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Hi All:

I am onto the next maintenance project on my wife's 81 380SL and would appreciate some advice and DIY information. The AC on her car does not work although when the AC is switched on, I can hear an audible click coming from the compressor (or a relay related to the compressor). When I checked the low side fitting, there is no pressure releasing from the valve. When I inspected the high side fitting with the car running, it has plenty of pressure when depressed slightly, although I did not have my gauge handy to check the level.

The system is the original R-12 set up and there is no evidence it was converted to R-134. I purchased several cans of R-12 from local sources and would like to get the system working again, but before I just start trying to add a can or two, I did some research on pressure levels of the low and high side hoses and came up short. I read a post where a member had a clogged part of the system and they replaced a metal filter. Another post had information on the expansion valve being the culprit but to access it required tearing the dash apart.

If I am having absolutely no pressure on the low side and plenty of pressure on the high side, is it as simple as charging the system to "equalize" the pressure and get things flowing again? Or is there something I need to troubleshoot before I hook up my gauges and attempt to add a can or two of this hard to get R12? I have not a clue why there is so much pressure on the high side if the system is supposedly depleted. Ie: does the high pressure indicate a sound system with no leaks, but then where did all the original R12 go? Once I tap a new can I have no idea how long I can safely keep it hooked up to my hoses/gauges without the fear of it leaking out.

I have no idea if there is a leak at the compressor or somewhere in the system since I am not the original owner and have no idea at what point the ac became non-functional.

Any advice would be awesome.

Ted :wink
 

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On my car the pressures equalize after a few hours with the system shut down. I had a look at the refrigerant circuit but I don't know if it is the same on your car (8 years newer than mine). If there is a blockage in the condenser, receiver dryer, expansion valve or evaporator, it could create the symptoms you have.

My expertise is limited on this system having only worked on one AC system.
 

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"If I am having absolutely no pressure on the low side and plenty of pressure on the high side, is it as simple as charging the system to "equalize" the pressure and get things flowing again?"

I agree with cushjbc, there should be equal pressure on both sides with comp not being on for awhile. Yours sounds like something is blocking the flow. I don't know of an easy way to tell what it is without removing components. If you know someone with a freon recovery machine you won't loose r12 to disassemble the system parts. I am not an ac expert, just relating what I have seen on my 4 mbs.
 

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when the AC is switched on, I can hear an audible click coming from the compressor (or a relay related to the compressor). When I checked the low side fitting, there is no pressure releasing from the valve. When I inspected the high side fitting with the car running, it has plenty of pressure when depressed slightly, although I did not have my gauge handy to check the level. If I am having absolutely no pressure on the low side and plenty of pressure on the high side...
The click you hear may be the magnetic clutch engaging on the compressor. Take a flashlight and watch the compressor when it clicks. You should see the engagement as the stationary studs forward of the belt will seem to disappear (as they are now spinning so fast). If not you have a clutch issue or a low pressure safety switch issue.

Not sure I agree with your method for diagnosis. Pressure gauges will tell how much low side or high side pressure there is... popping the schrader valve or "depressing slightly" as you say and not hearing or feeling any freon escaping is not a proper measurement, and is harmful to the environment. In short, there is reasonable uncertainty about your statement that you have absolutely no pressure on the low side.

I uploaded a video showing charging an empty system with R12. Let me know if you can't find it here.
 
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