Author Topic: Jennair Wall Oven broken Thermostat  (Read 7895 times)

Offline imzjustplayin

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Jennair Wall Oven broken Thermostat
« on: January 23, 2009, 05:38:28 PM »
I have a Jenn-Air W181 self cleaning wall oven that needs a new internal temperature probe/thermostat. This thermostat is used for determining how hot the internal temperature of the over is, it is not used for checking the temperature of meat or as a thermal cutoff switch. I contacted a parts supplier for whirlpool but they said that they don't manufacture the part I need anymore.

This is a digital oven albiet old and it works fine aside from this pesky old thermostat is giving invalid readings to the oven, making the oven think it's 900F when it's really 60F and 70F when it's really 400F. I'm pretty sure it's not a digital thermostat but probably some sort of thermo coupler that could easily be replaced by someone with electronics experience. Since this is a tech forum, I wanted to ask how I should approach such an issue where I need a replacement part/repair said part but they don't make it any more. So understanding the fundamentals of a temperature probe would appear to be a good idea.

Does anybody know much about temperature probes like how they work and how I should approach this issue? This oven isn't used that often and I really would like to NOT replace an entire oven because of a pesky thermostat as it has been trouble free since the day we bought it aside from this issue that is cropped up since last year.

Model W181

Offline Repair-man

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Re: Jennair Wall Oven broken Thermostat
« Reply #1 on: January 23, 2009, 06:08:43 PM »
I know a little about them...

That part you need is called the temp/clean thermostat, mounted thru the back wall. It was discontinued over 5 years ago, so start shopping for a new oven. No amount of tweaking wil resolve this issue. What is that , about 20 years old or more now?

« Last Edit: January 23, 2009, 06:11:22 PM by Repair-man »
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Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #2 on: January 24, 2009, 01:04:34 AM »
What do you know about this part? I'm not even sure if my part was actually broken when I took it out but now I'm sure it's broken because this little tiny wire that is used kinda like a Trottle Position Sensor on a car broke off. I'm trying to fix that. I think the reason why the thermostat broke or wasn't functioning properly (again I can't prove it wasn't working) is due to it being covered in soot. I think some other thermostat (which too is covered in soot) is the cause of the oven acting funny.  In low temperatures, the oven would say it's like 985F and cut off, once I heated the oven up with a blow dryer (I heated up the part listed in the OP and the one you have a pic of) with a blow dryer and the oven turned on, said it was 70F. Even at like 400F, the oven said it was like maybe 80F so I possibly had issues with two thermostats. I dunno, but again what do you know about this part? How can you tell if it's actually broken and why have you come to the conclusion that you can't tweak this part?

Offline Repair-man

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Re: Jennair Wall Oven broken Thermostat
« Reply #3 on: January 24, 2009, 05:26:28 AM »
Quote
Does anybody know much about temperature probes like how they work and how I should approach this issue?


You don't go to the hospital and ask the surgeon how he knows his stuff...he just does, and he's not oblilgated to explain himself.

Your oven has at least one problem that I can identify, and if there was a cheap, easy way for you to get out of buying a new oven, I would be happy to tell you. It is in our best interest to sell you parts anyway, so if we tell you there's no way around a problem you can usually take that to the bank.


To address one of your questions, the part which is now broken and obsolete  is a combination oven sensor and high-limit thermostat for the self-clean cycle. Not only does it monitor the oven temperatures by means of an internal thermister (now broken), but contains contacts that are set to open at 975 degrees F during the self clean program. Later models did away with this type of control since it was too difficult to replace in the field versus a standard oven sensor. GE made a similar oven in the 80's that proved to be cumbersome also.

The part number last recorded for this is 702812...maybe you can  find one on Ebay.
« Last Edit: January 24, 2009, 07:40:32 AM by Repair-man »
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Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #4 on: January 24, 2009, 02:45:40 PM »
Quote
Does anybody know much about temperature probes like how they work and how I should approach this issue?


You don't go to the hospital and ask the surgeon how he knows his stuff...he just does, and he's not oblilgated to explain himself.

Your oven has at least one problem that I can identify, and if there was a cheap, easy way for you to get out of buying a new oven, I would be happy to tell you. It is in our best interest to sell you parts anyway, so if we tell you there's no way around a problem you can usually take that to the bank.


To address one of your questions, the part which is now broken and obsolete  is a combination oven sensor and high-limit thermostat for the self-clean cycle. Not only does it monitor the oven temperatures by means of an internal thermister (now broken), but contains contacts that are set to open at 975 degrees F during the self clean program. Later models did away with this type of control since it was too difficult to replace in the field versus a standard oven sensor. GE made a similar oven in the 80's that proved to be cumbersome also.

The part number last recorded for this is 702812...maybe you can  find one on Ebay.


You're saying that thermostat has an internal thermistor as well as a mechanical thermostat? The only thing I saw was what I'd describe as a mechanical thermostat where you have some porcelain, a metal rod at the end inside of a metal tube that pushes on a mechanical switch. That mechanical switch on top has an electrode that rubs against a conductive surface that sort of looks like a rudimentary circuit board where you have two wires coming out of opposite ends and you have a circuit path on this board, the electrode rubs against this surface and acts like what I'd believe would be a throttle position sensor in a car.

Then supposedly when that mechanical switch gets far enough, I think shorts out a 600F limit circuit and then shorts a circuit for the 985F circuit which explains why there are 7 electrical contacts on the outside of this thermostat. 2 connects for 985F "circuit" 2 connects for 600F connect and 3 for the wierd thermostat thing I described where you have two wires connected to a pseduo circuit board and a third wire connected to an electrode that rubs in the middle of this circuit board, up and down and supposedly reading the potential between one of those two wires connected to the pseduo circuit board gives a temperature reading??

Are you telling me what I described above is a thermistor? I'm pretty sure THAT is NOT a thermistor. There are three other thermostats in the oven which I KNOW ARE thermistors so I'm wondering if you're thinking of another oven or I just have an older/newer version of an oven than you're anticipating.

Can you explain to me why there are 4 thermostats for the oven? One has a 600F limit, one has that 985F limit, another has a 210F limit and the last one has a 175F limit (the last one I think is just for telling the internal fan for the electronics to turn on or not. Oh and no, those various thermostats do not all refer to that 702812 thermostat but actually refers to 4 separate devices.

Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #5 on: January 24, 2009, 04:54:11 PM »
Quote
You're saying that thermostat has an internal thermistor as well as a mechanical thermostat? The only thing I saw was what I'd describe as a mechanical thermostat where you have some porcelain, a metal rod at the end inside of a metal tube that pushes on a mechanical switch. That mechanical switch on top has an electrode that rubs against a conductive surface that sort of looks like a rudimentary circuit board where you have two wires coming out of opposite ends and you have a circuit path on this board, the electrode rubs against this surface and acts like what I'd believe would be a throttle position sensor in a car.

I just realized what that bold is in the thermostat, it's basically a variable resistor! Now I guess the real question is, is a variable resistor the same as a thermistor! Or does a thermistor have a variable resistor in it.

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Re: Jennair Wall Oven broken Thermostat
« Reply #6 on: January 24, 2009, 05:16:11 PM »
You are beating a dead horse. I see no reason to continue this diatribe on a unit that the parts are no longer made for.
« Last Edit: January 24, 2009, 06:14:36 PM by Repair-man »
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Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #7 on: February 15, 2009, 08:37:07 PM »
Well for those who find this thread on some search engine, here are the results. As of now (Feb 15, 2009) the thermostat works, for the most part. I don't know too much about the thermostat and its calibration procedures and the calibration of the computer control panel at the front of the oven (it's mentioned in the paper schematic, something about rod length and angle) but what I ended up doing which I think would solve a lot of people's problems is to clean the metal rod with a scotchbrite pad. This rod should be smooth and like metallic silver, not very very dark grey or black. Over time, it will be covered with carbon from burning foods inside of the oven, making it slow to react to temperature changes.  In fact, Jennair specifically mentions scotchbrite pads in the user manual to use for cleaning the oven with; these pads are quite effective and won't damage the oven.

If the above solution isn't enough, I believe the issue is that the thermostat has lost calibration over time due to the elements inside the rod wearing out. So you'll end up having to move a pad that has two stationary electrodes on one side resting against it and another electrode that is riding against a black pad in between the two electrodes and it moves forwards and backwards in response to temperature changes. This pad might need to be carefully moved in order to recalibrate the thermostat. What made repairing the oven take so long was that I broke what seemed like an extremely fragile electrode (it's attached to a metal pad which is riveted to a brown plastic base that is attached to a metal rod that moves back and forth). Think of this arrangement like taping a fishing rod with a piece of meat on a hook to the top of your head, and you moving your head back and forth.

So I then had to devise a way of creating a replacement electrode and soldering it back on. Part of the problem was that the first electrode I made and soldered on was useless because it didn't flex as the electrode moved across the pad. So I picked a higher gauge wire (thinner) (26ga), soldered it farther back on metal plate, right behind and ontop of the rivet (which is attached to the brown plastic base), resulting in a wire that would flex against that electrode pad instead of bending and holding its position (i.e, it'd flex with the curvature of the pad opposed to bend, touch pad for a short time and then lose contact). Since the old electrode made sort of an upside down ohm sign, I bent the tip of the wire to resemble that/act like that where you have a smooth but fine point contacting the pad.

The only thing about the oven now that isn't how it used to be is that I've calibrated it so that it now thinks room temperature is 150F which is obviously wrong, however temperatures above 200F are accurate to within 30F according to my infrared temperature reader I bought online awhile back. Self clean works fine which means the 985F cutoff switch within that complicated thermostat works alright and so does the 600F "lock" work as well (tells the oven that the door in the front needs to be locked).

It took a really long time for me to figure out the schematic due to my limited electronics experience and while I didn't get the oven into 100% working order, I believe I did enough to fix it so that we can continue to use it for however long it will last from here on out.


I also feel it should be noted that this oven in its default, normal working configuration will not function properly when room temperature is below 65F. This is simply due to a design oversight and can be remedied by either opening up the thermostat, and basically uncalibrate it (make it think the room is warmer than it really is), or by heating up the temperature rod for a short time (until 70F) in some way like with a blow dryer before using the oven, or by keeping the room temperature at least 65F, though the last idea is most costly idea that I don't like but is what I think partially resulted in this fiasco.

If anybody sometime in the future suffers similar issues with the exact same oven as I have, feel free to contact my user name on sites that I frequent such as hardforum.com, genmay.com, or honda-tech.com. I just cringe at the idea someone has to toss a perfectly good oven away due to such a simple (yet complicated) issue.

Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #8 on: February 15, 2009, 08:49:13 PM »
Oh and one very important thing I forgot to mention.

Before you even THINK about disconnecting those 7 wires from the thermostat, besure to take lots of nice, clear, bright pictures of all the wires and where they connect to, etc. before disconnection. Look over the pictures before disconnecting anything and have no doubt in your mind that the pictures you have are everything you need because you'll need them as reference for when you go to reconnect them again.

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Re: Jennair Wall Oven broken Thermostat
« Reply #9 on: February 16, 2009, 02:33:07 AM »
So after all this fiddling around, you still have an oven that doesn't   function properly, yet you present this as though you conquered the problem by jerryrigging.

It is my hope that any one reading this will have too much common sense to tamper with electro-mechanical devices which have been built to factory specifications.  Now, if your house catches fire due to a runaway oven, you will have no recourse from JennAir since the unit has been 'modified'.
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Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #10 on: February 16, 2009, 06:52:56 PM »
So after all this fiddling around, you still have an oven that doesn't   function properly, yet you present this as though you conquered the problem by jerryrigging.

It is my hope that any one reading this will have too much common sense to tamper with electro-mechanical devices which have been built to factory specifications.  Now, if your house catches fire due to a runaway oven, you will have no recourse from JennAir since the unit has been 'modified'.

Heh, very funny. No what I did was I made an oven that doesn't work/inaccurate at low temperatures but is accurate at high temperatures. One thing I learned about this thermostat that should cool your anxiety about this is that when you ADD temperature, i.e make the oven think it's hotter than it really is, the oven effectively shuts off sooner. One problem that I did not realize I had until much later was that the oven never could reach 525F under bake rad, yet at 400f it seemed much hotter than it should be. So by me adjusting the thermostat, I actually made the situation much safer. I also with my infrared thermometer checked the temperature with the oven calibrated to "room temperature", i.e thinking room temperature is 70F (minimum computer reads is 70f), then tried to get the oven to 525F but it would stop at around 450F according to the display/computer. When I opened the oven and checked with the infrared thermostat, I found that the oven was actually at 550F, much hotter than the computer read. This is where I got the idea to calibrate the thermostat to 150F for room temperature, because had I left it at 70F, the oven would be operating much hotter than what the display would read out, and that in my opinion is far more of a firehazard.

However all of the above is superfluous for a slightly different reason. You see the thermostat does not entirely rely upon the 3 conductor variable resistor what ever in order to figure out the right temperature to 1. Lock the oven and 2. Stop applying heat when in the clean cycle. There are two other elements in the thermostat. There is the 600F lock, which has a pin that goes against the spring loaded metal plate and when this circuit is made (at 600F exactly) it tells the oven to lock the door. This function cannot be calibrated and nor can the 985F temperature cut off which works strictly off of the spring loaded metal plate where at 985F, it either breaks or makes a circuit, just like the 600F Lock circuit I mentioned. In fact, because of this arrangement, where it never needs nor is capable of calibration as they're designed with one temperature in mind and are a little bit less precise, I was able to confirm that the calibration I made to 3 conductor variable resistor along with using my infrared thermometer was most accurate.

So in conclusion, no I did not nor am I putting my self at risk and if anything I made the situation safer because from all of the data I've looked at and going by what I and others remember, the oven operated at much higher temperatures than the computer was reading out far before I even considered touching the thermostat. I can only imagine this is due the amount of time that has passed since this thermostat was made, that the elements do not expand as rapidly as they once did.

Offline imzjustplayin

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Re: Jennair Wall Oven broken Thermostat
« Reply #11 on: February 21, 2009, 04:41:28 AM »
I think I know why in order for the thermostat to be accurate at high temperatures it needs to be calibrated so that it is 100F off at low temperatures. I believe the reason is due to the wires that run from the computer (front control panel) to the thermostat. Since the wires are old and probably have slightly corroded or something of the like, their internal resistance has gone up. The computer does not compensate for that, and because of that, it makes the thermostat inaccurate at the lower temperatures but accurate at higher temperatures, when the wires are hotter and therefore have a lower resistance value to them.

More proof to this theory is that when I got the oven out of its clean cycle, I asked the control panel to tell me the internal thermostat's temperature and watched the panel as it cooled off. Using my infrared thermometer, I would periodically open the oven door and point the thermometer at the thermostat probe and compared the values of my thermometer and what the oven said, I found that the temperature readings by the oven were infact accurate for some time. However around the time the internal fan turned off and as the oven approached 200F, (according to the oven), I then noticed a change. Now the thermometer was reading somewhere around 150F but the oven was still reading a much higher value, and as it cooled down, the discrepancy grew and finally, when the oven got to room temperature (around 55-60F) the oven settled at 150F. This is where I got the number for 150F being "room temperature now".


I still don't know absolutely for sure if this is the reason as it's kind of sort of vague but it does make sense in a "I need to replace my sparkplug wires cause my car won't run anymore" kind of way. I don't plan on testing this theory out any time soon because it has been 3 long months and the oven functions for its primary purpose, baking things at 300F+. There is also the risk of making the thermostat entirely inaccurate due to too high or too low of resistance in replacement wires and connections. I feel it's important to point this new revelation out as a possibility since it would explain why it's not functioning properly at lower temperatures while being accurate at cooking temperatures and or when the Internal oven fan comes on which is at about 175F according to the schematic.

Once again, I hope this helps those who attempt diagnostics on such an oven or any electronic device for that matter.

 

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