Dishwasher Overflow Protection - Active Systems

[brisnat81]

Hi All,

My Mum rang during the week, to advise that her Mark 8 Dishlex Dishwasher from 1980 had developed this annoying habit, of running constantly as soon as power was applied. Regardless of what you do, where in the cycle the timer is, whether the door is open and whether the Knob is in or out, water enters the machine whilst pumping out.

My first thought was the timer had failed. I had a spare, so this afternoon I went over and swapped it in. Plugged the dishwasher in and the same problem occurs.

I took the front panel off and sat down with the wiring diagram and had a close look. The machine has a 3 level pressure switch, low, normal and high. I thought to myself, what sort of short, could be causing both the motor the drain, and the inlet valve to open.

On closer examination, the active wire, splits into two at the terminal block. 1 line goes to the master switch on the timer and then the door switch, the other line goes to the high level terminal on the pressure switch.

If the high level on the pressure switch is satisfied, it bypasses the timer switch and door switch and starts the drain pump.

I took my multimeter and low and behold, every terminal on the switch was shorted together. Thus why the machine is draining and filling at the same time.

Problem solved, I've ordered a $100 pressure switch kit and in a few days the machine should be back to normal.

This got me thinking though. What this means, is that there was an active antiflood system on a dishwasher design, going back 30 - 35 years.

For example, During a cycle, the machine gets a small piece of grit in the inlet valve, its a slow leak and it happens late in the cycle. The machine finishes, you unload and nothing appears unusual. During the night, the small leak continues. When the pressure switch reaches the High level, the machine turns on the drain pump and runs until the pressure switch resets. In theory this cycle continues indefinitely and prevents flooding.

If the valve failed catestrophically during the cycle and didnt close at all, when the pressure switch hits the high level, the wash motor stops and drains all water from the machine.

What surprised me about this system, is that it should work whenever power is connected to the machine, regardless of whether the machine is turned on or not.

I know this isnt a fully active, hose has broken water running out of the bottom protection system, but it took 1 extra wire and 2 extra terminals on a pressure switch to protect against inlet valve failure.

My question to everyone, does anyone know of another pre 1980 dishwasher design, that will stop and drain if an overfill is detected regardless of whether it is on or off?

When I fit the new pressure switch, I'll take pictures.

Have a nice weekend

Nathan

 

[toggleswitch]

Fascinating. As far as I know our US manufacturers' machines have no such feature.

Looking forward to seeing more!

 

[tomturbomatic]

Up to the 14 series, KitchenAid machines had an overflow pipe that was connected to the drain pump (or to the drain line in gravity drain machines) so that if the machine overfilled, the water would go down the overflow pipe at the front of the tank and into the separate drain pump powered by the lower part of the motor shaft. The drain pump was in operation anytime the motor was on so any water that went down the overflow pipe into the drain pump chamber was pumped out. The machine had a drain valve which opened to allow water in the tank to enter the drain pump during the drain sequences.
Most domestic dishwashers went to an emergency float that cut power to the fill valve if the machine filled above a safe water level. One of the easiest things to check in a machine that won't fill is to check under the safety float switch to see if something is lifting it. Normally, it can be lifted a fraction of an inch and you will hear the switch click. If it is stuck in the up position, you will not be able to lift it or hear the click of the switch.

 

[andrewinorlando]

The 1970's/1980's GE Potscrubbers had the same type of system. If the water level float detected that the machine was overfilled, the drain valve opened and began flushing the water down the drain until the water level inside the machine dropped or someone turned the water off. It was a neat system, and I'm sure saved some folks from some expensive home repairs. Of course, with those machines, the pumps ran continuously throughout the cycle, so it was just a matter of activating the drain solenoid, based on the position of the float, to divert the excess water down the drain.

 

[bajaespuma]

Now that you mention that, I do so remember the loud "chunk" sound of the GE dishwasher's drain solenoid activating.