No POWER to AC Compressor..Questions
 

I am working on my daughters '85 300D. I can't get ANY power to the AC compressor. I have NO POWER to the plug that goes to the compressor clutch.
I will tell you what I DO have.
1) System is charged.
2) Fuse is good, have power to blower and it is working. AC clutch on same fuse.
3) I have power to BOTH sides of LOW pressure switch on drier.
4) Compressor WORKS if I hook it up to EXTERNAL power source.
5) I exchanged the climate control push button assy with a good one.
6) Have power to both sides of freeze switch.

Is there a load relay somewhere that I can check or is that in the climate control switches?

That car has "Klima" relay under black cover just in front of fuse box...DOES the tachometer work?? If not then that is why the a/c doesn't work as well...

M.B Doc, Yes as a matter of fact the tach stopped working about 3 weeks ago. The AC has been down 1.5 yrs as I thought I had a bad hose because I couldn't get a vacuum. I must have had something hooked up wrong(guages) as It pulled a good vacuum and held a charge.

SO What is my next step? Does this Klima relay control Both the tach and the AC compressor? Seems like "Apples and Oranges "if so. Thanks for any help, Chris

The Klima relay need the engine RPM signal to turn the compressor ON..

Starting point would be the overvoltage relay. it is inside the car by the outer A/C vent on the passenger side..removal of the under dash panel is required on that side to access that relay.

M.B DOC, I did check the overload relay in that the fuse was good. Is this relay only on later 123's? I have an '83 parts car and colud exchange if it has one.
So is the overload relay for the tach and the klima relay for the ac? I have GOOD electrical experience.... the problem is the late factory manuals don't have wiring diagrams. Thanks for your expertise and patience!! Chris

I don't have this model of MB, but I think my low pressure switch works with "on" (that is, sufficient pressure to run the A/C and turn on the fan) = GND. Are you certain of the way your switch operates?

Maybe you are thinking of the Aux. Fan in front of the radiator. I measured 12volts at the LOW PRESSURE switch...Before the switch on one lead and then through the switch once I had the system charged.

Found the problem to NO voltage at AC comperssor.

M.B. DOC suggested I look at the overload protection relay that is mounted under the glovebox behind the passenger kick panel. He also asked if the TACHOMETER had quit and it had. The fuse was good but the relay wouldn't close like it should with the key on. I took apart the relay and "mechanically" closed the relay(with a small piece of injector return line hose)and WA-LA the TACHOMETER started working AND the COMPRESSOR engaged!! THANKS M.B.DOC and everyone else that helped!! Chris

After screwing around with the Klima on my 126 3.5L diesel for a few weeks a few years ago ... turned out it was the OVP which had fallen apart inside the relay case ... after reading for days on OVP and Klima, I think Klimas get a bad rap - statistically, at least.