Have a friend changing out his evaporator in his '91 124 (yup, another!) but before he tackled it his HVAC would not blow. The pushbutton unit has been rebuilt and it runs if you jumper the blower directly, but it will NOT run via pushbuttons. Where would the next logical place to look in this case?
Should have the dash back in by the weekend.

Thanks for the help....
Tobias MB
C280 Sport, 300CE, 400E, 190/5.0

The only thing the pushbutton unit does is send a reference signal to the blower regulator. The regulator is under the blower motor and would be easy to get to with the heater box out.

The signal is 1-8volts on the yellow wire.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

Donnie, thanks...there was voltage on the yellow wire so the regulator is defective.
This design is also used on Alfa 164 sedans and if the blower motor needs replacing it usually takes out the regulator with it. It probably overheats as the large heat sink attached has no air passing over it to cool
the regulator. Wonder how many other cars use that regulator as they are not rebuildable. List is over $210!

Tobias MB