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greygeek

BOSCH SHU 4306 CU dishwasher drain/load cycling

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greygeek

The BOSCH SHU 4306 CU dishwasher (DW) is 20+ years young. It did develop very unpleasant  behaviour. Any of the available wash cycle starts  in correct way.  These wash cycles are: POWER SCRUB PLUS,  SCRUB WASH, REGULAR WASH, RINSE & HOLD.

Here are step by step description of events.

  1. press ON switch
  2. the DW  starts with the short drain sequence
  3. the DW starts with wash mode which was used in previous washing cycle
  4. water valve is enabled, water flows into the DW
  5. after while the big high pressure pump is activated and dishes are getting washed
  6. water continues to flow into the DW
  7. the safety floater starts to climb
  8. when it reaches the top point, the drain pump kicks in
  9. the water valve is disabled, water stops flowing into the DW
  10. in this moment, the water level is a little bit lower (see (9))
  11. the safety floater  goes down, the drain pump stops running
  12. the water valve gets enabled, the water is flowing again into the DW
  13.  steps (7), (8), (9), (10), (11), (12)  get repeated 15-25 times. Approximately once per second
  14.  then, there is no cycling for approximately 15-20 seconds
  15. after these 15-20 seconds, the cycles 7-12 are again executed
     

This  goes for ever.

If I interrupt the process,  by simultaneously pressing POWER SCRUB PLUS and REGULAR WASH buttons, the DW goes into the DRAIN mode. All the water is pumped out from the  DW and the CLEAN LED is activated.  I did expect that  at the end of the DRAIN process, the water valve will  be disabled. But it isn't. The water continues to flow into the DW.

Please, help out to debug and find the root cause for this unpleasant behaviour.

Thanks

 

 

 

 

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greygeek

Very likely.,I did find the real culprit. Why "very likely"?  Because, I did order a three(3) spare parts from DigKey but the parts are not (yet) in my possession. I n/eed only one part but I ordered three. Why?  Just in case that there is a another  real reason for the parts to blow. I will know it after I  replace the blown part. 

Cost?

55 cents per part + $4.50 shipping

The control module  is no longer available

Control Module Part Number: AP2804247

 

Based on the following schematic and observation/analysis of the failure sequence, I  came to the conclusion that the main control board is at fault. To be more specific, I did forecast that one of the  TRIACs is dead.

 

wf.png

 

triac.png

tt.png

blown_triac.JPG

triac_skatla.JPG

P1011149.JPG

 

So, here you have it. The TRIAC was dead. Plain and simple/

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greygeek

zoomp.png

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Ghashaan
16 hours ago, greygeek said:

Based on the following schematic and observation/analysis of the failure sequence, I  came to the conclusion that the main control board is at fault. To be more specific, I did forecast that one of the  TRIACs is dead.

So, here you have it. The TRIAC was dead. Plain and simple/

It's valuable information for electronic technicians. 

Please do share with us the results after replacing the triac. If it works, perfect! But there could be other damages components that are not too obvious to the eyes.

By the way, it's good to know that a 20+ years young DW is still running.

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greygeek

Yes, I replaced the blown TRIAC with a good/new one. To my disappointment, the malfunction of the DW did not go way.  I also figured out WHY.

  The GATE of the TRIAC should be triggered/driven  with a pulse.  I was not able  to find out from WHERE this pulse should come from. I am assuming it should be generated in the micro-controller or some other IC on the board. Since, I do not have the schematic of the board available and I do not have time to reverse engineer the whole control board, I gave up. If I artificially create a pulse on the gate, the new TRIAC works exactly the it suppose to work.  The WATER VALVE gets activated.

CONCLUSION: I have to buy a new DW.  I am not happy about this.  All other parts of the DW are in good shape. Just lack of the availability of the schematic, I have to throw away perfectly good DW.  For the curious souls.  The controller is labelled  as SIEMENS GV630USA.

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greygeek

Forgot to add.  The misbehaviour of the "fixed"  DW changed in comparison to original failure mechanism. The original failure was: WATER VALVE was ALL time ENABLED.  After the "fix",  the WATER VALVE does not get enabled because the TRIAC does not get triggered

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Ghashaan

Most of the times when a component blows up, surrounding components get affected too. I think the decision that you've made is right. It will take you forever to do a complete reverse engineering of the board. 

What kind of device did you use to send the simulation signal? Just curious as I'm trying to set up my own repair shop.

Even if you buy a new DW, why not ask a specialist electronics technician to have look at the board? A specialist could come up a with an easy solution

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greygeek
On 5/5/2019 at 2:37 AM, Ghashaan said:

What kind of device did you use to send the simulation signal? Just curious as I'm trying to set up my own repair shop.

This is not a rocket science. First, read the TRIAC's datasheet
You will see that MAX GATE voltage is 2.00V

How TRACs, in general, work?

Most TRIACs can be triggered by applying either a positive or negative voltage to the gate. Once triggered, TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current.

So, I applied 2V to the gate  using piece of wire connected to the 2.00 V source. I did not connect the wire permanently to the TRIAC's gate. I just touched the gate with a wire and immediately removed it.

In other words, you do not need to gave any special device to perform this test.  Replace Lamp symbol with WATER VALVE coil.
NOTE: the offending TRIAC in my DW controls the WATER VALVE.

power-power21.gif

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greygeek

fixing  typo gave == have

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Ghashaan

@greygeek Thanks! That's a good explanation. 

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