Earlier in the week I posted a question about my continuing problem in diagnosing a a/c compressor that suddenly won't engage. To summarize quickly: I removed the Klima relay, opened it and cleaned it with circuit board cleaner and the relay contacts as well. Re-installed it, still nothing. I removed it again and jumped #5&7 on the receptacle in the car and the clutch engaged fine and a/c worked.

So, my question was how to diagnose whether this was a Klima relay problem or something else. Basically, how do you tell if the Klima is bad. M.B. DOC, one of our Moderators was kind enough to post a reply to help me. Someone had told me to check the "105/115" sensor. M.B. DOC explained my 1988 300SE doesn't have this "sensor". Here is what he said,

"To determine whether the problem is inside OR under hood, you must see if the pin number 10 on the "klima" relay gets a GROUND command signal from the pushbutton assembly. WITH the car running measure for battery voltage between pins # 5(power) & pin # 10, IF you have battery voltage then the problem is in the "klima" relay.
Your car doesn't have a 105/115 switch like the 1987 & earlier cars have. Your car has a blue 2 pin BLUE sensor that talks to the pushbutton & if the engine is too hot(or bad switch) the aux fans run & the GROUND command signal to pin 10 is shut off!!!"

Here's what I did: I started the car with the Klima removed. With the climate control system OFF I took a 12 volt test light and put the positive to #5 and the neg to #10. Result= no light.

I then turned on the climate control system to full minimum temp and pushed the button for the system to activate. Hooked pos back to #5 and neg to #10. Result= no light.

So, my first question is did I test what MB DOC was telling me to or was he telling me to use one of those hand held electronic analyzers that I would connect to #10 to test for a ground?

My second question is, if I did test it right and no light means the Klima is OK, what is the problem? Is it the switch in the car? I'd find this hard to believe as all three buttons that would turn the compressor on failed to make the light go on while testing (defrost button and the 2 buttons with the up & down arrows on them). It seems odd all 3 buttons would fail at the same time.

Other than this the car runs great. I should mention that many people have said a bad Klima results in poor tach gauge performance. I have not had this symptom at all. The a/c simply worked fine for the first week I owned the car and literally overnight, the clutch won't engage.

Thanks,

Ron B.

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Thanks,
Ron Brooks