I have done a lot of reading, but there are a lot of AC problems and I have not hit the exact one I am having. I do know I have some vacuum problems, because the 2 center vents are not opening.

I don't think vacuum would cause the problem I am having. First off I had a friend recharge my system and no cool air was coming out. He noticed that the compressor was unplugged. Well we plugged it back in and then we started getting cool air out of the outside vents. After a while though the compressor made a heck of a noise (released gas). What we noticed is that the compressor was on all the time it would not shut off until we unplugged it which is probably why it was unplugged to begin with.

What would cause the compressor to be on all the time? Where should I start the trouble shooting process?

Your compressor gets the signal to turn on from the temperature sensor in the evaporator. This sensor has a probe in the evaporator and it tells the compressor I need more freon. If the unit is telling the compressor to turn on. The evaporator is not getting cold, probably because the expansion valve is blocked. that would indicate a contaminated system. The small orafice that produces the colling effect with the freon is blocked. This was probably caused by the compressor going bad. The "sound" you heard was not a good sound.
I would say your system is contaminated due to a bad compressor. This would mean looking at replacing components, flushing out ALL the trash and reservicing with freon.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

MicroRacer,

Post your model and year on your profile so we don't have to look it up (or indicate mod & yr on the thread).

Compressor sounds shot. Its probably shot anyway after 21 years. Ditto to what Dave said.

Before you let all the gas out, leak test the entire system. Your system will have leaks at this age. You are looking at a fair amount of work here. Not hard work, but more of it.

(as compared to replacing the oil cooler lines AND both half shafts, I'd rather do the full a/c job, its a much less dirty job)

The vacuum diaphram that controls the center vent is probably broken. When it breaks, it breaks so that the vent stays closed. The diaphram may still hold vacuum, but it can't open the flap for the vent. (i'm a little fuzzy on this point)

You can reach the "flap" after removing the glove compartment and the rubber boot. Remove rubber boot. Create a double V sort of shaped thing from a coat hanger and jam that next to the flap to keep the flap open. I found that the V works best when placed between the flap and the rear of the vent housing. This is the quick fix.

The correct fix is replacing the diaphram which is about 2 or 3 hours and lots of skinned knuckles. A 10 year old with small hands helps I'm told. I've heard you don't need to pull the dash to replace the center vent on a 123 but that may be up for debate. I find the quick fix adequate, I close the vents manually when heat is needed.

Eric

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

Not to pirate the thread you started, but I have a related question to all involved:

My compressor locked up on me. Techs couldn't find any obvious leaks around compressors or hoses and there was zero freon (r12). Assumption is that the freon slowly may have leaked out to the point that compressor had no lubrication and died. I've owned the '84 300D for a year and don't know when r12 was last re-charged.

Techs cannot test for leaks if there is no freon or working compressor.

Anyone have an idea if the scenerio above could hold true and I could have A/C up and running by replacing compressor and just recharging? Thought is that the system may not have been contaminated without the freon.

Thoughts?

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Whoever dies with the most toys......Wins!

1984 300d
165k miles

NO NO NO NO, you have seized your compressor and have crap all inside the system. Search for the words "larry" and "flush" hehe. (couldn't help myself)

You too have fair amount of work to do.

20 yo car continues to leak R12 (AND LUBE OIL!!!). Compressor finally gives up because dumb techs only added R12 to top it off. System eventually has no oil in it, compressor overheats and seizes.

RIP R4

NEW tech adds a little R134 to leak test only. Finds leaks, evacuates and then the hard work starts (and the $$).

Later

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

Advice taken, my profile has been updated . I'll also look up those theads labeled Larry and flush.

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83 300D
84 190D

Actually you need to use " flush" on the left box of the search and check to see how Larry Bible spells his screen name for the right box... you are not looking for " larry" inside the posts.. but for posts BY him.... I assume.... LOL