SOLVED: Kenmore/Whirlpool Dishwasher

Thread Starter

MrChips

Joined Oct 2, 2009
23,515
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
  1. The dishwasher would randomly interrupt the wash cycle.
  2. Sometimes the Normal Wash LED would flash.
  3. Most times it would complete the wash cycle after numerous interruptions.
  4. Sometimes the control buttons would not respond.
  5. Sometimes the washer would be sitting idle and an LED would start flashing.
  6. Sometimes the flashing LEDS would go off on their own.
  7. Other times I have to turn off power to reset the system.
  8. In general, the fault appears to be very random.

Suspects
I made a mental list of all the things that can go wrong.
  1. Door switch
  2. Low voltage power supply
  3. Faulty sensor
  4. Bad control membrane keypad
  5. 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.

  1. Door switch was replaced with a solid jumper.
  2. Low voltage +5V supply looked ok.
  3. Since the washer was functional at times I assumed that the control board was ok.
  4. 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.
 

Thread Starter

MrChips

Joined Oct 2, 2009
23,515
Here is the console panel.

Kenmore Dishwasher1.jpg

The first button "Smart Wash" plastic overlay is broken. This was prime suspect but turned out not to be faulty.

The first thing to do is to determine if a push button is stuck. Here is the diagnostic procedure.

1. Disconnect AC power.
2. Disconnect ribbon cable at connector P1.
3. Open the door.
4. Apply AC power.
5. Close the door (or hold the door micro-switch in closed position).

The drain motor should run for 2 minutes. If this happens then the controller board appears to be working.
Repeat the above procedure except for (2). In other words, install the ribbon cable. If error lights flash and the motor fails to run then there is a problem with the control panel.

In my case a problem with the panel was detected.

Kenmore Dishwasher2.jpg
Door micro-switch

Kenmore Dishwasher3.jpg
Console ribbon cable at connector P1
 

Thread Starter

MrChips

Joined Oct 2, 2009
23,515
After three days of reverse engineering and trouble-shooting with an oscilloscope, I find signal leakage between pin-13 and pin-14 at the ribbon cable.

This leads me to suspect the control panel as being the prime suspect.

I remove the controller board assembly to get at a ribbon cable junction PCB below the controller assembly.
There are three ribbon cables connected to that PCB labelled P3. I disconnect the cables and search for a high resistance bridge between pin-13 and pin-14.

Kenmore Dishwasher5.jpg

Lo and behold, the problem is not in the console control panel!

The problem is in the ribbon cable that was connected to P1 on the control board.Kenmore Dishwasher6.jpg

I examine the tracks on the ribbon cable very carefully. The tracks and space between tracks look clean.
The only conclusion is that the bridge must be under the black plastic connector.

There are two ends of the cable. The bridge must be at one of the two ends, but which end?
These connections are not meant to be disconnected. There are metal tabs that hold the connectors in place. There are fourteen of them.

With the ohmmeter connected to pin-13 and pin-14 I try to wriggle the cable and see if I can determine which end is bad.
The reading is all over the place. Before I was reading 500kΩ. Now it is anywhere from 20kΩ to 5MΩ.

I take a guess and choose one end.

With a thumb tack I depress 14 metal tabs and manage to withdraw the connectors from the plastic housing.

Kenmore Dishwasher8.jpg

Bingo!
I see the crud between pin-13 and pin-14.
Kenmore Dishwasher7.jpg

I clean the ribbon cable with isopropyl alcohol.
After I let it to dry I cover it with conformal coating. I don't want to see this one again.

I put everything back together and this 10-year old washer is working again.
(I was getting tired of handwashing dishes after the third day.)

Thanks for watching.
 
Top