77 300D

Should I hear a hissing noise under the passenger dash when I engage the AC compressor? Is this a leak or does the expansion valve make this noise?

Well. If you ask me, I would say that it is the valves that open and close your vents. They are vacuum. Also behind your switch box for the climate control unit, there are a group of switches that run on electricity and vacuum and when activated they open and close, depending which button you hit. in turn they open and close little pink, crossover valves behind your dash. If one of those pink cross over valves leak, there would be your hiss. Just take off the bottom part of the dash, by your feet and try to find the hiss. If its the one behind the dash....that is a pain in the ass to change.

Yeah I'll have to go through all those, But it only hisses if I jump the Compressor Clutch to force compressor to run. I hear the hissing even when I run no vacuum to the AC Controls so I image it is the AC plumbing somewhere. I just wondered if the noise is normal or indicated the problem with the expasion valve or R12 leak.

if only when it is running, it is NORMAL.
the hiss you are hearing is the high pressure liquid refrigerant being forced throught the expantion valve. be glad the A/C is working.
John

It's got no cold air coming out the vents, and if I depress one of the valves on the compressor it does spit out a little R12, but not under very much pressure. I just got my R12 Certificate and ordered some cans to recharge. What are the chances of it just leaking back out? Hate to waste the stuff

so if you are putting some in, either there is a leak, or the coils are so dirty that the boiloff efficecincy is compromised.
on a old semi-hermetic system, Leaks are common.
expect to refill often. you can always remove all freon, charge with nitrogen, find all leaks (HA HA HA) and seal them up. if it is holding a charge at all through the winter, the likely hood of finding leaks are minor.
sorry to hear the system is not working well.
John

Isnt the 97 filled with R134? and not R12? I had the hissing pretty loud and the drivers side was warmer than the passenger. I just had it recharged and now both sides are cold. I was told it has a leak at the compressor and is $1500+ to replace compressor, receiver drier and recharge system. Hopefully this recharge will last through the summer

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD

I'm refering to the 77 300D I have.
BTW you can recharge your 97 system with with cans of 134a easily yourself I put a can in last year she's ice cold. No need to have a shop do that. Now if I can get the 77 to work the same way, I'll be taking the car back from the Nanny and driving it this summer.....

Ooops missed that you were referring to the 77. Thanks for the info on the r134. If it starts getting warm again I will try that. Cans are cheaper than the compressor replacement.

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD