Where is this module located on a 300SD?
Is it a relay?

I am still having trouble with my heat.

I only get heat if I unplug the AC compressor under the car.
With it plugged in I get hot air through the defrost and cold air through the vents.

Monovalve & CCU appear OK.
With ACC OFF and key ON I get 11.65V & good ground at monovalve plug.
Using Jeremy's trick of applying 12V to monovalve directly
I can hear the coil activating inside.

Thanks,
Jeff

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1984 300SD 300,000 miles--two tank WVO setup
2.88 diff & 500SEL anti-squat rear end