Today the blower motor in our '85 300D worked intermittently, independent of anything I could do with the push button controls. Reading the long series of discussions about the blower and its controls gives me some ideas, but a diagram of the blower control circuit would be really helpful. For example, I read, or think I read, that the monovalve is involved and that there is a sensor somewhere in the coolant. Hum ... how many parts are there in the control circuit? It's an California model of 300D, by the way.

Thanks,

Phil

The blower motor in your car is controlled by the pushbutton unit in the dash, an electronic switching unit under the dash, and a resistor box under the hood, on the right inner fender. The pushbutton unit and the fan motor are the two parts that are most likely to be at fault.

The motor brushes and/or commutator wear over the years and gradually make poorer and poorer contact. Symptoms you report are classic. Sometimes the fan will start (or run) only in high speed; sometimes kicking the fan helps it start. Sometimes you can replace just the brushes. I had to replace the motor in my '85 as the commutator was badly worn; I hunted for awhile to find just the motor as the motor plus fan is expensive (not that the motor was cheap). Sometimes you can find a working fan in a junkyard car – they are easy to remove (3 screws, 1 plug).

The pushbutton unit occasionally fails (fried pc board traces) due to a short circuit, especially in the auxiliary water pump. That failure sometimes also affects the fan. You can repair the fried traces yourself (I did mine) or buy a rebuilt unit – not cheap.

Jeremy

__________________

"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970