W124 Climate Control Issues
 

Greetings,

I just got a '93 300E 2.8 for a song. It's in great shape (just a couple of minor fixes) except for the climate control. I get no response when pushing any of the buttons or turning the temp control. The blower come on and the compressor doesn't kick in.
1. How do I begin troubleshooting the system?
2. Where can I find a FSM?
3. Barring that, is there any service manual available?

This is my first Mercedes and I could use any help at all.

Bob

Start out by checking fuses, sometimes they are blown and sometimes they are not making good contact. Also there is a separate strip fuse in a black plastic holder bolted to the driver's side strut tower top.

Welcome,
Since the BLOWER works, most likely issue isn't a fuse.

The compressor is controlled by a compressor (KLIMA) relay.
That relay gets signals from a/c pushbutton & sends command signal to the klima relay.
That relay gets RPM signal & refrigerant fill message & then turns on the compressor.

How good are you volt meter skills?

There is a detailed troubleshooting flowchart in the climate manual. It specifies use of a special tool for checking the climate system. The tests can be run using a simple DVM, if you are willing and able to simultaneously read the wiring diagram. It's a lot of work. :D

You haven't specified whether you are trying to heat or cool the cabin - it makes a difference. Since you're using USA model nomenclature, I'll assume the former. Try the simple test in the following paragraph.

A simple place to start - try unplugging the auxiliary electric water pump. It is located (I think...varies a bit on different 124 models) in front of the battery, with the monovalve. There is a connector on the pigtail that can be undone. The pump tends to short out after a dozen or so years, which causes the climate control pushbutton unit to go into a protection mode that more or less is what you've described.

I'm not sure since the post says he gets no response from pushing buttons, but also says "the blower work"? Figured I'd cover the strip fuse also.