How to know window motor blown?

[BonesDT]

Truck is an '89 Dodge Dakota. Both driver and passenger power windows stopped working, one died, then the other one slowed down and died a day or two later.

I disconnected the motors right at the motor. I tested the truck's incoming feed and it is +-12 Volts. I jumped the motor straight to the battery and it moves up and down; however, when I connect the motor back, it doesn't work and I check the feeds and no power when connected.

The truck uses a 30A circuit breaker in lieu of a fuse. I can't figure out if it is suppose to be a Type 1 Auto or Type 2 Modified, but I replaced it with a Type 1, and after some research, I'm thinking maybe it should be a Type 2, but the Type 1 should work better if anything. Still nothing.

Why would the 12-volts disappear when I hook up the motor? I took apart one of the motors and then asked myself, how can a motor blow out anyway?? Are these motors blown because they aren't powerful enough to run off the wiring harness (but not completely blow where a straight up battery jump would get it moving)?

 

[BonesDT]

Bump

 

[BonesDT]

Arg!

I "bench" tested the motors straight up to the battery and they turn in both directions. How could I be getting 12V when the motor is disconnected, but as soon as I connect it, there is no voltage?

 

[Joe Dirt]

You could have a broken wire somewhere between the switch and the motor. If there is a broken wire, it's like the wire isn't even hooked up...

Or, your switch(es) are junk...

 

[BonesDT]

How can it be the switches or wires if the 12V is coming through to the very end right up to the motor?

Here's a diagram. Yes, my finger looks like that when I push buttons.

 

[BonesDT]

Turned out to be the driver's side switch (which also affects the passenger side). Weird how it still eeked out a little bit of power to register on the voltmeter. Glad I didn't spend $160 on motors. This truck's old-school technology is so simplistic, yet it's an electrical gremlin nightmare.

 

[TedJ]

The meter draws a incredibly small amount of current, small enough that is negligible for our purposes here.

Now the most common failure and the one you experienced in auto electrics is high resistance in some connection or switch. Following ohm's law, the voltage drop across the resistance will be related to the current being pulled through it.

What this means for you is that you are pulling almost no current through the switch when it is connected to the meter, so the voltage dropped across the high resistance in the switch is also almost nothing. Hence, you get no noticeable difference from battery voltage.

Now you connect the motor and it's trying to pull 15a or whatever through that high resistance in the switch.... so the voltage drops to almost nothing.

This is why testing for voltage drops with the load in-circuit is the best way to troubleshoot auto wiring.

 

[BonesDT]

I see. I guess I was expecting, if the switch broke, it would produce a pure open circuit rather than increase its resistence suddenly - like a fuse, it just pops and nothing gets through it.

This truck also has fusible links. Those things have given me a headache in the past. I would expect them to act like modern fuses, they blow out, and become a pure open circuit, but they don't, they work like you describe, they blow out, but just become a large resistor. Much harder to diagnose (you can't just disconnect wiring harnesses and probe them)

I'm going to keep your info in mind next time the e-gremlins come around.