We're way past that. You already had 12 volts applied normally through the switch, and the motor didn't run. Applying 12 volts from another source won't change anything. Running external wires is done to verify the motor is okay after you found there is a switch or wiring problem, and you want to be sure to not overlook any other defects in the circuit before you calculate a repair estimate.
When you said you didn't get any reading, I take that to mean the meter continued to show an over-range condition, the same as when the probes are not connected to anything. As I recall, with this model of meter, that is a single "1". All that means is the meter is on, it's ready to take a reading, and what it's currently measuring is higher than the range you've selected. If you had the meter set to the 200 ohm position, it can read any resistance value up to 199.9 ohms. 200 ohms is WAY too much for a motor and will result in current flow that is much too small to make the motor work. You can switch to the 2000 ohm scale, (2k scale), or the 20k scale if you want to. At some point the meter will show a resistance value, but it's irrelevant at this point. You will be reading the excessive resistance caused by the arced contacts in the internal thermal circuit breaker. OR, if your motor doesn't have that circuit breaker, you'll be reading the excessive resistance caused by worn brushes in the motor. If the cause is worn brushes, you can sometimes get the motor working by tugging and poking at the window or motor. Worn brushes can just barely make good enough contact when irritating them to make the motor run for a little while.
If the circuit breaker is the culprit, you may be able to get the motor running by banging on it. The jarring might cause the contacts to move to a clean spot and work momentarily.
A former student showed me how his boss showed him how to take the motor apart and short out, (bypass), the thermal circuit breaker. I have mixed emotions about that. A window motor rarely runs for more than about five seconds at a time, so it is not likely to overheat and cause a fire. I've never seen a window motor short either, and draw excessive current. If it did, that circuit breaker could save the window switch from arcing at the contacts, but it won't save the switch if the wiring between the door hinges frays and shorts together.
The only value I see in that circuit breaker is if a switch were to stick on, the window would reach the end of its travel, lock up there, and the motor would continue to try to run. A stalled electric motor draws even higher current than normal, and that could overheat the motor if it weren't for that breaker. My concern is the liability issue if bypassing the circuit breaker led to a fire. I would be tempted to do this on my own car, but I would never attempt it on a customer's car. I wouldn't even tell a customer how to do that on their own time, at home, because you never know what some people will sue over.
Wednesday, May 3rd, 2017 AT 10:27 PM