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.
Interesting to read about a real bug in a circuit, but perhaps the problem was the bad pump and the insect didn't have any effect. Who knows; it's a good story.
Grace Hopper, a US Navy admiral, worked on many early computers and taped a moth in one of her lab notebooks. The story goes she found the moth between relay contacts in a Mark II computer, noted the problem, and told people she was "debugging" the circuits. Find more info and a photo of the mounted bug here: http://en.wikipedia.org/wiki/Grace_Hopper.
While inspecting circuit boards a friend of mine spotted an insect on the reverse side of a board. He turned the board over, but no bug. It was sandwiched between the fiberglass sheets used to create the substrate. I think I have a 35-mm slide of the encapsulated bugaround here somewhere. The board was "buggy," but thankfully the creature didn't affect any of the circuits.
While I agree with you, (lucky to be engineers) I've been forced in many instances to rise to a higher, broader perspective, and know firsthand that the general public (family members included) don't always share that perspective.A colleague of mine has a book on his desk entitled Design Engineering, and depicted on the cover is a bridge spanning a great chasm; one side of the bridge says "Design Engineers" and on the opposite reads "The Rest of the World".Yes, we are set apart, and by our perspectives, happily so; but many times from the perspective of the "The Rest of the World" it's their choice to isolate us.Everything is relative. But that's just an engineer's opinion.
Oh, and about the dishwasher – funny story;too bad the insect wasn't the real bug; that would have been poetic!
This story makes me wonder about the filters: why aren't they designed to prevent small items like pistachio shells and broken glass shards from entering the pump, or for that matter, from ever leaving the dish compartment in the first place?
I had a pistachio shell that made its way to the pump. The noise it was making suggested a bearing had gone bad and that a new pump was in my future. I'm glad I decided to open the machine up and take a look before ordering a pump. That made me wonder about the people that don't own screwdrivers and what they would do. Call repair service and buy a new pump? We're all lucky to be engineers.
How long before the pump stopped working was the broken glass discovered ? Did that person not look for all of the pieces ? And how small was the piece that blocked the impeller ?
Kudos to you for continuing to search for the culprit. I have to admit while I haven't experienced the bug problem, we've definitely run into similar issues with glass and other elements being trapped in places that block water flow and essentially wreck havoc on the dish washing cycle. Given that I hardly expect we're alone in this probem, it seems to me, a bit more engineering time could be spent on coming up with a better design for dealing with debris so it doesn't interfere with the unit's operation.
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