66 Heater Uncontrollable ON & OFF

[Project66]

The heater (original) in '66 coupe has started to turn its fan on-and-off (at what appears to be its highest fan speed) at random times. Duration for either mode is from 3 to 15 minutes. In either mode the fan on/off switch has no effect. I have never used the heater and don't know whether the switch ever did work. (I'm just getting to putting this baby into seviceable condition after eight years of storage following purchase.) Driving the car back and forth for initial motor service, then tranny service (30 min one way) the heater fan would stay off like expected. After the tranny service (many parts replaced after 137,000 miles) and gas tank sensor replacement, this wierd behavior started.

WHERE SHOULD I LOOK? All ideas welcome.

 

[2+2GT]

12V power is supplied to the blower motor at all times. The wire from the motor goes to the control, which is either off, or providing connection to the grounding resistor. If the wire from the motor to the control is grounded anywhere, the motor will come on full blast, even if the control is off.

 

[Project66]

To 2+2GT, Thanks for your reply, makes perfect sense. Hope I can get far enough into the heater to find the wobbling short.