I recieved a email from phreich asking me about this.
I'm posting my reply to his email here hoping it will help others.
What happens when the humidity sensor in an
"automatic" dryer goes out?
There is not a humidity sensor.
Does the dryer "think" that the clothes are staying
"wet" and continue to run, or does it think they are "dry" and then advance the automatic cycle towards completion?
There are two basic type of auto cycles on a dryer.
The first type has two fingers inside the dry. If you
take a look inside the drum you should see them on the
back wall or towards the front on the inside of the
The wet cloths short these two fingers together this is
how the dryer knows the cloths are wet. If the fingers
are being shorted the timer does not advance. Once the
cloths are dry they can no longer short out the two
metel fingers and the timer starts to advance to off.
The second type of auto cycle on a dryer does not have
the fingers or anything else to tell if the cloths are
wet or dryer.
This type works by only advancing the dryer timer when the
cycling thermostat is open.
The dryer the cloths get the more the thermostat is
open, thus advancing the timer to off.
The first type with the fingers inside work the best.
What I am trying to figure out is whether replacing the
sensor would get the "automatic" function working again.
The sensor (fingers in the dryer) don't really go bad.
There could be some coating or build up of softner on
them. Take some fine sand paper to clean them up.
There is often a small PC board up by the timer that
has to do with the auto cycle with the fingers, I have
maybe replaced two or three of these in 15 years. They
are hardly ever a problem.
What I most always find to be the problem is the dryer
is plugged up with lint inside the fan housing and lint
It may feel like it's blowing air out the vent ok, but
it's not as good as it should be.
The owner of the dryer may say the time dry cycle works ok,
but it really takes longer then it should and they have gotten use to it running this way.
Is there a test that can be done on the sensor to see
if it is functional (I have a volt-ohm meter)?
I would have to know the model and look up a wiring
Best bet would be to take it all apart right down to
the fan motor and blower. Clean everything not just the
front or under the drum, everything.