A wide range of products today use direct current (DC) electric motors, the power windows in your car being an example. These motors can be quite difficult to repair without pulling the whole application apart given their rather hidden placement. For this reason, you should always give a DC motor a quick diagnostic check whenever you have access to it.
Getting Started with the Troubleshoot
Begin by ensuring that the source of electric power has been removed. Once this is done, you can remove the DC motor from its mount. As some motors will be difficult to access, as they may be wedged tightly into position, it may be necessary to consult the manufacturer’s instructions to do this. Handle the motor very carefully, as it could pose a risk of electrocution.
Testing the Connection
The next step will be to attach the motor to a volt ohmmeter to test the connection (or continuity). Before placing the black and red leads into the connections, make sure the meter is in the ohms position. The black lead should be connected to the common point, and the red to the ohms. Touching the two leads together will determine if the meter is working correctly; the screen should read full continuity, or zero ohms.
Touch the leads of the motor to the leads of the ohmmeter to test the motor. Low resistance, falling somewhere between ten and thirty ohms should be indicated.
If the screen reads an open circuit or infinite ohms, proceed by rotating the end shaft of the motor. As the shaft rotates the ohmmeter should display different readings. This will point to a likely a problem in the electrical circuit, but indicates that the motor itself is good. The conducting brushes could be bad if an open circuit is still reading on the meter.
Inspect the Brushes
You can remove the brushes from the motor with a screwdriver. You’ll find the brushes at the opposite end of the motor to the drive shaft covered with the plastic end caps. Carefully inspect the brushes for any Look for any sign of breaks or cracks in the surface while inspecting the brushes. The part of the brush that is touches the commutator or conductor or should be curved and smooth. The motor will certainly fail if there are any broken springs or wires. If there are no signs of trouble on the brushes, the problem is likely to be with the commutator.
Take the rear end cap off of the motor with the screwdriver by unscrewing both of the screws that run the length of the motor. There should be an opening between each of the plates that make up the commutator assembly, so inspect them carefully. Look for any burnt varnish or broken wires. If you see either or both, your commutator has failed, and any damaged parts will have to be replaced.