Drive motors and starting relay
A motor that is trying to start, but can’t for whatever reason, is using one heck of a lot of electricity. If you think about it, essentially the motor is as overloaded as it can be. So much, in fact, that if it is allowed to continue being energized without the shaft turning, it will start burning wires. To prevent this, an overload switch is installed on motors to cut power to them if they don’t start within a certain amount of time.
If the motor is trying to start, but can’t, you will hear certain things. First will be a click, followed immediately by a buzzing sound. Then, after about 5 to 20 seconds of buzzing, another click and the buzzing will stop. The sounds will keep repeating every minute or two. This is the motor cutting in and out on the overload switch. In some extreme cases, you may even smell burning.
If you hear the motor doing this, but it won’t start, disconnect power and disassemble or remove the pump as described in Pump and Motor Service. Check for anything that might be jamming the pump. If so, remove it and you are probably back in business.
See if the pump shaft turns easily by hand. If it feels sticky or gritty, the bearings are probably bad.
Try testing for resistance across the motor windings. In most machines, the white motor lead is common. One at a time, test for resistance between the white lead and the other two or three leads. There should be some resistance. No resistance or no continuity at all indicates a bad motor. Also touch the test leads between each motor lead and the motor cage, or frame. Except for the green lead, there should be no continuity. If there is continuity, replace the motor. Continuity between the green motor lead and the motor cage is fine; the green wire is the ground wire.
If you’re not sure how to test the motor, take it to your parts dealer or to an electric motor shop. Have them test the motor.
If it won’t even start without a load on it, the motor is bad. If you have an ammeter, the stalled motor will be drawing 10 to 20 amps or more. Replace it.
NOTE: When replacing the motor, always use a new relay! They are a matched pair!
Motors use a lot of electricity compared to other electrical components. The switches that control them have to be built bigger than other kinds of switches, with more capacity to carry more electricity.
The switches involved in running an electric motor are too big to conveniently put inside the control console or timer. Besides that, there are safety considerations involved in having you touch a switch that carries that much electricity directly, with your finger.
The way they solve that problem is to make a secondary switch. A big switch that starts the motor, which is closed by the little switch inside the timer or one you push with your finger. A relay.
When you first start a motor, it draws even more power than when it is running. So you need to close a really big switch to start the motor, then open it back up once the motor gets up to speed.
A starting relay is simply an electromagnet that closes the bigger switch. When the motor starts, the electromagnet stops, and the “start” circuit stops. But the “run” circuit stays on, and the motor runs.
Dishwashers have a relay starting switch mounted either beneath the tub or in the control console.
You can usually figure out which terminals to test across by looking at the wiring diagram. If you’re not sure how, take the switch to your parts dealer. They can usually help you test the switch. If not, just replace it. They’re not too expensive.
If the motor is stalled (buzzing and/or tripping out on the overload switch) and the starting switch tests O.K., the motor is bad. Replace it.