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I replaced the heater fan motor and resistor on my 66 mustang. It has a 2 wire motor with a 3 speed switch. Now the fan comes on in the high speed mode when the ignition switch is turned on. Even though the fan switch is off. I thought that the new resistor was bad so I put my old one back in, same problem.
 

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The fan motor gets constant power with the key on through the 181 wire in the diagram. Fan on/speed is controlled by wire 913 to the resistors then grounding through the heater switch.
Sounds like the 913 wire is grounding somewhere it shouldn't. Could be a bad switch or wiring.

763217
 

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Discussion Starter #3
The fan motor gets constant power with the key on through the 181 wire in the diagram. Fan on/speed is controlled by wire 913 to the resistors then grounding through the heater switch.
Sounds like the 913 wire is grounding somewhere it shouldn't. Could be a bad switch or wiring.

View attachment 763217
The fan motor gets constant power with the key on through the 181 wire in the diagram. Fan on/speed is controlled by wire 913 to the resistors then grounding through the heater switch.
Sounds like the 913 wire is grounding somewhere it shouldn't. Could be a bad switch or wiring.

View attachment 763217
The fan motor worked as should except it made a lot of noise. The heater core started leaking so I replaced both hoses and fan while I had the heater box out. What could have changed? Thank you for your reply.
 

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Are the heater wires plugged up backwards or pinched on something under the hood or by the heater case?
 

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After moving the fan switch back and forth thru the settings the fan started working as it should. Must have been corrosion on the contacts. Thank you for your time and input.
Are the heater wires plugged up backwards or pinched on something under the hood or by the heater case?
 

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I've had some weirdness with my switch where if I move it too quickly it doesn't turn on, but if I do it more slowly then it works as it should.
 

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I used to sell a lot of used heater fan switches. I always hosed them out with electrical contact cleaner before shipping them. Never had a return. Made me wonder, though, how many of the switches being replaced needed only that.
 
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