Hot air blowing out of floor vents on 1990 GM when ATC is set to "Off"

[SomeoneWeird]

Hi, everyone. With the gracious help of folks on here, last November I replaced the heater core in my 1990 Grand Marquis and repaired the blower resistor.

These past few weeks, I've been having an odd issue. When the ATC is set to "Off", after I've been driving for a while, hot air starts to blow out of the floor vents at max intensity.

When I set the ATC to Norm AC, Max AC, or any other option (other than Floor itself, of course), the floor vent shuts off and the intensity of the blower is dependent on whether I have the setting on low, high, or anything in-between. But when it's set to "Off" -- full blower speed out of the floor vent.

This doesn't happen immediately when I turn the car on; it starts after I've been driving for a while.

Any ideas as to what might be wrong? I was thinking it might be a vacuum failure somewhere, but the fact that it only starts after driving for a while is a bit puzzling to me. I wondered if anyone had any experience with this issue or any suggestions. I couldn't readily find a past topic discussing this particular problem.

Thanks!

[sly]

Sounds like the off switch is bad. I've heard of this failure mode before, but can't remember the full cause, but I think it was the switch itself was failing on.