I have also been through the mono valve drill. Two things:

1. Replace with Mercedes. The Mtc does not work, and the common symptom is no heat at engine speed much over idle.

2. Be very gentle unplugging the wire to the solenoid. The pins broke on mine, and I had a hard time finding a plug or solenoid without broken pins. In fact, my woes started with broken connector pins. While dealing with that, I decided to throw in the new mono valve I had (MTC). I finally got a working solenoid from a parts yard with the wires cut so the plug does not have to be unplugged.

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1985 300TD. 378,000 daily driver