# Dishwasher Thermostat



## gsegal (Jan 27, 2013)

I'm trying to repair my daughter's dishwasher. It's a Frigidaire FDB750RCC0. The schematic shows the WASH boost thermostat connected to the Yellow/blk wire and when I pulled it out it was the 154227805 thermostat. The schematic shows the RINSE boost thermostat connected to the orange wire, which was the 154227808 when I pulled it out.

On another forum I found a post that stated the Rinse thermostat was the 154227805. Can anyone verify which is supposed to which?

Next I need to know what the trip temperature for each of these thermostats should be? I have no intention of paying $60 for a $7 themostat (the other one is $15). I can order any temp replacement I want for about $7 from digikey, but I need to know what the trip points are supposed to be.

Thanks for any help!
-G


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## hardwareman (Oct 9, 2010)

why are you replacing thermostats?


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## gsegal (Jan 27, 2013)

I was really just looking for an answer to the two questions I posted:
1) What are the trip points for the 805 and 808 thermostats
2) Which one is supposed to be the RINSE boost thermostat (connected to the orange wire).

If it helps me to get an answer to those two questions, then I'm replacing the thermostat because it's defective (i.e. timer halts waiting for thermostat to switch on). Yes the heater element and high temp thermostat are fine. I need to know the specs on the thermostats so I can test the precise trip point of each of the two boost thermostats, to verify if one/both need to be replaced, and also which replacement to order (I can get pretty much any temp trip point I want in the same case).


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## gregzoll (Dec 25, 2006)

These are all of the thermostats for that model http://www.repairclinic.com/Frigidaire-Dishwasher-Model-FDB750RCC0-ID-623375-Thermostat-Parts

Also there is no way you are going to find that one thermostat that is $60 for anything cheaper, unless it was pulled out of a old unit.
http://www.repairclinic.com/Frigidaire-Dishwasher-Model-FDB750RCC0-ID-623375-Thermostat-Parts


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## hardwareman (Oct 9, 2010)

normally the temps are printed right on the disc.


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## gsegal (Jan 27, 2013)

So the thermostat that was connected to the orange wire (RINSE boost) is marked as follows:
36TMV02 21442 S0341 
F122-25F 154227808

The other one is marked:
36TMV02 21439 S0343 
F117-25F 154227805

So do you think the F122 and F117 are the temp trip points,
i.e. 122 degrees F for the rinse and 117 degrees F for the wash?

Does it make sense that the rinse would heat to thigher temp than the wash?

If this is the case, then I would be an idiot to pay $50 more for the one thermostat, seeing as that's only a 5 degree difference in trip point.

I can get a 122 degree F thermostat for $8 from Digikey. They also stock a 110 degree F. So I will likely go with either another 122F, or the 110F as a replacement for the 117F. Again, I am dumbfounded as to why they would have a different thermostat for the rinse vs wash, especially within 5 degrees!? And even more curious as to why there is a $50 price difference (they are the same series part so is must be supply/demand). I guess their engineers have never heard of DTC.


Thanks!


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## ddawg16 (Aug 15, 2011)

Does your heated dry work? 

I just replaced the heating element in my dishwasher.


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## gregzoll (Dec 25, 2006)

It really depends on that thermostatic device. Yes, it could be that not too many people purchase that one item is why it costs more, it could also be the internals, which makes it more expensive than the other. Keep in mind that ordering parts through a non-certified supplier, that the manufacturer does not certify for their parts, you are leaving it in your own hands.

If the machine is over 5 years old, then you can find out that you are going to continue having to make repairs on it.


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## gsegal (Jan 27, 2013)

As stated, the heating element is fine (as would be the over-temp thermostat, for the element to work). The issue is a defective temp boost thermostat which will leave the timer un-powered until the heater heats up the water sufficiently to trip the boost thermostat.

Over this small a temp range the guts of these thermostats are nearly identical (simple bi-metal disc switch) - certainly nothing that would drive any cost difference, other than the usual supply & demand.

As I said, why a manufacturer would choose a less common part that is 5X the cost to get a few degrees different for a rinse cycle compared to the wash cycle is beyond me. I'm ordering two of the 122 degree thermostats for $7 each and the wash and rinse boost will heat to the same temp.

That unit is about 5 to 7 years old based on the house age. My own Kitchenaid dishwasher just past 10 years and I have only replaced the water solenoid in that unit.those And the piece of junk GE I had in my last house went 17 years without me ever having to open it up.

In fact, I sold my last house of 17 years with all the original (low-end) appliances (washer, drier, dishwasher, oven, A/C) still operational. And I'm now 10 years into my second house which I equipped with higher-end appliances. They all still work, but I did have to replace a $5 starting disc in the fridge and a control panel in my double oven. ($400 was way too expensive for a keypad & controller - next time I vow to build my own oven controller using a $100 Android Tablet - plus then I can surf the internet while I cook).

Zoll, you sound like the salesman at Sears when I built my new house ;-)
... I told him how my original Kenmore washer & drier were still on the job and without blinking he told me "no way these new units will last you that long!" I told him he should consider another line of work.

Now for my rant: They ought to flog the moron that would design that dishwasher circuit since leaving the heating element on indefinitely can be dangerous (yes I realize there is an over-temp thermostat, but relying on single point safety to prevent your house from burning down is just stupid - my friend's Maytag recently melted the racks inside). There are safer ways to boost the temp (I can think of one example with no additional parts that would have two failsafes to prevent an overheating condition).

-G


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## JasperST (Sep 7, 2012)

So you think the design is dangerous but you are willing to use cheap knock off parts?


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## hardwareman (Oct 9, 2010)

have you even tested these thermostats to determine which one if any is bad? Where does timer stick, in rinse or wash?


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## paintdrying (Jul 13, 2012)

The designs for the dishwashers are stolen for other designers, they are reverse engineered by unqualified engineers. Different engineers are working on different parts of your appliance. To the dismay of my family I will only run the dishwasher at the start of dinner. I realize these dishwashers are fire hazards. It will take a cute little blonde girl to die before our government is actually willing to protect us from something that may actually harm us.


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## gregzoll (Dec 25, 2006)

Wow, looks like someone forgot to take their meds today.


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## hardwareman (Oct 9, 2010)

yeah, that one is a head scratcher. Maybe to many paint fumes.


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## gsegal (Jan 27, 2013)

JasperST said:


> So you think the design is dangerous but you are willing to use cheap knock off parts?



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 ;-)


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## gsegal (Jan 27, 2013)

hardwareman said:


> 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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## JasperST (Sep 7, 2012)

gsegal said:


> 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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## hardwareman (Oct 9, 2010)

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.


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## Protocol. (May 31, 2012)

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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## gsegal (Jan 27, 2013)

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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