1990 300SL A/C Compressor problem - PeachParts Mercedes-Benz Forum

bergmjh · #1 04-17-2011, 10:41 PM

The A/C compressor will not turn on. I am having trouble isolating the problem on an old 129 chassis SL. I do not know how the system is supposed to work? I have a Chilton's wiring diagram that has helped me out but now I am stuck. The A/C compressor gets its power from an engine control module. I know the wiring between the engine control module and the compressor is good because I removed the engine control module and jumped power to the A/C compressor clutch and it works. I think my problem is no ground signal at the push-button assembly. I have a 12V signal that comes from the engine control module through the A/C low pressure cut out switch to the push-button assembly on a blue/red wire. I get no change in the voltage signal when I change the mode the push-button assembly is in. No voltage difference between asking the compressor to run and not to run. I think I can try grounding the wire and get the compressor to run but I do not want to risk damage the engine control module if the push-button assembly is not supposed to provide a ground? Does anyone out there know how this system is supposed to work? Or any other direction I can try to fix this problem?
Thanks John

RadioTek · #2 04-19-2011, 09:16 PM

If you have low coolant, there is a sensor to prevent the compressor from coming on.

bergmjh · #3 05-02-2011, 06:08 PM

I did find the wires switched between the engine coolant temperature sensor and the idle control valve. This resolved the auxiliary fans running all the time problem but not the compressor. Improvement as the compressor now will come on but only for 3 seconds. I tried a new compressor thinking the speed sensor in the back was bad with the same result. I also tried to load test the 3 wires between the MAS relay and the compressor just in case there was a wiring harness problem. No help with that one. Every time I turn on the compressor I do get a clutch slippage code #13 in the computer. Sometimes I also get code #12 clutch open circuit? No clue as to where to turn next? Any thoughts will be appreciated. Thanks John