Compress: Yes, I may be reinventing the wheel. You've been a member here a lot longer than I have. I think I did a thorough search before I started this thread but don't remember.

Zebellis: I don't know why I said that; you could be right.

Sixto: Yes, you could run the PBU's speed control output (which varies between 0 and about 6 Volts) through a unitary op-amp, so the output is exactly the same as the input. That would allow the PBU's 'automatic' speed control to function normally.

The potentiometer (whether Mercedes radio fader or something else) could then be used to adjust the control of the blower speed controller up or down, depending on your wish to have the blower speed higher or lower than the speed PBU has chosen. If PBU changed its output voltage, that would be passed through the circuit and proportionally increase or decrease the blower speed. If you didn't like PBU's choice, you could dial it up or down as you wished. The circuit would need to have an upper limit for blower current of no more than 28 Amps.

As far as circuit design goes, that is pushing against the limit of my abilities and I would have to ask the advice of others. But it is possible.

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