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Estate Dryer Starts Itself

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TechnicianBrian:



Model Number: TEDS840JQ2 , this dryer would run a full cycle, sound the complete buzzer, then after about 10 minutes would turn itself back on. A ghost in the machine? Maybe, but lets walk through the circuit diagram for this unit and see what makes this machine want to keep running.






Here is the wiring diagram for this unit (I took the liberty of removing all the components that are not apart of this circuit) and as you can see, there really are only two components that are used to start the motor circuit, the timer and the push to start switch. So lets look at what happens when the timer is turned to a cycle, and the start switch is pressed.





Once started, current begins flowing through the double make/break timer switch, on to the start switch, and then to the motor (see yellow). The neutral path to the motor is provided through timer switch #4 and then on to the door switch. At the same time, current flowes through the push-to-start relay (see red) resulting in a magnetic field which keeps the start switch contacts closed (that's why it stays running when you release the switch). Also, a circuit is set up through the timer motor (see green) causing the timer to start advancing. Now a somewhat common failure that gives this exact symptom is the start switch contacts become fused shut so it acts just like holding the switch closed all the time, but that wasn't the case here. Let me show you what happened using the next diagram.



When the timer came to the end of the cycle, the double make/break contacts opened as well as those of time switch #4 which shut off the L1 current to the motor, and opened the neutral line causing the motor to stop. But the switch contacts on timer switch 0 remained closed allowing current to flow from the L2 side (this timer motor is 240vac) through the motor to the start switch contacts which were still closed because of current flowing through the start relay maintained the magnetic field which held them shut. Since the timer motor was still running, once it reached the beginning of the next cycle, timer switch #4 and the make/break contacts closed and the dryer started off on another cycle.

Knowing now which component had failed, I installed a new timer and now the ghost has been removed from the machine.





TEDS840JQ2

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