I just finished repairing a Kenmore Dishwasher Model 665.77962K700 (not exactly sure) made by Whirlpool that took me three days of trouble-shooting in order to find the problem.
Symptoms
Suspects
I made a mental list of all the things that can go wrong.
Diagnostics
So I got out the oscilloscope and started probing around at the control board. Since I could not locate a circuit schematic I had to do my best and reverse engineer the control board.
I was able to read the part number of the MCU, Freescale MC908AP64. Good. I know how to program those chips.
I printed the pinouts for the MCU and started to trace the signals from the MCU to the control panel.
I put the oscilloscope on the 14-conductor ribbon cable at connector P1 from the control panel looking to make sense of what's going on. I suspect that the buttons are multiplexed. There are 12 control buttons in all. On top of that there is an LED above each button. They are multiplexed as well.
I can't make out much of what is going on by looking at the waveforms. I can see six phases in the cycle. I can see some phases going low when a button is pressed.
By day 3, I map out which pins on the ribbon cable connector P1 are inputs and which are outputs. I see pins 11, 12, and 13 are inputs. The others are outputs, some are NPN driven, others are PNP driven.
Good. If I disconnect pins 11, 12, and 13, these should all float high, pulled up with 10kΩ to +5V.
Pins 11 and 12 float up.
Pin 13 shows the same signal that is on pin-14 which is an output signal. Wait a minute. Something is wrong here.
I disconnect the power and unplug the control panel ribbon cable at connector P1.
I measure the resistance between pin-13 and pin-14. I read about 500kΩ but it is not steady. We're on to something here.
The rest of the saga will follow.
I will post pictures later and let you know what was the problem.
The problem was not obvious and very difficult to locate.
Symptoms
- The dishwasher would randomly interrupt the wash cycle.
- Sometimes the Normal Wash LED would flash.
- Most times it would complete the wash cycle after numerous interruptions.
- Sometimes the control buttons would not respond.
- Sometimes the washer would be sitting idle and an LED would start flashing.
- Sometimes the flashing LEDS would go off on their own.
- Other times I have to turn off power to reset the system.
- In general, the fault appears to be very random.
Suspects
I made a mental list of all the things that can go wrong.
- Door switch
- Low voltage power supply
- Faulty sensor
- Bad control membrane keypad
- Bad control board
Diagnostics
So I got out the oscilloscope and started probing around at the control board. Since I could not locate a circuit schematic I had to do my best and reverse engineer the control board.
- Door switch was replaced with a solid jumper.
- Low voltage +5V supply looked ok.
- Since the washer was functional at times I assumed that the control board was ok.
- I focused on the control panel looking for a stuck or intermittent membrane push-button.
I was able to read the part number of the MCU, Freescale MC908AP64. Good. I know how to program those chips.
I printed the pinouts for the MCU and started to trace the signals from the MCU to the control panel.
I put the oscilloscope on the 14-conductor ribbon cable at connector P1 from the control panel looking to make sense of what's going on. I suspect that the buttons are multiplexed. There are 12 control buttons in all. On top of that there is an LED above each button. They are multiplexed as well.
I can't make out much of what is going on by looking at the waveforms. I can see six phases in the cycle. I can see some phases going low when a button is pressed.
By day 3, I map out which pins on the ribbon cable connector P1 are inputs and which are outputs. I see pins 11, 12, and 13 are inputs. The others are outputs, some are NPN driven, others are PNP driven.
Good. If I disconnect pins 11, 12, and 13, these should all float high, pulled up with 10kΩ to +5V.
Pins 11 and 12 float up.
Pin 13 shows the same signal that is on pin-14 which is an output signal. Wait a minute. Something is wrong here.
I disconnect the power and unplug the control panel ribbon cable at connector P1.
I measure the resistance between pin-13 and pin-14. I read about 500kΩ but it is not steady. We're on to something here.
The rest of the saga will follow.
I will post pictures later and let you know what was the problem.
The problem was not obvious and very difficult to locate.