# Help with recurring thermal cutoff failure on electric clothes dryer



## T_R_Oglodyte (Sep 29, 2013)

We've got such a diverse community that I"m sure that someone out there will be able to offer in informed opinion. So here's the situation.

About one week ago the heating element on our electric clothes dryer stopped working.  The blower operated, the drum turned; everything operated normally except no heat being delivered.  This has happened a couple of times before when the heating element has failed, so I keep a spare heating element on hand.  But this time when I pulled the heating element it was fine.  The resistance was on spec, and just to be sure I compared resistance of the heating element with my unused element and the two matched perfectly.  

So I started looking at what else might be going on. For reference, here is the wiring diagram:






I determined that the 350 °F thermal cutoff had popped. It showed an open circuit.  I also verfied that continuity on the red/white back to the power supply and on the black back to neutral. So the only open element on the circuit was the cutoff. The high limit thermostat shows zero resistance. The reinstalled heating element shows resistance on spec.  

Replacing the thermal cutoff is an easy repair, so I ordered replacement parts - Whirlpool OEM.  The cutoff ships with a replacement hi-limit thermostat; the recommendation is that both units be replaced whenever one is replaced.  I made the replacements, reassembled everything, started up the dryer and verified that the heating element was working, and told DW that we were back in business. 

A couple of hours later she comes back to me and says that once again the dryer isn't heating.  So I open it up again, and determine that the brenad new thermal cutoff has popped.

There are only two things I can thing of here.  My first thought is that since the hi limit thermostat is supposed to cut out at 250 °F that unit might be defective, even though it's a brand new OEM part.  So I get out the electric skillet that we haven't used for years, set it to 260 °F, let it come to temp, and put the high limit thermostat face down on the skillet surface.  After taking about 30 seconds to come to temp, the high-limit thermostat pops open; I hear the click and confirm with the ohmmeter that the unit has gone from closed to open. So that appears to be working fine.  I also verify that there aren't blockages in the air flow from the dryer to the outside vent.  

Now I am at a loss to figure out what is going on.  Perhaps the thermal cutoff I received was defective, except that now two OEM cutoffs have blown in short order.  That indicates to me that somehow, someway, there are excessive temps in the heating unit. But that being the case, why isn't the hi limit thermostat shutting off the heating element first? 

The air heating unit is about 10-inches in total length.  The hi limit thermostat is located about three inches from the inlet end of the unit, and the thermal cutoff is about 1.5 inches further downstream.  What could possibly be creating a 100 °F temperature differential over that distance when there is proper airflow through the unit?

I am baffled.  Anyone have any thoughts?


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## csxjohn (Sep 29, 2013)

It seems this is a common problem.

"The thermal cutoff will trip if the venting is blocked, poor air flow or has a shorted to ground heating element. This could also be caused by a bad operating thermostat."  From Manage My Life dot com.

You've covered the heating element and air flow.  Could it be the operating thermostat malfunctioning?  Still doesn't explain how the problem gets past the high limit thermostat.

If that heating element is turning on before the thermostat tells it to, you won't have any airflow and plenty of heat.

http://www.managemylife.com/mmh/que...at-thermal-cut-off-bad-again-what-next-thanks


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## T_R_Oglodyte (Sep 29, 2013)

csxjohn said:


> It seems this is a common problem.
> 
> "The thermal cutoff will trip if the venting is blocked, poor air flow or has a shorted to ground heating element. This could also be caused by a bad operating thermostat."  From Manage My Life dot com.
> 
> ...


I thought about the operating thermostat, but based on the wiring diagram I can't see how that is the problem.  It looks to me as if the system is designed, both in wiring logic and physical arrangement, so that the hi-limit thermostat shuts off the heating element, and if that unit fails to operate properly the cutoff shuts the system down.  If the operating thermostat malfunctioned "on" the hi limit thermostat should catch that.

In any case, the operating thermostat is opening and closing on spec.

I didn't check for a ground short on the heating element.  I'll need to do that.


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## csxjohn (Sep 29, 2013)

In thinking more about this, if there is a lot of heat it could shut down the one and still have enough heat to melt the other.  Checking for the heating element shorting to the frame looks like the next stepl


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## Phydeaux (Sep 29, 2013)

Have you checked the venting, all the way from the origin at the dryer to its exit?


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## Sea Six (Sep 29, 2013)

Two main causes of that - the vent is clogged with lint, or the lint trap is obstructed, which also restricts air flow.  I'm told fabric softener sheets are a leading cause of blocking the lint trap.  Take it out and see how well water flows through the screen.  In some cases, the screen is so blocked up that water does not flow through it.  Also remove the collapsible vent hose from the back of the dryer and totally clean it out, including inside the dryer.


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## T_R_Oglodyte (Sep 29, 2013)

csxjohn said:


> In thinking more about this, if there is a lot of heat it could shut down the one and still have enough heat to melt the other.  Checking for the heating element shorting to the frame looks like the next stepl


Checked. No short.



Phydeaux said:


> Have you checked the venting, all the way from the origin at the dryer to its exit?


Did all of that. Even ran a pig through the duct to be sure there are no obstructions and ran the unit with no clothes to confirm air flow.



Sea Six said:


> Two main causes of that - the vent is clogged with lint, or the lint trap is obstructed, which also restricts air flow.  I'm told fabric softener sheets are a leading cause of blocking the lint trap.  Take it out and see how well water flows through the screen.  In some cases, the screen is so blocked up that water does not flow through it.  Also remove the collapsible vent hose from the back of the dryer and totally clean it out, including inside the dryer.


see above - did all of that.


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## AwayWeGo (Sep 29, 2013)

*Lint Build-Up In Dryer Vent Ducts Downstream Of The Dryer.*

The air has to flow freely not only through the dryer's internal airway but also through the external dryer vent all the way to the outside of the house.  

Otherwise it can get hot enough inside the dryer to cycle the safety thermostat & pop the 1-time-only thermistor. 

If all that checks out OK, then you're dealing with a thermal mystery. 

One unorthodox approach is to replace the fan housing thermistor with a safety thermostat like the 1 on the heater box.  That won't fix the overheating problem, but it will let the dryer keep on drying while keeping the heat level within the safety thermostat specs.   

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## T_R_Oglodyte (Sep 29, 2013)

AwayWeGo said:


> The air has to flow freely not only through the dryer's internal airway but also through the external dryer vent all the way to the outside of the house.
> 
> Otherwise it can get hot enough inside the dryer to cycle the safety thermostat & pop the 1-time-only thermistor.
> 
> ...



It's at the thermal mystery stage. What could cause a 100 degree change in temperature in the space of 1.5 inches in the heater box?


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## Passepartout (Sep 29, 2013)

T_R_Oglodyte said:


> It's at the thermal mystery stage. What could cause a 100 degree change in temperature in the space of 1.5 inches in the heater box?



Air leak? Perhaps the failure of the replacement thermal cutoff was just a defective part? I know how reluctant one is to just throw new parts at it- especially with electricals being non-returnable. Just a thought.


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## T_R_Oglodyte (Sep 29, 2013)

Passepartout said:


> Air leak? Perhaps the failure of the replacement thermal cutoff was just a defective part? I know how reluctant one is to just throw new parts at it- especially with electricals being non-returnable. Just a thought.



Air leak isn't the problem. The heater box is in good condition. 

********

But I think I may have figured it out.

I think the heater element may be on the verge of failure. When the heater fails the wire breaks. What happens is that as the wire breaks down the resistance progressively increases at the failure point. Which means there must be a localized hot spot in the heater box at that point where the failure is occurring. If the place where the failure is developing happens to be right next to the thermal cutoff that could trip the cutoff while everything else is working fine.

I do have a reserve heater unit so if I order a new cutoff I can replace both items and see if that solves the problem. If that doesn't work it's probably time for a new dryer.


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## AwayWeGo (Sep 29, 2013)

*Broken Heater Element Freak Heating Scenario.*




T_R_Oglodyte said:


> I think the heater element may be on the verge of failure. When the heater fails the wire breaks.


Sometimes the broken heater wire -- the wire that forms the actual tungsten-alloy heater element, that is -- breaks in such a way that a broken end makes contact with the grounded inside wall of the metal heater box.  That can complete the circuit for at least part of the heat element & thus make the element get hot even when it's not spozed to be on.   In that case, replacing the heat element fixes the problem. 

Good luck. 

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## T_R_Oglodyte (Sep 29, 2013)

AwayWeGo said:


> Sometimes the broken heater wire -- the wire that forms the actual tungsten-alloy heater element, that is -- breaks in such a way that a broken end makes contact with the grounded inside wall of the metal heater box.  That can complete the circuit for at least part of the heat element & thus make the element get hot even when it's not spozed to be on.   In that case, replacing the heat element fixes the problem.
> 
> Good luck.
> 
> -- Alan Cole, McLean (Fairfax County), Virginia, USA.​


Heater element is not broken.  As noted above, resistance is to spec. I also removed it and physically inspected.  Also checked ground short from both terminals.

My working hypothesis, though, is that there is a spot where failure is beginning to develop, which should create a localized hot spot as resistance increases at that point during operation.  And that hot spot would be adjacent to the thermal cutoff.  It's actually the only thing I can think of at this point.  And it actually does comport with observations.


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## ricoba (Sep 29, 2013)

T_R_Oglodyte said:


> .... If that doesn't work it's probably time for a new dryer.



You are obviously more talented and tenacious than I am.  I would have been scouring the Home Depot and appliance ads on line by now!


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## T_R_Oglodyte (Sep 29, 2013)

ricoba said:


> You are obviously more talented and tenacious than I am.  I would have been scouring the Home Depot and appliance ads on line by now!



Rick - right now this is personal between me and the dryer. And I'm not yet ready to surrender.


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## AwayWeGo (Sep 29, 2013)

*I Resemble That Remark.*




T_R_Oglodyte said:


> Rick - right now this is personal between me and the dryer. And I'm not yet ready to surrender.


Me neither -- & I don't give up often. 

But a few times over the years -- not many, just 4-5 times -- I have declared certain repair-resistant dryers unfixable after repeated attempts & have scrapped'm, saving the potentially reusable parts for future zombie dryer projects.  

Waste not, want not. 

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## csxjohn (Sep 29, 2013)

T_R_Oglodyte said:


> Rick - right now this is personal between me and the dryer. And I'm not yet ready to surrender.



I know exactly how you feel.  That is how I've learned how so many things work and how to fix them through the years.

After reading a few more articles I tend to think you are right and something is going on with the heater element.  Too many others have had the same problem for it not to at least be a possibility.

It could be getting so hot so fast that the one does turn off and the other melts pretty much simultaneously and of course the one resets itself and the other is fried.


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## ricoba (Sep 30, 2013)

T_R_Oglodyte said:


> Rick - right now this is personal between me and the dryer. And I'm not yet ready to surrender.



:rofl:LOL!:rofl: Don't give up the ship!


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## T_R_Oglodyte (Sep 30, 2013)

csxjohn said:


> I know exactly how you feel. That is how I've learned how so many things work and how to fix them through the years.
> 
> After reading a few more articles I tend to think you are right and something is going on with the heater element. Too many others have had the same problem for it not to at least be a possibility.
> 
> ...



Except it's been functioning for 20 years without that being a problem.  

But the more I think about my theory the more it makes sense to me.  I pulled the heater element, and the coil appears to be coated with oxides.  The coils are have a dark grayish to black coating - not bright metal.  And while the resistance across the terminals is to spec, if I measure resistance by touching the coils the resistance is about 1000x greater - unless I scrape the surface a of the coil a bit with the tip of the probe. Then suddenly the resistance drops back down to normal.  In contrast with the new element that I have on hand I can touch the element anywhere and get a good reading.

So it looks to me as if the heater element may be in it's death throes.  

I ordered new OEM parts, which should arrive on Wednesday.  At that time I'll put in the new cutoof, replace the heater element, and start it up.  If the thermal cutout pops, then I'll figure there is something much more fundamentally wrong, and I'll surrender.  

****

In researching I didn't see anything about imminent heating element failure being a cause of thermal cutoff failure.  Thermostat failures, yes.  Blocked duct, yes.  But not localize hot spot within the heating element.  

****

And I hear you about learning things through persevering.  At my age (where I now have my National Parks lifetime senior pass - obtained at Halaakela this last summer), I can look back and say that in my life I have learned the most where I have decided to burrow in and stick with something. 

Whether that had to with sales, parenting, screwed up lab experiments, personnel problems at work, technical challenges at work, or fixing machines, the process of burrowing has helped.  Even if it led me to a point of realizing that the best thing is to just simply start over from scratch, there has been great value in the process involved in reaching that conclusion myself instead of taking the easy way out and doing that in the first place.


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## AwayWeGo (Sep 30, 2013)

*Cleanliness Is Essential To Fixliness.*




T_R_Oglodyte said:


> I'll put in the new cutoof, replace the heater element, and start it up.  If the thermal cutout pops, then I'll figure there is something much more fundamentally wrong, and I'll surrender.


While it's all apart, it's a good idea to take the shop vac to it to snorkel dust & lint out from all the internal nooks & crannies.  

Even high-power shop vacs can't get it all by suction alone.  I take _el fixo_ dryers outdoors & attach the shop vac hose to the "blowing" port & aim the stream of air all over the dryer insides, specially at the motor from every angle I can.  Even after vacuum cleaning, I get a huge clouds of dust & lint out of the dryer by shop-vac blowing -- & not just clouds, but also chunks & tufts that the force of air dislodges from their hiding places.  So much lint goes flying far & wide that I always take the dryers outdoors for shop-vac blowing.  When I'm done, there's plenty of dust & debris on the ground to vac up.    

On any Sears-Whirpool-style dryer with top-access lint screen, I remove the lint chute & hose it out & dry it before re-installing it.  That means taking out 4 screws where the bottom of the chute attaches to the fan housing.  (The bottom of the chute forms the fan housing cover.)  It also means removing the 2 screws attaching the top corner of the internal bulkhead on the motor side so that the bulkhead corner can be pulled ahead enough to extract the loose lint chute.  

On newer models, with back panels that go all the way to the top of the dryer body & with just 1 screw at each bulkhead top corner, you can leave the bulkhead alone & take the lint chute straight out from the back after unscrewing it from the fan housing -- lots easier than on older models.  

While the fan housing it open, I like to remove the bobby pins, paper clips, brassiere underwires, collar stays, earring fasteners, screws, nails, etc., (if any) that have lodged inside, & clean out all the lint that I can from around the side of the fan housing.  Shop-vac blowing helps.

Let us know how your project turns out. 

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## Elan (Sep 30, 2013)

T_R_Oglodyte said:


> At my age,... I can look back and say that in my life I have learned the most where I have decided to burrow in and stick with something.



  Totally agree.  Twenty years ago, I'd be somewhat intimidated when it came to tearing in to things I knew little about.  Now, I'm just the opposite.  There's very little I can't fix, and I almost find it insulting if I can't fix something.  Two thoughts allowed me to cross the mental barrier associated with repairing things:  1) If I'm going to replace it anyway, it can't hurt to try to fix it first. 2) Even if I can't fix something, I'm guaranteed to have a better understanding of how it works when I'm done.  #2, in itself, justifies the effort, IMO.


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## AwayWeGo (Sep 30, 2013)

*I Semi-Resemble That Remark.*




Elan said:


> Twenty years ago, I'd be somewhat intimidated when it came to tearing in to things I knew little about.  Now, I'm just the opposite.  There's very little I can't fix, and I almost find it insulting if I can't fix something.  Two thoughts allowed me to cross the mental barrier associated with repairing things:  1) If I'm going to replace it anyway, it can't hurt to try to fix it first. 2) Even if I can't fix something, I'm guaranteed to have a better understanding of how it works when I'm done.  #2, in itself, justifies the effort, IMO.


By me, _Thought No. 1_ is all the justification I need.  That is, if it's already busted there's no harm in trying to fix it.  If I succeed, then it's fixed.  If I don't, then it's still busted -- but it was _already_ busted, so what's the harm ? 

We differ also in that I don't take it personally if I can't fix it.  And though I find I am able to fix lots of things, the limits of my _el fixo_ skill are far short of finding very little that I can't fix.  Music stands.  Gas & electric dryers.  Lamps.  My grandson's Harry Potter wizard wand.  Occasional miscellaneous items around the house or at our dinky non-traveling travel trailer.  That's about it.   

In my case, an added _Thought No. 3_ also applies, to wit*:*  I'm just an old retired guy with nothing to do & nowhere to go, so the success or failure of attempted repairs is _mox nix_.  

When I was young, all I needed was a hammer, a screwdriver, & a pair of pliers.  Now I need exactly the right tool for precisely the right application or I can't fix anything.  So it goes.  

The family I married into is extremely self-sufficient in DIY matters.  The Chief Of Staff & her sister grew up without ever encountering the concept of taking stuff in for repairs or of calling in home repair services of any kind -- plumbing, heating, air conditioning, electric, electronic, carpentry, roofs, windows, concrete, brick work, appliances, automobiles, you name it.  Their dad did it all, period, mostly single handedly.  That's how they grew up, so that's how they thought everybody grew up.  

Years later, they found that the DIY competence of their respective husbands never came close to matching that of their late father.  Their dad, my late father-in-law, was an example in temperament & willingness & responsibility as well as in wide technical skill & know-how.  He is still an example to (try to) live up to.  Most of whatever ability I have to fix stuff I gained 1 way or another from him.  May he rest in peace. 

-- Alan Cole, McLean (Fairfax County), Virginia, USA.​


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## T_R_Oglodyte (Sep 30, 2013)

AwayWeGo said:


> The family I married into is extremely self-sufficient in DIY matters.  The Chief Of Staff & her sister grew up without ever encountering the concept of taking stuff in for repairs or of calling in home repair services of any kind -- plumbing, heating, air conditioning, electric, electronic, carpentry, roofs, windows, concrete, brick work, appliances, automobiles, you name it.  Their dad did it all, period, mostly single handedly.  That's how they grew up, so that's how they thought everybody grew up.
> 
> Years later, they found that the DIY competence of their respective husbands never came close to matching that of their late father.  Their dad, my late father-in-law, was an example in temperament & willingness & responsibility as well as in wide technical skill & know-how.  He is still an example to (try to) live up to.  Most of whatever ability I have to fix stuff I gained 1 way or another from him.  May he rest in peace.
> 
> -- Alan Cole, McLean (Fairfax County), Virginia, USA.​


When I was a junior in college, three of moved into an apartment together as roommates. When simple things went wrong I fixed them, because that was how we did things, as did most of the families in the neighborhood.  

One time my two roommates were watching me do something and one of them thought that what I was doing was pretty interesting. He commented that in his family when something went wrong, they called in someone to fix it.  The other roommate laughed and said his facility was the exact opposite. When something broke they just ignored it.


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## Htoo0 (Sep 30, 2013)

I'm SO glad my father was a fixer. However, I did decide there are times to just 'call the man'. I don't replace my own home shingles for instance. It's just too much work for one guy to get it done quickly. In Oklahoma insurance usually covers most of the cost anyway due to storm damage. I don't do major repairs on vehicles either. I've rebuilt engines and transmissions but the time, effort and the need for the car makes it a losing proposition for me these days. For $20 I also let someone else change the oil/filter and dispose of the waste. But I replace my own water heaters, repair washers and dryers, etc. Recently designed and constructed a 10' x 12' x 8' walls storage shed, replaced the trim and gable siding on my home, put down new flooring and figure out how to do many other small projects. Really saves some money but guess I'm not doing my part for the economy.


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## T_R_Oglodyte (Oct 3, 2013)

Replacing the heater element solved the problem, even though the element hadn't failed and actually was exactly on specification.

This supports my hunch that there was a hot spot on the heater element located between the high limit thermostat and the thermal cutoff.  Being downstream of the high limit thermostat, the thermostat didn't pick up the overheating.


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## Passepartout (Oct 3, 2013)

Good Job! You probably saved a few $hundred$ to spend on vacation.


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## T_R_Oglodyte (Oct 3, 2013)

Passepartout said:


> Good Job! You probably saved a few $hundred$ to spend on vacation.



Sometimes taking a shower leads to inspiration.  I had been wracking my brain, trying to figure out what could be causing the cutoff to fail without having the first shut down the system. The cutoff should only fail if the thermostat fails, since it is the safety backup to the thermostat.  Since the heater hadn't  failed, and actually tested to spec, I hadn't been thinking about that. 

Then in the shower it suddenly seemed clear. There had to be a temperature differential between the two locations. And that would only happen if the heater element was not heating uniformly.  Ergo, the problem must be in the heating element.  Then when I got the old element outside I could see that it was oxided and likely wasn't too far from outright failure.


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## csxjohn (Oct 4, 2013)

T_R_Oglodyte said:


> Replacing the heater element solved the problem, even though the element hadn't failed and actually was exactly on specification.
> 
> This supports my hunch that there was a hot spot on the heater element located between the high limit thermostat and the thermal cutoff.  Being downstream of the high limit thermostat, the thermostat didn't pick up the overheating.



Glad you got it worked out.  My theory may still hold some water, that the heat was high enough fast enough that it tripped the high limit thermostat but still had enough heat to melt the thermal cutoff before the power to the heating coil could be cut and the coil allowed to cool.  Once the coil cools the high limit thermostat resets itself and there is no evidence that it tripped.

So I'm thinking if it's an air flow problem the HLT will cut off before it gets too hot.  If it's the heating coil giving off way too much heat they will both trip and the one resets.

Either way, other TUGers will know to replace the heater coil if there is not an air flow problem.


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## Elan (Oct 4, 2013)

There could have been electrical heating of the thermal cutoff as well.  Doesn't really matter at this point, problem solved.  Nice job, Steve!


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## T_R_Oglodyte (Oct 4, 2013)

Elan said:


> There could have been electrical heating of the thermal cutoff as well.  Doesn't really matter at this point, problem solved.  Nice job, Steve!



I'm not sure what you're saying here. Are you suggesting that the electrical current passing through the thermal cutoff heated the cutoff to failure?

If that's what you are indicating, my answer is that's why I paid more to get OEM parts.  Those types of failures are common in the cheapo aftermarket products that are listed on Amazon and eBay.


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## Elan (Oct 4, 2013)

T_R_Oglodyte said:


> I'm not sure what you're saying here. Are you suggesting that the electrical current passing through the thermal cutoff heated the cutoff to failure?
> 
> If that's what you are indicating, my answer is that's why I paid more to get OEM parts.  Those types of failures are common in the cheapo aftermarket products that are listed on Amazon and eBay.



  Yeah, that's what I was thinking.  That when the heating element turned on, the impedance changed in such a way to cause excessive current through the cutoff.  That, _coupled_ with the ambient temp from the element, caused the cutoff to open (Ti + Ta > Tcutoff).  Either way, the issue would be with the heating element itself (not a defective cutoff), which is what you ultimately replaced to fix the issue.  

  As I said, this was my thought initially, but when you said you were going to replace the element, I was pretty sure you'd fix the problem, so it didn't matter.


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## T_R_Oglodyte (Oct 4, 2013)

Elan said:


> Yeah, that's what I was thinking.  That when the heating element turned on, the impedance changed in such a way to cause excessive current through the cutoff.  That, _coupled_ with the ambient temp from the element, caused the cutoff to open (Ti + Ta > Tcutoff).  Either way, the issue would be with the heating element itself (not a defective cutoff), which is what you ultimately replaced to fix the issue.
> 
> As I said, this was my thought initially, but when you said you were going to replace the element, I was pretty sure you'd fix the problem, so it didn't matter.



Yeah - but to the extent there is any electrical heating of the cutoff that is considered in the design, at least in the OEM part.  In the secondary market, it's entirely likely that there could be added thermal heating, but not likely. 

BTW I checked resistance of the elements under full load, and all of the impedance in the heater circuit is in the heater element. None of the other components measures even as much as one ohm.  


 I didn't measure impedance of the old heater unit under load.  Had I done so, I think I would have found that while it was on-spec in cold state, under load the impedance would have been much greater.  

In any case it all comes down to a failing heating element causing uneven heating in the heater box, with the area near the high-limit thermostat being in a cool zone and the thermal cut-off located in a hot zone.


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