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?

Thanks



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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