The '79 300SD does not have a monovalve, that started in '81. Check the auxiliary heating water pump, which is next to the climate control servo. The servo is mounted near the passenger side firewall. The water pump has two heater hoses going into the servo(the thing with all the vacuum lines going into it). Unhook the two wire connection from the aux. water pump and hook it up to the battery directly to see if it is working. If it is not working it will not pump hot coolant through the heating system. With the car cool and the pump hooked up directly to the battery, undo the reservoir lid, start the car,run the defrost, and see if the coolant level drops. This could also be the problem, no coolant in heating system. The pump is around 90-100 dollars, and I do not think it is repairable. If it is I hope someone will discuss that.

[Edited by patsy on 12-27-2000 at 11:15 AM]