I replaced a failed blower motor resistor with an aftermarket part. The blower motor now works, but not on high, which is important down South. I ordered an OEM part (Motorcraft) and installed it. The heat sink on the part got hot and the blower stuck on high. I took it out and reinstalled the aftermarket part and now the blower motor goes on and off as the heat sink heats up and cools down and resets itself. My question: Has the blower motor itself gone bad, or am I just getting unlucky with a series of faulty parts? Thanks for any informed answers.

 

There is a good possibility the blower motor is pulling too much current and overheating the circuit that controls the blower motor.
If you want to try this, you can remove the blower motor and see just how hard it is to turn. Sometimes you can feel the extra drag and that answers your question.
There is another option and that is to connect an amp meter , easy at the blower fuse , and see just how much the motor is pulling in hi speed position. They generally pull about half of what the fuse is rated for. If more than that The motor bearings could just be dry or you just will need to replace the motor. If you decide to replace the motor you might need a long allen wrench in order to get to the screw that secures the squirrel cage blower wheel to the motor shaft.

 

[QUOTE=hanky;114442]There is a good possibility the blower motor is pulling too much current and overheating the circuit that controls the blower motor.
If you want to try this, you can remove the blower motor and see just how hard it is to turn. Sometimes you can feel the extra drag and that answers your question.
There is another option and that is to connect an amp meter , easy at the blower fuse , and see just how much the motor is pulling in hi speed position. They generally pull about half of what the fuse is rated for. If more than that The motor bearings could just be dry or you just will need to replace the motor. If you decide to replace the motor you might need a long allen wrench in order to get to the screw that secures the squirrel cage blower wheel to the motor shaft.

Hanky: Thanks for your reply. Someone at another site solved the problem. When I pulled the bad resistor out and plugged in the OEM part, I tested the blower motor on all speeds and it worked fine. I then left it on for a couple of minutes on high and that's when it locked on high speed and the switch would not turn it down or off. Steve (the guy that answered on another site) laughed at me and told me to let the OEM resistor cool down and to go ahead with the full install (it's kinda hard to reach and I was too lazy to install it just to test it). Turns out that the aluminum heat sink fins require a minimum air movement to remain cool. That's why the test worked fine with the part hanging out, but a 2 minute extended run with the part plugged in, but not in the airflow of the blower chamber, overheated. I am an idiot. But it works perfectly now.

 

Glad you found the answer and shared it with us. Thanks for the update.