A/C not kicking on
 

On my W124, the a/c wasn't blowing cold the other day so I thought it was low on freon. My mechanic looked at it the next day, it was blowing fine, full of freon, nothing wrong with it. He said it must have just been some glitch that it didn't work the other day.

Today I went somewhere, a/c is not working again. I checked and the compressor is not running when I push the button. It could be the pushbutton control panel but the heat works. Just not the a/c.

What do you think it could be and if you think the control board is messed up, how do you fix it?

1991 300d 247k

If like my 1985, the auto temp control is often quirky. In mine, rotating the temp set wheel to max blue (w/ a click) forces the AC clutch on. It will then only be disabled by the low-pressure switch (at filter-drier) or the anti-ice switch (below glovebox), as least by the push-button box.

But, one other thing can disable it since my year began the funky Klima AC relay box. It also works on engine rpm. If the dash tachometer isn't working, Klima won't turn on the AC clutch. That can happen if it loses the signal from the EGR box (right kick-panel) which happens if it doesn't get power from the over-voltage relay (OVR, right of glovebox). The OVR commonly fails. It also has a fuse on top which could blow. I bypassed the EGR box at the connectors, using a post here w/ photos. Clear as mud?

When it doesn't work, confirm ground and 12V to the AC clutch. Could be a bad clutch or that the clutch isn't getting power.

Good luck!!!

On the 124 the Klima relay sits between the a/c control and the compressor. It looks at the tach feedback from the compressor to make sure the compressor isn’t seized (to save the serpentine belt).

I would highly recommend getting hold of the schematics and WIS, it will take you through a step by step how to figure out what’s wrong.

+1
The electronics on the 124 pay close attention to the compressor. If it is old, getting a replacement BEFORE it fails and leaves a mess inside to clean out is wise.

But if the ac quits, AND you are getting rough idle, and odd lights on the dash, and the tach stops working, try a new OVP relay or clean the fuse in it first.

The tachometer works, but....
 

I had my mechanic look at it. He thinks it's the klima kickdown relay behind the battery. I will ask him if he looked at the OVP to see if he tested that. Being that my tach works, could the OVP still be the problem?

No, the ovp has specific symptoms when it fails. I suppose it is possible the relay could have a weak contact on one port that causes the ac, but not he tach etc to drop out. The klima is likely.

Since your dash tachometer works, the Klima box should be happy with that signal. Another thing which might disable the AC clutch is the kickdown switch under the accelerator pedal. If that switch stuck on, Klima would think you are accelerating hard so disable AC. Would likely also make the transmission stay in gears to higher rpms.

But test points in the wiring diagram to find which culprit in disabling the AC clutch. Some might be on the low-side (pressure switch?, forget). It isn't necessarily the Klima box. You could also remove it and actuate the clutch pin there with a jumper in the base.

Are you guys talking about the Klima relay?
 

I just got a new replacement part, 001 545 80 05 it says Klima Kickdown on it. Got a refurb one. Did not fix the problem. Still no a/c.

If you have a known good OVP to test with, try that. If you don't but have a soldering iron, try reflowing the main pins to the board on the OVP. Worth ruling out a bad connection there as you have already eliminated the Klima as a culprit.

Good luck