Yup. The fan speed switch is what I'm referring to. When an electric motor is running it has the same things a generator has. That is a magnet, a coil of wire, and movement between them. Spinning motors develop a "back EMF" or voltage that opposes normal current flow and causes it to be reduced. To say it a different way, when you load down a motor, as in when the bearings or bushings get tight, the speed is reduced, therefore the back EMF that is produced goes down. With less back EMF to oppose current flow, current flow goes up. That's why a locked-up motor will blow a fuse even though nothing is shorted.
There's two things that will cause the melted resistor connector you found. Too much current or a high-resistance connection. A tight motor will draw too much current which will overheat the terminals in the connectors. You found the ones at the resistor but the same thing likely has happened at the switch.
A high-resistance connection occurs when there's a film of corrosion between mating terminals or when the female terminal has lost its tension and isn't holding onto the male terminal with sufficient force. Either of those conditions will cause a small drop in voltage, and the current going through that resistance will cause heat buildup. Heat further degrades the connection which causes more resistance, and more heat buildup. Eventually the point is reached when the motor slows down or won't run at all, or the connection gets so hot the plastic body melts.
The poor connection can develop arcing between the terminals too, and they can burn away to the point no current can get through. It sounds like that's what you have now.
What you need to do is follow the path to see where you're losing voltage. Imagine a golf ball stuck in a garden hose. If you could take pressure readings at various points, you'd have normal pressure between the faucet and the restriction, and no pressure after the restriction. It's that restriction you're looking for in this circuit.
To be accurate for this type of problem, everything should be connected normally when you take voltage readings. You might want to start right at the motor and see if you have voltage there on one of the wires. If you do, you may just have worn brushes in the motor and a new one is all that's needed. If you don't find voltage there, we'll have to work our way backward toward the resistor, then the switch to see where it is being lost.
Thursday, November 20th, 2014 AT 4:26 PM
