I just solved my Kenmore Series 90 problem, and wanting to pay back the help I received from ALL you good folks out there, here is what I found out. Hopefully this info may benefit you too. First, before reading my success story, you MUST check out the pictures at:
Kenmore Series 90 Electric Dryer No Heat ?!? - Appliance Blog Forums
These pictures were at the bottom of the thread when I found it today. Please refer to the pictures, I use the terminology of these pictures to describe my success.
My Kenmore Series 90 has Model number 110.78972892, somewhere of a late '90, early 2000 year vintage. The problem: Clothes began to take forever to dry. I read multiple web sites - most started off with cleaning your exhaust ducts, then the answers got squirrely from there. I cleaned my flex duct, and opened the dryer and vacuumed everything in there (note: the dryer opens from the front - use a flat head screwdrive to press down on two retaining tongues). I ran the dryer, minus the flex duct, just to see if there was any improvement. NO LUCK. I then wondered if it could be one of the temperature sensing elements. This dryer has at least three temperature sensing elements. The two I considered significant were a "thermal fuse" and a "cycling thermostat (or thermistor)". Note that the pictures referenced above show these two sensors right next to each other on the exhaust side of the fan. I ruled out the thermal fuse sensor because from another website I learned that when the thermal fuse goes out the dryer will not run at all! Mine ran, it just dried slowly and didn't seem to throw off much heat, as measured when I placed my hand near the exhaust. I also ruled out the "thermal cut off" temperature sensor because if that failed, the machine would run, but no heat ( remember, mine heated, but poorly). The more I looked at the thermistor (a.k.a cyclic T-ostat) and its roll to call for heat, the more I felt it was the problem. I called up my local appliance parts place with the model number of the machine and the part number of the thermistor (although the parts place should only need one or the other part number). The price was $23. I figured it was an educated gamble, bought the part, put it in the machine, and it solved the problem. If it failed, I would have begun to wander if it was the high limit thermostat (approx. $13) or the circuitboard that the thermistor sends a signal to.
After putting in the good thermistor (or cycling T-ostat), I verified using my wifes jam thermometer that it cycled correctly. To check this cycling I placed the sensing element of the thermometer about dead center of the dryer exhaust, about two inches away from the opening. Sure enough, I saw the gas ignite, then the temp climb to 150 deg F. The gas would then shutoff and then the temperature would drop to approx 125 deg. F only to have the gas re-ignited for a climb back to 150 deg. F. This cycling went off like clockwork, and now, as I type, my wife and I are quite happy, knowing all is right in the world because our dryer once again works.
Help! Kenmore Dryer just stopped heating! 
