Automatic climate control behavior
 

My 85 300d's Automatic Climate Control is on the fritz. Before I started the last round of AC work, I had everything functioning correctly except for the actual AC not getting cold. I've got a factory rebuilt HVAC control module/switch panel, and a good sensor, but it's still blowing hot air all the time - even in max cool. I realize that might be mono-valve, but it's been rebuilt and was working before I did the AC work. The automatic regulation is behaving strangely (fan speed is not being controlled accurately and doesn't really seem to respond to the temperature wheel at all in a meaningful way). The auxiliary water pump works, though I pulled the 2amp inline fuse I added to it last night when I was testing some stuff with the ignition set to run. I was also filling the coolant at the same time and hadn't run the engine yet, so it sounded like it was trying to push air through.

When we were pulling the evap housing out, my friend accidentally dropped the bracket with both the electronic regulators attached that normally lives above the glove box. Does anyone know the functions of those two boxes? I don't have them handy, but I believe one is temperature regulation and one is fan speed - does the fan speed unit actually regulate the automatic fan speed feedback circuit, or does it just act as a series of relays for the temperature regulator unit and/or control unit? I have spares for both units, so this is more out of curiosity, since I'll just swap both units.

Also - any recommendations for grease for the blower motor's bearings? That's one original part of my system that works well, but I did notice driving today with the kick panel off that it's squeaking a bit. I might as well try to fix that and save headaches later on.

Check out this tech article from our site on replacing the climate control; it'll defintely help you troubleshoot and hopefully resolve your issue. If you have any other questions, feel free to post a Comment just below the article and someone will get back to you with more detailed info. Best of luck! :)

Pelican Technical Article - Mercedes-Benz W123 - Climate Control Replacement


-Dmitry

Your version of the W123 may be similar to the W126. Startek is having some issues at the moment so I was unable to look at the W123 diagrams.

The Blower Speed Controller controls all speeds for the blower, including Hi-Low-Auto. It is also on its own fuse. The Pushbutton Controller is mainly for switching.

The same is true for the Temperature Controller. The Pushbutton Controller sets the mode and supplies the rheostat for variation. Everything else is controlled by the Temperature Controller.

Maybe you will have better success at opening the W123 diagrams than I did.
http://www.peachparts.com/shopforum/general-information/248948-mercedes-service-manual-source-cd-dvd-paper.html

A couple of things to check; make sure you have a hose in place horizontally to the left of the glove box for drawing an air sample from the little grill on top of dash, and pinch off the heater hose near the mono valve and see if you get cold air. If you do, then the mono valve is either flooded or getting a signal to open.

Hope this helps.