Just confirmed that the blower control is a DC voltage between 1 Volt (low speed) and 6 to 9 Volts (high speed) on the yellow wire from the PBU to the controller module. The control current is almost negligible, less than 1 milliamp, so the controller must be a Darlington pair or an op-amp with a gain of 10,000 to 100,000. With 1 VDC control voltage, the blower runs at low speed, drawing 5 Amps from the battery. The air comes out of the defrost vents and the side dash vents, as I expected. Access to the wiring is at connector X64, located above and behind the power brake booster.

So the homemade REST circuit will work. I want to get out of this thread and let you go back to drag racing. I'll post a new thread in Tech Help or Diesel Discussion (since my circuit will go in my 300D) when I get a little farther along. The engineering is done; if I had a technician, I'd "leave the rest of the work to the technician."

BTW, for folks who don't like the way the BPU controls blower speed, it would be easy to break the yellow wire at X64 and wire in a potentiometer to provide a control voltage between zero and about 6 Volts. Sixto, among others, is interested in this.

Jeremy

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"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