I often tow cars on long trips with the a/c on. I noticed that when the engine would kick down to the lower gear to climb a hill or pass, there would be a lot of vibration while the compressor was on, and when the system cycled off, the vibration went away. Finally about a year ago on a long trip, my a/c died, the compressor would no longer engage. When I got home and checked it out I found that the metal pipe/hose that bolts onto the compressor, and runs to the dryer, had a big crack in it right where the pipe runs into the flange/base that bolts to the compressor.

I chalked it up to maybe me being too rough on it during the engine swap and ordered a new a/c line, and also put a new compressor on just in case the old one was going bad and causing the vibration at higher rpm.

Once refilled the system worked perfectly but the vibration at higher rpm with the compressor on, was still there. I figured that was how it was supposed to be. The vibration feels similar to a really bad u-joint in a driveshaft, but only happens with the a/c system engaged above about 3500rpm. There does not seem to be an RPM based, or WOT based a/c system cutout on this truck.

Yesterday I got back from another long trip and about 30 minutes from home the a/c died. Same thing, the metal pipe (less than 9 months old new from GM) has a crack at the base of where the pipe meets the flange, that bolts onto the compressor itself. IT is not being pressured by anything around it, it basically sits in the air by itself.

The only thing I can figure is that for some reason there is vibration at higher rpm with the a/c on, and it is causing this metal a/c pipe to vibrate and eventually crack and let all the freon out. Kinda like an oil dipstick tube will vibrate at the top if it is not bolted to the head with a bracket.

Has anyone else had such an issue? Mine is an 02 denali with the rear a/c.

I think I am going to see if the pipe can be welded, and maybe strengthened that way. If not I will have to spend about another $175 for a new hose from GM.

I am thinking of making an additional support bracket from the front of the passenger head, to bolt onto this metal line and keep it more stable hopefully.

I have also thought about ordering an MSD rpm switch and putting a 3500rpm pill in it, to cut off the a/c system and prevent damage. In my mind this is something that should already be in place...does anybody know if this is something that can be programmed in the PCM?

 

nobody?

 

Is it possible that you have a loose or broken motor mount causing the engine to torque too much and stretch that line? May be the source of your vibration...

 

the vibration is not present at any rpm with the a/c off. The motor mounts looked normal enough to me last time I had the engine out.

I was able to get the cracked line welded up and reinstalled it along with a brace leading from it to the passenger head, maybe this will improve things.

I still wonder if the PCM can be programmed to cut the a/c off above a certain rpm or throttle %. Anyone know?

 

I think that's possible....doesn't the compressor shut off at WOT with the stock settings?

 

mine doesnt seem to...or perhaps it does at WOT, but not at 80-90% throttle. The vibration only seems to occur from about 3500rpm up with the compressor engaged, regardless of throttle position.

 

I checked a stock tune on a 2002 Silverado, and the A/C disable rpm is 5250, re-enable is 5000. Disable throttle position is 100%, re-enable TPS is 0%. You can have your tuner program it any way you want.
As far as the vibration goes, it sounds like the compressor might be pumping liquid refrigerant, which means it might be way over charged. Another possibility is mechanical damage inside the compressor, like a broken piston. This might allow it to pump oil. Over charging can cause the mechanical damage, and similar damage, maybe from a different cause, could make a less experienced mechanic over charge it.
Regardless, the pulses from pumping liquid are plenty powerful to crack a pipe.
Maybe a different mechanic, or further diagnosis would determine if the compressor is bad, or something else in the system bad.

 

Well, this behavior occurred when I got the truck with the stock compressor and a/c charge in place (67k miles, probably never serviced or messed with prior to that).

Then the first time the pipe cracked last year, I replaced both the pipe and the compressor, and recharged it myself by pulling -28 vacuum for 1 hour then adding the prescribed 2.7-3lb of 134a/charge oil for rear a/c equipped trucks. It ran that way until last week when the pipe cracked again.

I've since had the pipe welded and reinforced, added a stabilizer bracket to the pipe to hopefully keep it from cracking, assuming the cracking is a result of vibration forces. The crack is across the pipe right at the joint where the flange mates to it, where it bolts onto the compressor. If you imagine your shin as the pipe, your foot as the flange, and the floor as the compressor, the crack occured at your ankle, cross-ways and not up and down. So in my mind this means it was from movement stresses and not from internal pressure, because if that were the case it would be an up and down crack.

After I repaired the pipe, again I charged the system same as before using a 3oz can of charge oil and 3.5 12oz cans of 134a (which seems to be right in the 2.7-3lb range recommended by the manual for rear a/c models) and it works normally again, but the vibration above 3500 is still present.

It should be noted that a/c system pressure with the system running at idle is in the normal 50-55psi range with the full charge stated above, so I dont think I have overcharged it. Besides, would not a seal in the compressor or an o-ring at a pipe joint somewhere give out well before a metal pipe, if overcharging/high pressures were to blame?

thank you for the information on the tuning aspect. I have already had my pcm tuned by wheatley, but the next time the truck has downtime or something I will have this part of the tune corrected. I'd like to have the compressor disengage at 75% throttle input or higher, OR 3500rpm or higher, because in my mind anytime you reach or exceed either of those values, the a/c compressor has no business being on because you obviously need more power right then...plus higher rpm cannot be good for the compressor, even if it is 100% in working order.

 

Refrigeration is a weird animal. Overcharging doesn't necessarily mean higher pressures. It just means that there is more refrigerant in the liquid state than the system was designed for. There is still a chance that the conmpressor is damaged.