Caution. Always practice safety first. Before attempting troubleshooting or any repairs to your dishwasher motor, disconnect the appliance from the power source. Unplug the unit from the wall outlet or shut off the power at the circuit breaker or fuse box. Wear rubber gloves and rubber soled shoes when working with electricity. Only pick up tools by their insulated ends.
Check to see if the lid switch "clicks" when you close appliance. If the switch is defective, it cannot signal the dishwasher motor to engage. If the switch is defective, a replacement can be purchased online from websites that specialize in appliance replacement parts or from your local appliance dealer.
Check the wiring to the dishwasher from the main power source and from the door switch to the timer and motor. If wiring is brittle or broken, it cannot conduct power to the motor and the motor will not start. Replace defective wiring. Use the proper appliance-grade wire and connectors.
If the motor on your dishwasher hums, but doesn't turn, it may need to be replaced. If you haven't used your dishwasher for sometime, the motor may be stuck and will need to be manually turned. If it does not respond to turning, it is defective and it is time for a replacement.