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Power cutting out intermittently on Neff/Bosch Dishwasher S58M40X0GB/07

Discussion in 'Troubleshooting and Repair' started by sebwills, Nov 14, 2020.

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  1. sebwills

    sebwills

    2
    0
    Nov 14, 2020
    Hello,

    I have a Neff S58M40X0GB/07 integrated dishwasher (same company as Bosch I believe), manufactured 2012.

    It has developed the following fault. About 1h45m into the cycle (usually when there is 1:05 remaining), the power cuts out completely. If left to its own devices at this point, the power cycles on and off with approx 2 minute period (1 minute on, 1 minute off). Since it remembers that its mid cycle, it tries to resume, starts running a drain pump, but gets cut out before it can make progress, so is stuck. The machine plugged into a normal mains socket and the rest of sockets' supply is normal, so the issue is internal to the dishwasher.

    If I open the door while it's in this power-cycling state, the power continues to cycle on and off, even though the machine is not trying to continue the cycle (because the door is open). i.e. the display comes on and the controls respond normally for 1 minute, then it cuts out completely (blank display, unresponsive controls). This cycling will continue for a long time (hours), even after the dishwasher has cooled down etc, and even with the door open so the machine isn't trying to continue working.

    If I switch the machine off using its 'soft' power button during one of the 'on' periods, it remains off. If I then turn it on again the power cycling continues. However if I leave it off for several hours, before turning on again, it will sometimes run for a longer period. The next day, for example, it's able to start a new cycle and run for 1h45m before resuming the error state.

    My initial assumption was that some component that is activated at a particular point in the cycle is causing it to cut out - especially since it always seems start at the same point in the cycle - but it's not quite that simple because the 'cycling' continues even with the door open and the machine not attempting to do anything.

    Could it be an error condition that the controller is detecting and deciding it should cut the power completely rather than display an error code as it usually would? And what would be causing it to turn on again after one minute? Or is it more likely an analog electronics issue with the power switching circuit?

    I have dissembled the machine and removed the Melec 9000 683 387 control PCB. Visually it looks fine. It has a TNY264GN power switching chip on it (as discussed here https://www.electronicspoint.com/forums/threads/pbc-bosch-dishwasher-spi50e05eu-07.289432/ )

    There are 100uF 400V and a 470uF 16V electrolytic caps on the board. They look OK, but is it worth replacing them speculatively? What about the (more expensive) S14300K varistor?

    Any suggestions? Thank you!
    Bosch PCB small.jpg
     
  2. Tha fios agaibh

    Tha fios agaibh

    2,115
    713
    Aug 11, 2014
    Yes very likely.
    It sounds like the power supply is being overloaded or its faulty and unable to deliver enough current.

    Its also likely that the drain motor tries to run even with the door open to prevent flooding.

    I'd check the pump motor and monitor its voltage when in use.

    Sometimes adding jumper wires to the pcb helps to monitor voltage and current remotely.

    The caps are worth testing because their essential for the smps, but the varistors are not likely a problem.
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    10,801
    2,435
    Nov 17, 2011
    Also verify the issue is not on the mechanical side. If the drain hose is clogged, the drain pump may not be able to remove water as quickly as it should.
    Leaving the machine sitting for a few hours, enough water may have seeped through the blocking collection of debris and the water level sensor may indicate successful draining.
    Just speculating, however.
     
  4. sebwills

    sebwills

    2
    0
    Nov 14, 2020
    Thanks for the replies. I don't have a cap tester but will replace the electrolytics just in case.

    I appreciate what you're both saying about the mechanical side, Harald, though I know the machine pretty well and I've never heard it doing anything at all with the door open. So still very puzzled why it would continue cycling on and off in that state.

    I guess my line of attack is
    1) replace the caps since that's cheap
    2) maybe try to add some jumpers so I can monitor the board while it's in the machine and operating. Pretty awkward though.
    3) failing those attempts, just hope it's something on the board and buy a replacement control module.

    I'll let you know how it pans out!
     
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