How can I find out why a small DC motor won't work?

I bought a broken remote control car that I am re-architecting and trying to get to work. The steering for the car is done by a small DC motor. Although the motor is tiny, it runs off the 9.6 Volt battery that powers the rear wheels as well.

The motor worked fine for a couple of months, but now does not turn on at all. It goes through a gear box that actually does the steering. The design of the steering is a little strange. The motor's either full on right or left, or it's off in the center.

So to turn right, for example, the motor turns until it is stopped by the mechanical resistance of the wheel bumping against a block, and the motor then holds that position until the current is turned off. A spring then pulls the wheels back to center. So I would guess that the motor is stalled the whole time it is holding the wheels in a turned position, and pulling the stall current for the motor.

My guess is that the motor burned out because of pulling stall current for too long a time, but how could I check that? If I hook it up to a power supply and check for voltage across the two motor terminals, I get nothing. If I check for continuity across the two motor terminals when no power is being applied, it is an open circuit. That's all I could think of to test.

A visual check reveals nothing unusual. The two motor terminals are attached to a small PCB board that seems to have a few surface mount resistors on it. I have not taken the motor apart.

I cannot see any writing on the motor. I can take a picture of it if that would help.

Any suggestions would be appreciated.

My guess is that the motor burned out because of pulling stall current for too long a time, but how could I check that? If I hook it up to a power supply and check for voltage across the two motor terminals, I get nothing. If I check for continuity across the two motor terminals when no power is being applied, it is an open circuit. That's all I could think of to test.

That is odd because it suggests you have two faults. No power to the motor and a burned out motor. Are you sure of those tests? It could be it burned out causing a short and that short killed the motor's driver.

The surface mount components are probably noise-suppression - ferrites or capacitors or both.

Have you measured the resistance across the motor terminals to see if its burned out?

Cut one of the motor leads to isolate the motor from the rest of the circuit, then apply voltage, 6 or so, to the motor terminals to see if it will run. This will verify if the motor is defective or the motor drive circuit is defective. Note that if the motor is defective its drive circuit might also be defective.

I measured the resistance across the motor terminals, and it is over 100k Ohms.

I have also removed the motor from the gearbox and the motor driver and hooked it up to a 5 Volts power supply, and it does not move at all. But I was wrong when I said that there was no voltage across the terminals when I hooked it up to the 5 Volts power supply. There is a voltage. I was touching my meter probes to a solder blob on each terminal, not realizing that the solder blobs were not making a connection with the probes.

I measured the resistance across the motor terminals, and it is over 100k Ohms.

That is very high, are you sure that is not just your skin resistance? Were you touching both terminals of the meter when you made the measurements?

It sounds like it is open circuit and quite burned out.

Grumpy_Mike: That is very high, are you sure that is not just your skin resistance? Were you touching both terminals of the meter when you made the measurements?

I get varying results, but it's usually above at least 80 k Ohms, and I am not touching the probes. It's funny, though. I can get the meter to settle on a resistance with the faulty motor, even though I get a little different reading every time. But with a similar motor that works fine the meter will not settle on a resistance, but jumps around from 20 or so Ohms to 200 or 300 Ohms. Almost always less than 0.5 k Ohms, but I thought it would settle down to a small number.

It sounds like it is open circuit and quite burned out.

Yes, I'm afraid the motor is burned out. That's a shame, since I have no idea how to replace it. I wonder if the 9.6 Volts I used (which usually measured out at well over 10 Volts) was too high for this motor. Though it did work for quite a long time.

But with a similar motor that works fine the meter will not settle on a resistance, but jumps around from 20 or so Ohms to 200 or 300 Ohms.

Motors with brushes have contact resistance on those bushes that can change with microscopic movement of the rotor. It is probably that you are seeing.

I see. When my motor is burned out, does that mean that the brushes are no longer in contact with the rotor? With this small a motor I suspect that is irreparable.

I ask because I'm surprised that this motor burned out, and wonder what caused it. These little motors seem to be indestructible in the various toys I've scavenged them from. I wonder if I abused it somehow. Maybe too high a voltage. Or too much current (stall current) for too long. Those are the two possibilities I've thought of.

Could be worn out brushes in the motor, especially if its fairly old. Usually you can pull small DC brush type motors apart and that will allow access to the brush assembly. If the commutator is open circuit then its unreparable.

Daanii: I see. When my motor is burned out, does that mean that the brushes are no longer in contact with the rotor? With this small a motor I suspect that is irreparable.

Along with open brushes, It could also mean that the motor wire windings have burned open.

I ask because I'm surprised that this motor burned out, and wonder what caused it. These little motors seem to be indestructible in the various toys I've scavenged them from. I wonder if I abused it somehow. Maybe too high a voltage. Or too much current (stall current) for too long. Those are the two possibilities I've thought of.

Yes, those are methods of damaging a motor.

I'll try to take apart the motor tomorrow to see what what happened. I suspect the motor is toast, though, and destined for the garbage can.

I need to be more careful. Not only have I burned out a motor, but I seem to have burned out a 1 1/2" speaker as well. It was working fine (I added it to the car for a horn) until I suddenly saw a wisp of smoke. I seem to have fried it. Perhaps I needed a resistor on it.

If you simply wired the speaker to the car horn circuit then you will indeed have destroyed it.
A car horn circuit is a simple 12V DC system which is either ON or OFF.
A speaker operates from an AC signal source (amplifier) matched to the speaker impedance, usually 4, 8 or 16 ohms.
If wired directly to DC, all that will happen is the cone will give a single click as it moves to a new fixed position and excess current will flow through the voice coil windings.