I can't remember exactly, but I did have a problem with my heater on an 83 380sel and it was the monovalve. As I recall, my symptoms were excessive heat (my blower just worked hard to try and cool, but it just blew really hot air). I removed, inspected, and reinstalled the valve and the problem went away for awhile. When the problem returned sometime later I disassembled the valve and found that it did not move freely when power was applied. I cleaned it to ensure that it moved freely, reassembled and never had another problem.

__________________
07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126