This is probably an easy one for someone...

I've got a problem with the operation of the heater/defroster system. The car is a 1982 242 turbo. I live in a place that relatively cold, so in the morning when I first take off, I usually have the defroster activated (switch pushed in). It works just fine (warm air blows only out the defroster ducts) until I start driving... specifically, until I start accelerating. At this point warm air also blows out the heater ducts. When I back off the accelerator, the warm air quits blowing out the heater ducts (but still blows out the defroster ducts).

I think that this is a turbo-only problem: It appears that this anomaly occurs when the engine speed is increased, which causes the turbine to spin up and create positive manifold pressure. The positive manifold pressure (somehow) allows the warm air to get to the heater ducts.

Since the switching for the defroster/heater system is vacuum-controlled, I am assuming that I probably have a problem with a seal or a diaphram.

Anybody got any experience with this? Any thoughts?

Thanks.