Scott:

Several possibilities:

Monovalve (behind the battery, I think). Usual failure, however, is to refuse to close, causes too much heat. Can also stick shut. Easy to check, just unplug it, see what happens, then apply battery voltage (carefully!) to the pins on the valve. Should be closed with no voltage and open with applied voltage (someone tell me if I have that backwards, please!). You can also remove it -- four screws that hold the cover on, pull cover off and remove valve. Torn or badly bulged bellows will cause problems here, so will having the armature stuck in the solenoid winding in the cover. Easy to replace, just make sure you get the screws back into the original thread in the plastic housing.

Second possibility is that the temp control is bad, and you don't have good control. Bad hose to the temp sensor is possible here, too, as hot air from the vents will get drawn from under the dash directly, fooling the temp controller.

Third possibility is poor heater core circulation from a bad aux pump (also by the battery, you should feel it running when head is called for), bad engine water pump, or plugged heater core.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!