My heater fan was working fine a few days ago.

Now it wont spin up at all.

The motor is okay because i can jump it and it runs fine

Im getting 12V to the motor. I can stick my multimeter right in the yellow and black wires (at the motor) and get 12.00V at high speed and 10.35V at low speed

But im getting no amps... it doesn't even develop enough current to run a small electric motor.

I think its a very weak ground. As far as I can tell the black wire is hot and the yellow is ground.

I checked the resistor in the heater box by placing one lead where the yellow wire connects and checking for continuity between the upper three prongs that go to the wiring harness. All are okay

Does someone have a wiring diagram? Where the hell does this thing ground too?

-Gun

 

The blower motor gets its ground through its case. the Yellow-black wire goes to the heater switch. You can download wiring diagrams here. http://www.themustangshop.com/resources.cfm

 

You may want to clean the connectors and tighten the screws and insure a good ground first (spray with CRC or some other electrical non conductive displacment). Then use the ole wiggle and wack method untill you find the loose connection and youu know exactly what to fix. It sounds like its the ground though. Good luck.

 

According to those diagrams the ground for the heater is attached as a 4th wire to the switch.

Getting those scews out for the controll box is going to be fun..Yippie

Thanks for you help hopfully it matches the daigram and that ground can be cleaned up.

Gun

 

It may ground through the wiring, butIt more than likelygrounds through the case too. if you can get a jumper wire from a good ground to a bare spot on the motor you can verify if that is the problem pretty easily.

 

On my 66, the motor sinks a ground thru the resistor to control the fan speed and always has a constant 12 volts applied to the fan, thereby controlling the fan's amperage. Thus, the ground wire has to be attached to the switch which, selects what portion of the resistor to use. (The resistor varies the ground resisance)I'm not sure what year your car is so I can't comment further.

You may want to jumper around the resistor to see if the fan will run that way. It would then point to a bad resistor or switch. (it shouldn't matter which terminals you jumper to as long as one is the fan input)

 

Well, it is apparantly very year specific. Now if I strain my brain cells and think really hard, I remember that my 70 mustang only has one wire going to the blower motor. This wire comes from the switch/resistor and the motor gets its one and only ground from the case of the motor. What year is yours??

 

I got it.

Once i got the switch out the ground wire is not really a wire at all but simply a little tab that sticks out from the back of the switch and grounds to the metal switch case. The case is held to the switch mechanism by little tabs one of these tabs presses on the ground tab.

I sanded all the metal parts inside the switch and the ground tab...and the part of the switch case that touches to ground tab. Then i sanded the part on the switch case that secures the switch to the control box and the mount on the control box and the screw that holds the switch to the control box case.

That fan has never ran better.

Thanks for the help guys.

-Gun