The main causes for the ac fan motor not working basically are: winding circuit, lack of oil holding shaft, abnormal capacitance, etc. From the maintenance of fan motor, the last two kinds of fault is much more common. The maintenance steps are: test plugs, motors, capacitor, and then processing.
The first step, measure the running condition of electric fan and control winding. First rotate the electric fan timer. When hearing a clicking sound, it means the timer is normal. If not, it means the timer get broken. If the timer is normal, set the multimeter to R x 100 Ω files, using one hand holding two multimeter to clamp fan power plug, another hand to push electric fan gear switch and measure the operation of the fan motor and speed regulating winding resistance. Most of the electric fan normal resistance value is between 600-1000 Ω. The difference of resistance value in each gear is 100. If resistance value is consistent, the electric fan running speed winding is normal. But, it can't say fan motor is at work because motor, start winding, as well as winding have the possibility of a partial short-circuit. If there is no resistance, the possible reasons are: a motor winding burned and caused circuit break; thermal fuse get open circuit (disassemble the motor and have a look, and if it just open circuit, joint thermal fuse and can get it back to normal; all gear switch are broken, but it not likely happens.
The second step, measure the startup capacitor: if the motor shaft rotate, measure startup capacitor of electric fan motor. First of all, using the multimeter R x 100 Ω to measure the both ends of the motor starting capacitor (the entire electric fan motor running, start, and speed winding resistance), that is the black and yellow wire. If resistance value is around 1.3 K Ω, it means fan AC motor start-up, operation and the control of motor speed winding is normal. However, if the resistance value is infinity, we can determine that the electric fan motor startup winding circuit is damaged. If the user doesn't want to change motor, the maintenance work is done. Second, the fan rotation speed is getting slow down or it not rotates at all. It also could be the result of an abnormal startup capacitor. In the cases of normal winding resistance value, start capacitance should be further measured and handle the problem according to the actual situation. (If there is no capacitance table, the same model of capacitance can be an alternative). If everything is all right, the motor should work when powered on. If the motor shaft can be held by fingers, the fan motor is normal. If not, and even you can easily change the rotation direction of the motor with the hand, it is usually caused by winding short circuit in some parts. In this case, nothing can be done but to replace the motor.
Through the above troubleshooting steps, you can find out the basic malfunction of ac fan motor and repair electric fan. Ac fan motor wiring diagram enclosed for reference