One recent morning, I opened my dishwasher expecting to find clean, dry dishes, but instead I saw that the water had not drained.
I removed the top panel to expose the control panel and dishwasher controller. The dishwasher cycles were driven by a timer mechanism with contacts that opened and closed as the timer rotated.
Behind the panel, I also located a full working electrical diagram on which I located the drain pump and the signal wires that drove the pump. On the exact set of contacts that controlled the drain motor, I found the remnants of some type of water bug that had made its way into the timer and decided to reside in the contacts. I decided this unfortunate creature had shorted out the contact with another contact and shorted the motor.
After cleaning the contacts, I emptied the washer basin and pulled out the unit. The drain pump was easy to access, and I pulled off the pump and hooked it up to an auxiliary 120V power source, which still did not turn the pump, so I bought a new pump and installed it. The pump worked, and I reinstalled the unit.
The problem was solved until three days later, when the water again did not drain. I checked the contacts, and there were no bugs, so I pulled the unit and pump out again. This replacement had a removable casing covering the impeller blades. I removed the casing and found a piece of glass that was blocking the blades from turning.
I removed the glass, and the blade moved freely. I also checked the original and found another piece of glass. I then checked the trap on the bottom of the washer and found additional pieces of glass that had passed through the debris filter. I removed these pieces and re-installed the original pump.
It has worked great since this last repair. Evidently, a glass had broken during the wash cycle at some point, and the broken pieces had worked their way through the system, jamming the pump. This was one situation where an obvious problem was not the actual problem.
This entry was submitted by Tim McNulty and edited by Rob Spiegel.
Tim McNulty is an engineering graduate of Penn State University. He works as an engineer in the plastics manufacturing industry specializing in injection molding.
Tell us your experience in solving a knotty engineering problem. Send stories to Rob Spiegel for Sherlock Ohms.
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