Andrew -- this is on your '91 240, correct? Your temp control is a direct control. Early 240s used a thermostat to regulate the temp. A fluid-filled tube sat in the heater box and sent info to the thermostat valve which would open then close the valve. Sort of like a kitchen oven. Your 240 uses a disk which has openings of increasing size as you adjust the slide control to warmer. That allows more coolant (heatant?) to get to the heater core. Interestingly, these valves are more desireable as they have longer lifespans than the earlier thermostat design in which the thermostat dies after several years. I replaced the thermostat valve on my '81 with a newer disk design this summer, but my son has this car in Vermont this winter and I suspect he has it fully on most of the time. He had it down here in Phila. over Xmas vacation, and I had no trouble getting it to 'throttle down' to lukewarm.

To give some advice, I wonder if your cable has become disconnected. It's a stretch, but you might want to check each end to make certain that it's functioning properly. That's about the best I can do...

Rob