have a dedicated condenser fan (that works)?

[decal]

Perhaps you should consult a professional.

[thunderbrick]

Perhaps you should consult a professional.

   

[JerryM]

Perhaps you should consult a professional.

From Bob's diary notes above, I kinda' felt he had consulted with professonals and was now *digging*. 

The symptoms really are quite classic of an overheating condenser. I'd try to rule this out. 

[Bob in St. Louis]

Thanks to all for posting. 

Jerry, I've checked the fan clutch on hot days, but not on the days when it's "hot enough". You recall the old trick where you stick your towel wrapped fist into the blades of the fan to see if you can slow it down?......
Yea, It's strong alright. 
But again, I've not had a towel handy on the really hot days. It's supposed to be 90-95 everyday for the next ten days. That should give me plenty of time to sweat my ass off test the system further.
I have, by the way, power washed the condenser and made sure the fins aren't bent over.

Decal and Bizarroterl ...... My quarter century in the auto repair business, combined with the vast knowledge of the 12 guys that work in the dealership service department that I oversee, not to mention the knowledge of the mechanic friends I've made over the past 25 years has gotten me to the point where I'm posting on an audio forum for assistance. 

Suffice it to say, that I am the professional. The end of the line.   
Please re-read the first sentence of my first post. Hopefully then you'll understand where I'm at with this problem. There is easily a couple hundred years of combine knowledge that I've "picked brains with", and so far, I'm no better off. 

Bob

[TomS]

Thanks to all for posting. 

Jerry, I've checked the fan clutch on hot days, but not on the days when it's "hot enough". You recall the old trick where you stick your towel wrapped fist into the blades of the fan to see if you can slow it down?......
Yea, It's strong alright. 
But again, I've not had a towel handy on the really hot days. It's supposed to be 90-95 everyday for the next ten days. That should give me plenty of time to sweat my ass off test the system further.
I have, by the way, power washed the condenser and made sure the fins aren't bent over.

Decal and Bizarroterl ...... My quarter century in the auto repair business, combined with the vast knowledge of the 12 guys that work in the dealership service department that I oversee, not to mention the knowledge of the mechanic friends I've made over the past 25 years has gotten me to the point where I'm posting on an audio forum for assistance. 

Suffice it to say, that I am the professional. The end of the line.   
Please re-read the first sentence of my first post. Hopefully then you'll understand where I'm at with this problem. There is easily a couple hundred years of combine knowledge that I've "picked brains with", and so far, I'm no better off. 

Bob

Bob,

You mean GM didn't say "they all do that"? 

[Bob in St. Louis]

No, they didn't. 
I worked at a Chevy dealer for six years and all my (mechanic) friends know that.
So basically, they know they can't blow smoke up my ass. 

[thunderbrick]

No, they didn't. 
I worked at a Chevy dealer for six years and all my (mechanic) friends know that.
So basically, they know they can't blow smoke up my ass. 

Or cold air, apparently.................

[JohnR]

It's the sensor.

[Bob in St. Louis]

 Thanks guys. I knew if I waited long enough, my questions would be answered. 

Bob

[JohnR]

Seriously, what could be that sensitive to temperature?

[youravhandyman]

My wife!  *rimshot*

[Bob in St. Louis]

Seriously, what could be that sensitive to temperature?

Exactly. 
85F and it works perfectly, 90F and higher and it does this. Obviously, something has reached it's threshold, but "what" is the question. It sure acts like a restriction in the high side line, but I can't see a few degrees making a restriction come and go. 

Bob

p.s. My wife takes meds to regulate her temperature. I'll get you the name of the prescription for your wife. 

[JerryM]

...You recall the old trick where you stick your towel wrapped fist into the blades of the fan to see if you can slow it down?......
Yea, It's strong alright. 

I use a sturdy welder's glove.    Also, rather than just shoving a hand in, I have a sober friend shut the at-temperature car off. I grab the fan with the gloved hand and have my buddy turn the car back on. It's way easier to test that way and, if it goes wrong, my sober buddy will know what to do. 

There is a whole lot of stuff to cool up there, and the cooling system's not getting any younger. It seems as if it worked fine for five years. Out of curiosity, what was the warranty period on the cooling system, et al? Does the condenser have a dedicated electric cooling fan?

[Bob in St. Louis]

Nothing electric up there. Regarding the cooling system....Think of 1980's technology.
Just a bigassed belt driven fan with a thermo fan clutch. No wires, nothing electrical.
What's the five years on cooling stuff you're talking about? 

Everything under the hood is 10 years old and has 170,000 miles (except for tune up stuff, a/c compressor, dryer, and orifice tube). The warranty on all that has long since expired.


By the way....It was 95F today. Vent temp at an idle was 90F. Vent temp while on highway was 55F (ideally, vent temp on highway should be 40F).
And that's with one fifth of a charge of refrigerant. That's liveable, except the 90 degrees at idle. 

Bob

[JerryM]

Nothing electric up there. Regarding the cooling system....Think of 1980's technology.
Just a bigassed belt driven fan with a thermo fan clutch. No wires, nothing electrical.
What's the five years on cooling stuff you're talking about? 

Everything under the hood is 10 years old and has 170,000 miles (except for tune up stuff, a/c compressor, dryer, and orifice tube). The warranty on all that has long since expired.


By the way....It was 95F today. Vent temp at an idle was 90F. Vent temp while on highway was 55F (ideally, vent temp on highway should be 40F).
And that's with one fifth of a charge of refrigerant. That's liveable, except the 90 degrees at idle. 

Bob

Given the timeline, and that this has been happening for four years, I was guessing that the warranty period was five years. 

Overheated refrigerant will always vent; that's what the vent is for. As one of the symptoms is refrigerant keeps venting, it makes condenser cooling a primary consideration.

I asked about the dedicated electric fan because it's a good solution. Something like this. Cheap, effective, and easy to add if your wiring harness has the port. Adding a dedicated fan and a full refrigerant recharge should make things right again.

Best of luck,

Jerry