I was trying to repair one of those coin sorting banks.It runs on two "C" cell batteries. A toy really.

It has a small DC motor that drives the mechanism. It seemed to have stopped working.

I'm a bit of a do-it-your-selfer and decided to try and fix it.

I decided that the two wires connected to the dc motor needed to be soldered back on better as my multi-meter showed no voltage in either position of the switch, but voltage before the switch was about 2.5 volts and 1.0 volts after the switch when on. I wasn't getting good continuity between the negative battery terminal and where that wire connects to the motor.

Once I soldered them back on I installed the batteries and hit the on switch. Nothing.
But my meter said I was getting good continuity on the negative lead and voltage this time across the motor terminals. About 1 volt DC. It just wasn't turning.

I decided to get some fresher batteries. When I came back the motor was running.

Experimenting some more noticed that once switched on the motor would start on its own after about 3 minutes 43 sec. It got warmer in the meantime, but not hot. When running I got about 1.9 volts DC and about 0.5 amps across the motor terminals. Prior to start I got 1.0 Volts across the motor terminals and zero amps on any setting of my meter (micro Amps, milliamps, Amps all zero) even after flipping the switch back and forth.

What going on ?
The only circuitry in the switch appears to just be an on off switch.

I'm a mechanical engineer and not an electrical engineer so seeking an answer from the latter if possible.

These things are cheap but I have fun trying to fix little things like this.
I'm also really curious about what's going on here.
The motor is only getting slightly warm before it starts. I know conductors INCREASE in electrical resistance with temperature, and not the other way around. Insulating materials will have lower resistance with increased temperature but I'm sure it has to be much much hotter than the slightly warm temp change I'm seeing.

Anyone have an educated guess at what's going on here?

I suggest that your problem is mechanical, not electrical. Corrosion between the shaft and the bearings along with accumulated dust and dirt over the years is creating to much resistance for the motor to start. Of course the true repair would be to disassemble, clean and lubricate. That would be a very delicate and tedious task.

I think you could confirm my diagnosis by warming the motor with a hair drier prior to attempting to start it. The heat liquidizing and thinning any lubricant remaining in the bearings.

I do small home repair jobs for people. I am often ask to replace bathroom exhaust fans. While this is a simple task, but it often requires removal of the ceiling drywall because access is not possible from above. These fans are powered by small shaded pole AC motors which are extremely reliable. Indeed the only moving part is the armature. Very much like the little DC motor you are working with. And like the DC motor, very little starting torque. I use to replace the motors. All of these motors are pretty much the same but the mounting brackets often differ.
Then I began dissembling the existing motors, sanding and lubricating the shafts. I have used this approach any number of times and it has not failed to correct the problem.

These shaded pole motors, like the DC motor you are working with, are simply coils of wire. As long as you have electrical continuity there is nothing that can go wrong electrically.

Since this type of motor is not designed to be repaired, the question is can you successfully dissemble and reassemble the it.

Before dissembling you might try cleaning between the shaft and the bearings with something like acetone or lacquer thinner. Letting the cleaner seep in between the shaft and the bearings. Then let the motor run for an extended period. Hopefully friction will remove any corrosion.

I'll try that. Taking the little motor apart won't be hard but getting it back together it might be.

It could very well be clogged with dust etc or just need lubrication. Would be interest to open up just to see.

For $2 I can just replace the motor. I'll probably do that. You can get ten of them online for $20. Might come in handy repairing something else in the future.

But I am curious why the motor began to turn on it's own after about 3.5 minutes each time. I'd think it wouldn't turn at all if it was just in need of lubrication

Again. This is not an important repair problem The whole unit could be replaced for about $27.
I just like tinkering with stuff like this and seeing if i can fix it. Just a curiosity for me.
Also once in a while a gadget like this just isn't made anymore and can't be replaced.

I understand that completely and am often guilty of the same.