Heater-A/C suction fan

The (modern) symbol on the schematics identifies it as a resistor.

I think that this resistor running hot is a result of the motor not running (not that this component being hot shows that it's bad, and is preventing the motor from running). When the motor isn't running, it is not generating any back EMF, so the current thru the resistor will be much higher than if the motor was running (and, since the heat generated by the resistor is I^2*R, it is at a much higher temperature than if the motor was running).

Does the motor rotor motion feel "good" (i.e., it rotates smoothly and easily by hand)?

You might also measure the resistance between the two bare leads of the capacitor after you disconnect one of the leads from the motor terminals (it should be infinite ohms). If this capacitor is internally shorted (and reconnected), it would give the symptom that you have (no motor rotation, but current still flowing thru the resistor). Alternatively, for a short test, you could just disconnect one of the capacitor leads, and see if the motor works in that condition.

 

No, I don't have any specific information for the nominal R or C value.

The 283 KOhms for the capacitor isn't great, but, since your motor still won't run without the capacitor, that's not a good sign for the motor itself IMO. You might try measuring the resistance between the two terminals on the motor while you slowly turn the rotor by hand (when the capacitor is removed). If all was OK, this would always be a fairly low value regardless of rotor position (although you might get some random noise spikes in this measurment since the brush to commutator contact won't be perfect and the voltage used by the multimeter for the measurement is very low). On the other hand, if you find that for a large portion of each revolution the resistance is low, but over another large portion of each revolution it is always infinite, that would be a sign you have something wrong in the commutator/brush or one of the windings.

 

No trouble -- always glad to lend a fellow Owner a hand if I can. You've posted an interesting, new problem that hasn't been much discussed before.

Certainly a result that doesn't show an obvious problem. I don't think that there are any other electrical components inside except for the commutator/brushes and the rotor windings, but I've never had one apart. When you say "The motor makes 1 twitch when put under power without capacitor, then stops", does the rotor go to a fixed position and stay held there while the power is applied, or does the rotor twitch and then is just free to rotate while the power is applied (the same as when no power is applied)?

Also, if you can measure the current while the power is applied this might be interesting. I've got one motor in my side mirrors that isn't working, but its resistance measures OKish (similar to the working motors); however, when I apply +12V power, the DC current is ~0.5A; whereas, the other working motors draw ~0.1A when running (and, if yours does the same, this would explain why the external resistor is getting so hot).

I wouldn't be too worried about the external R and C (as long as they are not totally dead/bad). My guess is that they are only there for EMI suppression and arc suppression at the commutator/brushes, and the motor should run OK without them for a brief test. If you bend the metal tabs back holding the white plastic end cap in place, it should come apart, but reassembly is tricky (you need a way to hold the "springy" brushes back away from the commutator when you reinstall the white plastic end cap, and a way to release them afterwards. Also, there are probably shim(s) and/or thrust washers on the shaft ends to set the end play so don't let them fall out on the floor and note who goes where). With where you are now, I see little downside to trying to disassemble, and would be glad to suggest some further tests if you can post photos of what's inside.

 

I did a little more digging, and the resistor in series with the motor windings is a crude speed control device that has two drawbacks:

1. it's inefficient, because a lot of heat is dissipated in the resistor (this is less of a concern on small motors in this type of application as the primary goal is not useful work; however, the resistor will run kind of hot and needs the appropriate power rating), and

2. if the frictional drag of the motor changes over time, you can fall off a cliff where the motor can never get itself started (which is where you seem to be -- and this really overcooks the resistor, since the motor isn't rotating, it isn't generating a back EMF, and the current in the resistor is even higher than it would usually be).

Although, your motor seems to run "normal" when you run it without the resistor (i.e., R=0), I'd bet that it is actually running faster than it was designed for. You can try using a lower resistor value (somewhere between what you have now and zero) to see if you can reach a happy medium, but it would need to have a big power rating (like 2W). Also, if you can lubricate the bearings, this might help reduce the friction.

 

You may want to check clearances on the reassembled rotating components.

Theoretical power dissipation of a 7.5 ohm resistor at the starting current of 0.5A is 1.875W: just under the 2.0W rating. Your measurement of the continuous current of the motor alone was 135mA -- or 0.137W at 7.5 ohm. Assembled, the motor must be drawing over a half amp to burn out the 7.5 ohm, 2W resistor.

That may imply that the motor is trying to turn something that may not be turning freely.

A motor under load will draw more current than a freely running motor. 2W may not have been sufficient for the resistor under load. But given the size of the original component, it looks like your motor is drawing more current than the design intended.