（1）. Remove the motor from the car for inspection.
（2）. Measure the cold insulation resistance of the motor with a 250M ∏ megohmmeter, and the resistance value should be greater than 0.5 M ∏.
（3）. Rotate the shaft head by hand to check if the armature rotates flexibly, and there should be no jamming or rubbing phenomenon.
（4）. Open the protective cover and observe the degree of wear of each electric brush. If one of the four brushes has a height less than 12mm, all brushes need to be replaced.
（5）. Observing the surface of the commutator, a normal surface should be smooth and purple black in color. If the surface is rough or uneven, it is considered that the surface of the commutator needs to be polished again.
（6）. Under normal circumstances, when measuring the armature coils (A1, A2) or excitation coils (S1, S2 or F1, F2) of the motor coil with the on/off range of the multimeter, the multimeter should display that it is conducting and the resistance value should be close to 0 (at room temperature, it is generally only a few ohms). If the measured resistance value is greater than 1 π or does not conduct at all, it can be determined that the motor has a fault.
Reason for malfunction: a. Motor coil malfunction
b. The motor carbon brush is severely worn or has poor contact
c. Severe contamination of the commutator or no open circuit in the motor coil
Inspection of motor insulation resistance: At room temperature, the insulation resistance of the motor coil to the housing is above 1 M ∏. Use a multimeter to measure the insulation resistance of the motor armature and excitation coil to the motor housing in the 2 M ∏ range. It should display infinity, otherwise it can be judged as a motor fault.