325-350 psi at 2500-3000 RPMS on the A/C high pressure line? What could cause this?
 

The car is a 1987 TD

The Mercedes shop - a very very well respected indy shop here in Northern Va - has had my car for over a week now. I brought it in because the A/C was blowing hot once again. Well the clutch on the compressor was fried. Mercedes warrantied the compressor, they put it on, and filled with R12.

Now this is the 3rd compressor I'm on so they wanted to keep the car and drive it for a while to see why they keep going bad. Today i spoke to them and apparently at idle the pressure on the hide side is fine, right where it is supposed to be. But once the car gets going and the tach hits 2500 RPMs the pressure reads at 325 psi and hits 350 psi at 3000 RPMs.

They seem stuck at this point on how to get the pressure down. I trust these guys completely - they have done great work on the car and are always honest with everything. Just curious if any of you guys have ever experienced this and if so what your solution was (if any). Thanks.

Expansion valve.
 

My money's on a clogged expansion valve. The smallest bit of debris can clog the valve, then the refrigerant has no place to go.

SteveM.

That could be it. Did your indy tell you the low side pressure at 2500 rpm??

Usually high pressure on the high side is air in the system. But it could be the TX valve or even the compressor.

I'll place my bet on air in the condenser.

Danny

The mechanic did not give me the low side pressure at 2500 RPMs. But both of those things sound like possibilities. I know they evacuated the system obviosuly when they replaced the compressor and put the r12 in - you would think that would have removed any debris. But air in the condenser could be it.

I am hesitant to call and suggest a course of action - they typically hate that... what do you guys think?

Depending on temperature and humidity it doesn't sound that far out of spec. I don't have the 124 service manual handy, but it runs higher than you would expect on the high side.

If the expansion valve was clogged you would get reduced cooling due to the restricted refrigerant flow. If it cools OK, doubt you have a problem there.

No such thing as air in the condenser. Any air in the system would circulate, along with the refrigerant. Definately not a good thing having air in the system, but doubt this is an issue.

Sounds like the system is overcharged. What is the low side pressure?

Mechanic believes it is about 50 to 60 PSI too high. pressure should high 200s so I'm told.