heater blows on low but does not blow on med. or high. changed out blower switch but didnt work any ideas on whats wrong and how to fix it. have 77 f-250 w/ac.

 

The resisitor is shot.
It is usually located in the heater plenum.

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[updated:LAST EDITED ON 30-Sep-02 AT 08:23 PM (EST)]I wish I still had my diagrams for 77 Ford pickups. But I would suspect that you have a blower resistor problem. It should have four wires on it I believe. You should have continuity between all of the terminals.