Gas dryer thermostat question.

I have a Kenmore 110.76912692 gas clothes dryer. The 186F non-resettable thermal fuse opened which caused the burner to cut out. There was a considerable amount of lint built up so I cleaned it all out. I'm about to replace the fuse but I also had some other questions regarding the operation of the thermostat.
When I disassembled the dryer, there was a nice schematic (more of a block diagram) hiding under the control panel. According to the diagram, the series of devices in line with the heating circuit is as follows:
186F thermal fuse. Operating thermostat, 150F or 155F. High limit thermostat 205F.
Then from there, it goes to the gas burner assembly.
If the thermal fuse is set at 186 what's the point of having a high limit switch at 205?
The operating thermostat is a 4 terminal device which sits right next to the thermal fuse. Two of the terminals make up a single pole switch. The other two terminals have a resistance of 7k ohms and from what I can figure out at the company's website (see link below) that makes this device, it uses some sort of biasing to alter the cutoff point. Is this correct?
http://www.thermodisc.com/productdetail.asp?ProductID=8
Thanks for your reply.
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David Farber
David Farber\'s Service Center
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The physical location of the high limit and thermal fuse might answer the question. Not really sure about that. The thermal fuse usually dosen't blow until the airflow is restricted by lint build up anywhere in the stream as you found out. I think the fuse is further from the burner, again not too sure.
The other two terminals connect to a small strip heater beside the bimetal switch inside the thermostat. This can be biased with AC voltage to bias the temperature at the switch (making it think it is warmer than it actually is). I believe they do this during the gentle cycle (or delicate - whatever) to get a lower temp without needing to have a different thermostatic switch.
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I guess it makes sense that the sensor nearest the burner would have a higher cut off temperature.
Using that AC voltage bias is a neat trick. The schematic shows a thermostat heater resistor with a resistance of 5600 to 8400 ohms. It's not shown linked to the thermostat. Perhaps a dotted line between the thermostat and the resistor would have made things a bit more clear.
Thanks for your reply.
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David Farber
David Farber\'s Service Center
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Thermostat/fuse placement on gas dryers is only slightly different from electric dryers. A thermal fuse opening on the flame housing might indicate poor airflow, causing heat buildup in that area. If there is a thermostat in that area, it backs up the thermal fuse.
A thermal fuse in the blower housing area guards against overly high temperatures in the drum, and hence the blower housing itself. Since there is no electric element to short, runaway temps in the blower housing point more to an operating thermostat problem. Poor venting tends to reduce, not increase, temps at the blower housing.
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I'm going to replace the thermostat along with the thermal fuse.
Thanks for your reply.
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David Farber
David Farber\'s Service Center
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