Mornings here are getting cooler and the last few days I've had foggy windows so I decided it's finally time to deal with the blower fan. Hasn't worked since I bought this 81 custom cruiser with tempmatic (temper-mental? could be) heating at any speed. I've known from day one that the electrics on this have been abused, so nothing would surprise me on this. Pulled the motor and put 12V to the input and grounded the housing .. nearly flew out of my hands ... so we know that works fine. Onto the next obvious thing ...eyeball the resistors .. nothing obvious there, not that I expected there to be. Pulled the relay and swapped in the one from my '81 parisienne ... no change. Curious. Notice that the motor housing is installed in plastic and not self grounding so I check the ground lead ... aha .. no contact to ground. Install a jumper to ground ... Hmmmm. Low 2 and 3 all work now ... but Hi only runs at low. Curiouser and curiouser. So I pull the power wire from the fan and check the voltage in the 4 switch positions .... WTF ... 12.5V in all positions. That can't be right. Hit the books ... finally get a clue that the ground lead from the fan housing and the ground lead from the relay are joined somewhere in the wiring harness and it's probably on the far side of that that the break can be found ... Well bugger, lunch is over and it's time to clean up and get back to work. At least I've got some fan power to clear the window tomorrow morning ... I'll chase the break then.


But if the voltage is the same in all four positions ... what's changing the fan speed?

 

The different sized coils on the blower motor "resistor" is what controls the speed of the fan motor.

 

Yeah ... that's obvious ... but usually running a voltage through a resistor causes the voltage to drop. Mine didn't. See the problem?

 

The voltage will only lessen when current/amperage is flowing through the resistor. No electrical load equals no current/amperage flowing equals no voltage decrease.

 

^^^ This!

You are probably using a low-impedance VOM. Try a test light or a high-impedance VOM instead.

 

Thank you. Thought I was losing my mind for a bit.