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Old 02-01-2013, 02:30 AM   #16
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Originally Posted by hardwareman View Post
have you even tested these thermostats to determine which one if any is bad? Where does timer stick, in rinse or wash?
My daughter said it was hanging in two places in the cycle. Based on her info of how far into the cycle it hangs and looking at the cycle diagram, I believe it is hanging in the two rinse cycles. First thing I verified was the heating element, over-temp switch and timer - all were fine.

To verify the thermostats I needed the expected trip temps, thus the reason for my original post.

I tested the thermostats using a pot of water with the thermostats sitting in a stainless steel measuring cup with the cup partially submerged in the water (so the thermostats were dry and the only heat hitting the thermostats was conduction from the water through the steel cup.

The thermostat labeled F117 tripped at 122 degrees F, which seems reasonable assuming the F117 marking indicates a 177 degree F trip temp. The one labeled F122 tripped at 136 F, which seems a little high.

So there are a few possibilities: 1) I couldn't see exactly how they thermally coupled the thermostats underneath through the plastic tub, but possibly there is too much thermal resistance. 2) Bi-metal discs can be temperamental and possibly my heat test may have thermally shocked the unit into working properly. 3) 136 F might be too high for that dishwasher to achieve.

My plan is to replace the F122 with a Cantherm 122 degree F thermostat (same package) and use a graphite thermal pad to be sure there is good contact to the mounting location.


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Old 02-01-2013, 07:01 AM   #17
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Originally Posted by gsegal View Post
Where did I say I was going to use "cheap knock off parts"? Cantherm is one of the leading manufacturers of thermostats - nothing "cheap" or "knock-off" about them. They are inexpensive, not because of their design, but because I am choosing to buy them from an electronics supplier rather than from an appliance parts house. The price has nothing to do with cost to manufacture or quality, and everything to do with supply & demand.

I suppose there are people out there who may sleep better at night knowing they paid $60 for a thermostat - expensive has to be better, right? Just don't tell them the manufacturer of that thermostat sells the exact same part to a different market area for $7.

And yes, I absolutely characterize a design that can allow a heating element to remain energized indefinitely, in an appliance that is commonly set to run while people are sleeping or out of their homes, as "dangerous". Particularly since the timer could easily be used as a secondary failsafe with zero impact to cost or parts count. But, I could be wrong since I've only been working as an electrical engineer for 30 years ;-)
I went by your comment that you can buy a $60 replacement part from someone for $7. Replacement part isn't the same meaning as same part. I don't know if it is or not, just beware that there are many Chinese knockoffs for much cheaper but the quality isn't always there. For some things the gamble is worth it, sometimes not.

If you are an electrical engineer you could install some kind of thermal fuse, seems like it would be worth it to me.

I, for one, do not run any appliances while I'm away or asleep.
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Old 02-01-2013, 08:08 AM   #18
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good luck to you, I hope that solves the problem but if its hanging in 2 places I have to believe there may be another problem somewhere. I don't believe it is a thermo problem but keep posting with your results. If that does not solve the problem upload the wiring diagram here.
................."I want a house that has gotten over all its troubles. I dont' want to spend the rest of life bringing up a young and inexperienced house"...........
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Old 02-01-2013, 09:54 PM   #19
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I doubt 140 is too high for the unit to achieve. Most dishwasher detergents are designed to only work at higher temps. Please keep in mind many models of dishwashers will actually wait for the water to come up to temp for rinse and sani. Most of them can wait for up to 45 minutes.

How many watts is the element putting out? Elements, especially in water can be tempermental. What kind of voltage do you see? Is there any voltage drop when the elements engage? Current draw?
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Old 02-02-2013, 10:16 AM   #20
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The mfgr's cycle diagram shows a wash-rinse-rinse-wash-rinse-rinse-dry sequence (for all but the light cycle). So the wash thermostat could cause the timer to hang in two places and the rinse in up to 4. The rinse cycle are in pairs, so 4 stuck points may have looked like only two to my daughter.

I verified the heating element has continuity as well as the over-temp switch. I asked my daughter if it was putting out heat and she said it was and there was steam venting while it was hung. The duration it hung for was multiple hours (they were out of the house a couple of times), so it wasn't just a matter of more time.

I suppose there is some chance the heater is on its way out and can't put out enough power to boost the water temp high enough. I didn't measure the current draw or the water temp sitting in the tub while it was in the boost pause (the latter is probably the easiest and most informative test). But I don't live there and I already have the thermostats out so I figure I'll replace the high one for $7 just in case it was stuck/intermittent.

I wonder how hot the water in a tub would eventually get with a working heater? With such a large surface area, maybe not super high? If it's only 150 or so, then possibly it is the thermal resistance between the thermostat case and the water in the tub. I reached under the unit to remove the thermostats so I couldn't really see how they conduct them to to a plastic tub. A metal plate maybe? To be safe, I have some graphite thermal pad material I plan to put between the thermostat and mount.


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