Not a new topic at all but I could use some brainstorming.

Just returned from a trip to Charleston SC, a total of 1250 miles. Drove the 1983 300CD. Car ran great as expected but ... froze our arses off on the Virginia to SC border leg.

For some time I had been having the common heater issues at highway speeds. Since I was using a cheap aftermarket monovalve solenoid, I fell for the urban legend about them "blowing closed" at high speed and just before leaving bought a high buck OE solenoid for about $130.00. After installing it there was absolutely no change in operation.

The system regulates perfectly at around-town velocities so I do not suspect the control pushbutton assembly or hose blockages. I tried pulling the connector completely off of the monovalve solenoid expecting it to default to full heat. It did not. There was no change, that is heat at low speed and no heat at high speed. The thermostat appears to be regulating the coolant temperature o.k., in the 82 to 84 degree range as expected this time of year..

I know there are a number of you out there with similar issues. Any discoveries to share?