I've searched and searched, but can't find this specific problem.

I have an '87 560SEL with 91,000 miles. Last winter it started to act up on putting out heat. With the temp set anywhere from warm to max, it would just blow cold air out the defroster vents. Eventually I figured out that setting the temp to max before I started the car resulted in heat, but even that quit soon thereafter.

So now, once again this winter, no heat (a/c worked great during the summer).

I have a monovalve rebuild kit that I will install as soon as it's warm enough for a coolant flush. Could a defective monovalve cause this? Or is it more, like the ACC unit?

Thanks for any input.