The two central components in an electric motor are the armature and the wire windings fixed to the inside of the motor's casing. Electrical energy passes to the windings on the armature via two brushes which induces a magnetic field while electricity also passes to the wire winding fixed to the casing. The two magnetic fields oppose each other, similar to two "north" or two "south" poles on magnets repelling each other. The opposing magnetic fields force the armature to rotate and they make the motor operate.
Put on rubber gloves, if the armature and other parts use high voltage electricity to operate, such as your domestic electricity supply. It's not necessary, if the parts operate on low-voltage electricity from a battery.
Use a multimeter to test the armature and other parts. A multimeter registers electrical energy so you can find out if electricity is getting to all the electrical parts.
Set the multimeter to register voltage. Voltage is the force of electrical energy such as 110 volts from your domestic supply or 12 volts from a car battery.
Locate the two wires that connect to the brushes on the motor. The brushes are on the front or back of the motor and touch the armature enabling electricity to get to the electrical parts.
Turn on the motor so you can test if electricity is getting to the armature. Place the metal prong on the end of the red wire from the multimeter onto the red wire that connects to one of the brushes. You need to find the bare part of the wire that connects to brush. Place the prong on the end of the black wire from the multimeter onto the bare part of the black wire that connects to the other brush. If you get a reading, electricity is getting to the brushes. If you don't get a reading you have a fault between the electrical supply and the armature.
Place the prongs on the end of the two wires from the multimeter onto the commutator of the armature; this is metal ring around the axis of the armature where the brushes connect. If you get a reading, electricity is getting to the armature and other parts. If you don't get a reading you need to replace the brushes.
Turn off the electrical supply to the motor. Remove the screws holding the top part of the motor casing in place using a screwdriver. Carefully lift off the top casing so you can access the armature and other windings.
Place one of the prongs from the multimeter onto one end of the wire windings on the armature and the other prong on to the other end of the winding on the armature. It doesn't matter which prong attaches to the armature windings and you don't need the electricity turned on. The multimeter records a very low voltage reading, if the wire winding circuit is complete. If you don't get any reading then the windings on the armature have burnt out so they need replacing.
Repeat the process by placing the two prongs on the opposite ends of the windings fixed to the top and bottom parts of the casing. If you get a reading, the circuit is complete. If you don't get a reading the outer windings have burnt out and need replacing.
Test the bearings. These are fitted to each end of the motor where the armature sits. Lift the armature from the lower casing. Look at the bearings on the top and bottom parts of the casing to see if they show signs of wear. Feel them using your fingers. If they are smooth then they are fine. If they have grooves or feel pitted they need replacing.
Replace the armature and then replace the top half of the casing. Insert the screws and tighten using the screwdriver.