How do dryers sense moisture?

I'm having a problem with an Estate gas dryer, model # TGDS740PQ.
One of the symptoms is that the Timer knob moves when in Timed Drying mode, but not when in AutoSense mode. The other symptom is that it takes a long time to dry a load in any mode. Even when the clothes are completely dry, the AutoSense knob, which should move towards Off once the clothes are dry, never moves. The dryer would run forever if I let it.
I should point out that the clothes are not always warm when I check them, whether they are wet or dry. Sometimes they are dry and toasty warm, sometimes they are dry and cool, sometimes they are wet and warm, sometimes wet and cool. My understanding of the way the cycling thermostat works, they should *never* be wet and cool. But even if the cycling thermostat was bad, why would that cause the knob to not move in AutoSense mode?
I looked at all the parts for my dryer at the following site and can't find anything that looks like a moisture sensor. How does a dryer sense the moisture and decide when it's done?
http://www.appliancepartspros.com/partsearch/model.aspx?model_idQ38494
BTW, before anyone mentions it - I tore the dryer apart this weekend and cleaned it from the opening in the back of the drum all the way out to the vent. This didn't help, but there wasn't enough lint to have caused a problem in any case.
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On Mon, 17 Nov 2008 07:08:24 -0800 (PST), DerbyDad03

advance the timer
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The first thing I'd try is to take it outside, and blow out all the internal air passages with compressed air or a leaf blower. Gathered lint can cause all kinds of problems.
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Perhaps you misunderstood my last paragraph.
As I said, I cleaned it from opening in the back of the *drum* all the way out to the vent, not just from the back of the dryer where the exhaust hose attaches.
This included the internal ductwork, the fan housing, the lint trap holder and any other place where dust could collect. Air flow is not the issue.
And for my trouble, I am about a $1.50 richer in loose change.
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And for my trouble, I am about a $1.50 richer in loose change.
It costs $2.50 to repair. Give it more time, or leave more change in your pockets.
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I wonder how much change is trapped in old, scrapped appliances. Figger $1.50 average?
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.
This was a $1.50 in a 4 YO dryer. Older ones might have much more.
BTW - The very nice lady at the appliance parts store said it's probably the gas valve coils. If they won't keep the burner on long enough, the clothes will eventually dry, but if it never reaches full temp, the Cycling Thermostat won't advance the AutoSense Timer.
She said it's $32 for the coils which go bad pretty often and $27 for the Cycling Thermostat, which almost *never* goes bad. She said if it was her money, she bet the extra $5 to replace the most likely cause.
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"DerbyDad03" wrote:

They go bad all the time, and are a five to ten dollar part. There are two to three (sometimes more) thermostats in a dryer. It's easy enough to remove them (one at a time, label the wires, after unplugging the unit, et cetera), hook them up to a VOM, then dunk them in hot water to check for a state change. Take you maybe 15 minutes, and will give you actual information that you can use, instead of throwing parts at it.
Jon
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wrote:

$5 to $10 where?
The best I've seen is ~$20, online with shipping.
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"DerbyDad03" wrote:

Look over at ebay.
Jon
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wrote:

No need...the lady at the parts store was right. I replaced the coils tonight and the dryer is as good as new.
Thanks anyway.
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