Bosch Dishwasher Fills then Drains Repeatedly

[TechnicianBrian]

Model #SHU43C06UC/17, this dishwasher would fill with water and then start to drain over and over, but would never start the wash cycle. When I arrived, I could hear the fill valve come on, then shut off and the drain pump would start to run. This continued until the unit was shut off or the door was opened. Thinking I was dealing with a flood switch problem, it was time to pull the unit from the cabinet and take a look.



This dishwasher uses a fill valve like most other dishwashers, but instead of the control board or timer turning on the valve for a set period of time, this one will turn on the valve and wait for a pressure switch to close indicating the tub has filled. There is also an overfill switch used to prevent flooding and to detect water under the unit if there is a leak.

Water from the valve will enter an inlet system which contains a pressure switch along with a mechanical float for the overfill switch. Water continues on into the dishwasher sump assembly filling the tub from the bottom. As water enters the tub, the water level inside the level assembly will rise accordingly until the pressure switch closes shutting off the fill valve and starting the wash cycle. If to much water were to enter the tub, the overfill float will rise, actuating overfill switch, which will close shutting off the fill valve and turning on the drain pump.



With the unit pulled from the cabinet, and the side panel removed, it was easy to see what was making all the noise, and I could visually see how it was happening. Because the pressure switch was not closing, the overflow switch would close shutting off the valve and turning on the drain pump. Very quickly, the drain would lower the water level within the tub enough to open the overfill switch turning the fill valve back on. This pattern would just continue, but the cycle would never start.

Using my meter, I did a quick resistance check of the pressure switch and found it was no longer working. But after I installed a new level switch assembly (the switch is part of the whole unit), the dishwasher would fill with water, and then wash the dishes.

[Penguin45]

Hi Brian,
We see loads of these type of problems over here. It's normally caused by the transfer hose (bottom right of the piccie) becoming choked with food deposits. it causes the water to back up into the pressure chamber. A quick look at your vid also suggests a very high fill pressure - the machine may be unable to cope with the flow rate. The pressure switch area looks absolutely spotless - which is good!

Penguin45.

[JWWebster]

Penguin do yall replace the whole thang or just clean the food deposits out?

[TechnicianBrian]

Hi Penguin,

We see the same thing and often the result is water leaking from the air gap and running into the bottom tray.  Add on a high pressure water source and the fun begins.  I usually pull the hose from the sump and flush out the gunk just to save me some hassles in the future.   Would be such a big deal if everything was easier to get to.

Brian