KUDD23 saves the day

[johnb300m]

So I noticed some washability and drying issues creeping up shortly after re-installing the KUDD23.
After some monitoring and temp measurements, I noticed that the main wash was just not heating up in the main wash like it used to.

Previous observations showed that the thermal hold would trip around 135F and by time the rest of the 6-7min wash period was advanced, the water was about 140F.
This time, I noticed that the thermal hold would not hold anymore in the main wash, or maybe hold for like a minute, then advance. Leaving me with end-of-wash temps only around 130-135F.
With some ChatGPT sleuthing, and knowing this original thermostat was over 30yrs old, it was looking like it just reached end of life, and GPT said that these snap-disc bi-metal thermostats fatigue with old age, tripping earlier than required.
I was able to confirm this by filling the machine, letting the water cool to below 100F, and letting it run a wash cycle. It did thermal-hold and heat till about 115F, and then it advanced, giving me water temp of just over 120F after the main wash.

So I started a hunt for a new thermostat.
I tried looking for other OEM dishwasher thermostats, but by this time in history, they're all only "high limit" thermostats 180F to over 200F. Everything has moved to digital thermistors now.
I found some HVAC snap disc thermostats, and found one that's in the "normally open" 120V range with my amperage, and it has a 140F trip point. Which means I should be able to get at least 140F or more, water temp in the main wash again.

• So I got this model:
  https://www.supplyhouse.com/White-R...trol-Cut-In-140-Degrees-F-Cut-Out-120-Degrees

When I ran it for it's initial test, I certainly got my thermal hold, but it was for over 25 minutes! And the wash water reached reached about 160F by end of wash period. In my research, I know these snap discs can have a tolerance range of +- 5-7deg, but I didn't expect it to be that wide.
I also thought there might be some thermal lag, even though the steel tub very quickly takes on the temperature of the water.
So I got another one, at lower temp, and what's neat bout this one is it's adjustable.

• https://www.supplyhouse.com/White-R...-F-Includes-Tab-to-Screw-Terminals-14699000-p

This one adjusts from 90 - 130F with a screwdriver dial on the back. I figured, if there's an amount of thermal lag, then 130F should be a good max.
When I installed it, I set it to 125F, so as to avoid any temp overruns again.
Lo and behold, when I ran another test wash, it was cutting out right at about 125F, and only getting to the low 130Fs again at end of wash. So this thermostat was more accurate, and maybe I didn't have the thermal lag I thought.
I bumped up the temp setting to 130F, the max this unit will now no, and re-ran the test.
It advanced from thermal hold almost as early as before, not getting to the ~140F I wanted.

At this point, I thought, ok, I don't really have a thermal lag problem, I'm either fighting a switch tolerance span, or maybe I didn't mount the 140F t-stat properly. So I re-installed the 140F, and ran it again last night, in fact.
I got similar results as with the first time. The thermal hold ran for over 25min, and by the end of the wash period, the water was 154-155F.
Just a tad too hot for what I'm consistently looking for.
This confirms to me, I think, that the tolerance on this 140F unit is almost 10F too high on the + side.

Mounting the Thermostats
Something I didn't mention yet is how I'm holding these vastly different thermostat models.
I had to devise a 3D printed holder that would mount with the current thermostat mounting bracket under the tub.
You can see that here: This is actually Rev. 2, to hold both versions.


These are photos of Rev 1:


Here is the photo of model Rev 2, that mounts the same way:
I even was able to get the legit KA logo typeface on this one.



Data
Here's the data recorded from the various tests.
If you do a little math, you can see that the 800w heater is working well, and heating the tank of water about 1deg, per minute.



Steps Forward
I think I'm going to look for yet another thermostat that can get me more at the target ~140F temp I'm looking for.
I'm not really in favor of running this 30+yr old machine to 155-160F temps on a regular basis. Seems harsh and I don't want to subject some of my dishes to that, nor pay for that energy constantly.
If I can dial this in, this will get me good 140-145F washing temp, and 20-25min total main wash times, which is what seems to be in-line with OEM design.
White Rodgers has another series of adjustable thermostats that go from 140 - 180F, I could maybe try that and hope it's tolerance range is tighter and closer to the 140 limit.
Or get another 140F single temp thermostat, hope the tolerances are better on that one.
Or keep searching for different types......

 

[combo52]

Fun problem you might try putting some heat paste for better conductivity between the thermostat and the bottom of the tank. I’ve also had luck when we had trouble with whirlpool dishwashers that ran half the night by putting insulation under and around the thermostat so it’s sensing the tank temperature more than the ambient temperature. You might find one of these tricks will help you get the results you want.

Whirlpool used to recommend putting insulation under the thermostat in a service pointer when there were complaints of the dishwasher running half the night when the area under the dishwasher was cool or cold because of leaky sea around walls and floors, etc.

John L

 

[neptunebob]

When did Whirlpool stop calling KA dishwashers KUD? I think the last was KUD25, then they went to the tall tubs with different model numbers.

 

[johnb300m]

@combo52 Hey thanks for the tips.
I was actually thinking the same thing, of getting some thermal paste to see if that would help.

I did a couple extra tests though, I can't believe I didn't think of this sooner....
I wanted to, in a controlled way, figure out when these thermostats were closing (heating limit).
So I took the old OEM one, and the adjustable one and set them on a cast iron pan, for it's measured heat conduction, while measuring temp with a IR thermometer. Controlling the temp rise of 1deg over at least 30sec, I was surprised to find that the OEM thermostat was triggering regularly around 125F, which would not allow the water to get to 140, with the remaining 7min after that trigger point.
The adjustable thermostat, which can be set up to 130, was triggering regularly just above 125 to almost 130F, which is well within it's +- 5deg tolerance spec. But that's still not enough for it to get to 140 with the timer programming.
What I learned is, that the pan test is very much close to the trigger points I've observed in the machine, so I think I'm getting good contact sensing as-is. So I will forego the goopy thermal paste.

I think a thermostat at 135F would be PERFECT, however, I cannot find one at all. 120, 130, and 140 seem to be the only intervals.
The specs on the thermostat I have that's a 140 unit, has a tolerance of +-5deg also, but it's regularly triggering well above 140, closer to 150 or more.

What I did is order a new 140F thermostat, hoping this one will be better calibrated. And then if the new one works better with the pan test, I will swap that into the machine and re-test installed. Then I can return the faulty one and hopefully be on my way.
If for some reason I don't get the behavior I want, I can maybe play around with using some kind of plastic insulator between the tub and thermostat, and use my adjustable one that goes down to 90F, and do some kind of temp gap correlation between trip temp and water tank temp with the insulated offset.
Sort of like they do with sensors and plastic tub dishwashers.

I did another data gathering on a Normal wash this morning.

Wash time: 42m 58s
Start Temp: 118F
End Temp: 157F

 

[Chetlaham]

The specs on the thermostat I have that's a 140 unit, has a tolerance of +-5deg also, but it's regularly triggering well above 140, closer to 150 or more.

What I did is order a new 140F thermostat, hoping this one will be better calibrated. And then if the new one works better with the pan test, I will swap that into the machine and re-test installed. Then I can return the faulty one and hopefully be on my way.

I would try go this route as it is the least complex.

If the new 140*F thermostat has a similar or higher cut in than the previous 140*F White Rodgers thermostat I would re-install the previous 140*F White Rodger thermostat and leave it as is.

The point I've been making all along: a 150-155*F main wash will not harm anything while giving the benefit of increased wash performance.

As is the thermostat will probably begin cutting in at a progressively lower temperatures as it ages so that would also be taken into account.