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Old 06-07-2000, 11:07 PM
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Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,315
Today I launched my effort to fix the AC on my "new" 300E. The system has been dead for some time.

A very competent mechanic put a pound of freon in the system along with dye.

1. Compressor clutch would not engage. No voltage on the blue wire. Jumped the low pressure switch. No help.

2. Auxiliary fan works if power applied directly.

3. Applying 12V to the compressor got the clutch to engage, but a horrendous noise started and smoke quickly appeared. Conclusion: compressor is siezed.

4. While making these unpleasant discoveries, approximately half a pound of freon leaked out. We checked with the black light very carefully, including putting the car up on a lift. No sign of leak.

Conclusion: expansion valve or evaporator leakd

My plan at this point is to check the expansion valve and the rest of the plumbing behind the firewall. If the leak is not there, I will probably go without AC because the only thing left is the evaporator and replacing it is a huge amount of work.

Assuming that the leak is not in the evaporator, I will go ahead and replace the compressor, drier bottle (and switches), and expansion valve, and convert to 134a.

My question:

Can anyone suggest a likely reason for the lack of power to the compressor, and a troubleshooting sequence? Or do I start from the control unit and work my way forward on the schematic?. For some reason the AC control unit arouses my suspicions.

Other ideas?


Chuck Taylor
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,
4 250C's
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Old 06-07-2000, 11:54 PM
J.HIDALGO's Avatar
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Join Date: Jul 1999
Location: Jax, FL
Posts: 1,785
I am a DIY and just finish replacing the evaporator myself with the help of the MB manuals. My recommendation is to buy the MB CD at this site, if you are to trouble shoot yourself.
I found the evaporator leak with the dye but, it took some driving to finally show up at the drainage hoses on each side of the transmission, the compressor front seal was also leaking (probably for working overtime with low freon). Since your compressor is not working I do not know if it will show up, may be somebody else can help you with that. I posted some of my experiences on this site, do a search. The evaporator (copper core is recommended)is relatively cheap, the labor is expensive and hard. I kept R-12 and it is working fine, so far.
Good luck!
86 300E
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Old 06-08-2000, 07:36 AM
Posts: n/a

There are several things that could be preventing the compressor from being actuated by the climate control amp, including sensors, cutout control unit, or the climate control unit itself. I agree with the previous suggestion in this case, get the CD and work through the control problem. Hopefully you won't have to replace the evaporator, good luck in finding the leak.

I thank my lucky stars every time I get in my 300E and the A/C works one more time. I have a nightmare that the same problems that you are dealing with will show up on mine.

I will be in Stuttgart Sunday and Monday. While I'm at the MB Museum, I'll see if I can find the alter where I can offer a prayer to the Benz gods for you.

Best of luck with it,

Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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