85 380SE approx. 125,000 miles converted professionally to R134a by previous owner about 10 years ago. System had never been touched again, never even topped off according to the PO and it was blowing cool when I got it. I topped off approx. 1.5 months ago when the weather was in the high 70s to low 80s, and pressures were all in line with max of approx. 250 to 275 on the high side.

I hooked up the gauges a few minutes ago to see if everything still checks out OK. With engine hot after a 45-min. highway drive, had my wife idle the car at 2,000 RPM in the driveway. Low side reads almost 50, high side reads about 370 (EDIT: I put 375 in the thread title, it is actually 370, but with pressures so high, who's counting?). At idle (650 RPM) the high side drops to approx. 250 and there is a high-frequency oscillation in the gauge needle that smooths out immediately once gas is applied. Ambient temp is approx. 94 degress with 42% humidity (according to both weather.com and the thermometer in my kitchen window).

Am I going to kill this compressor? Or are the pressures just unusually high due to idling in the heat (i.e. way less airflow through condensor versus when driving around). Should I evacuate some R134a out to bring high side back down under 300? I've never left a system with such high pressures before, and want to make sure I don't kill the compressor.

Does the vibrating needle indicate any kind of problem?

Incidentally, the A/C is quite comfortable, but not what I would call "cold" and yes, I am aware of the 80% efficiency of a converted system. It's nowhere near as cold as the R12 system in my 300E. The suction hose does not sweat and is not particularly cold...

Any thoughts? Thanks!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

is the auxillary fan coming on? seeing if it is would be a good place to start. it should come on with pressure that high

Yes, it's definitely on, no problems there.

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

have you tried cleaning the condenser area with the water hose?, may have debris not allowing air to flow

Condenser is new, was replaced just a few years back and is still clean and shiny. A few bugs in the fins, but nothing out of the ordinary.

Again, the question is not "why is the pressure so high" as much as it is "is the high pressure OK." I'm trying to determine if 375 is safe for the system, or should I evacuate some refrigerant out?

I guess I should just come out straight and ask what is the maximum safe (for the compressor) high-side pressure on this car?

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Sounds too high to me and seems like it is flooding the evap....
On a 134 conversion, you should still see bubbles at eye-sight w/cond. fan on . If the sight is clear, it is most likely over-charged