Author Topic: Jennair Wall Oven broken Thermostat  (Read 8308 times)

Offline imzjustplayin

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  • Member Since: Jan 2009
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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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  • Member Since: Jan 2009
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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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