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Too much heat, CK1500

I have a CK1500, 2001 with AC. After driving 30-45 minutes it goes to full heat. Turning the heater control knob has no affect. Everthing works fine up to this point.

The heater/ventilation control module can be easily replaced but befor forking over 175 bucks for this I hope to verify whether this is indeed the likely culprit.

As cars and electronics go, it could well be something else hidden deep in the bowels of the dashboard.

Thanks.

This could be a vacuum problem. If the temp control door, or flap, is vacuum actuated, and there is a vacuum, leak, the door will move to one extreme, which, in this case is full heat.

The temp control door directs airflow through the heater core or the AC evaporator, or a combination, depending on the desired temperature.

Does the throttle position have anything to do with this?

The temp control door is electrically operated by a motor. It appears to work fine ‘in the garage’ and changes position depending on the heat control knob position.
Throttle has no apparent bearing.