I actually have 2 problems but maybe they are related.
1. While driving with the AC on, periodically the system switches to full heat out the vents. If I turn it off for about 5 minutes and then turn it back on everything is fine.
2. When I have the AC (or heat) on(the button with both upper and lower arrows) the air comes out the two side vents and two center vents across the front and thru the defrost. Nothing thru the driver/passenger wells. When I push the button for lower arrow only, the air comes out the two side vents and the defrost but nothing in the driver/passenger wells. This doesn't seem to jive with the owners manual.
Does anybody have any ideas on this one?? Thanks.

Steve,

I am not familiar with this particular model, but I suspect that it is very similar to the 124 cars.

The problem is likely to be in the pushbutton control unit. It is very common for these units to be fixed by heating the solder joints. Many times a cold solder joint is the culprit.

Also, the CD sold by merecedesshop will contain the needed troubleshooting information.

If it comes down to replacing the pushbutton unit, make sure you pay attention to the information that comes with it. There will be several checks to make, as outlined in the instructions that come with the unit. Check the Aux. water pump, and the heater control valve to ensure that the new unit is not overloaded causing a followup failure.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

A vacuum leak can cause this...You may have a vacuum leak if the vents are not working properly. Don't overlook the mono-heat valve. It will cause the heat to kick in or not come on at all at random.

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Benzmac:
1981 280GE SWB
1987 16V
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Thanks to Benzmac and Larry. I think I'll start with checking vacuum leaks although that is such a pain under that dash of a W107 body type. Thanks again. I've solved several problems with your insight and knowledge over the last year.