Hi.
I'm having trouble with our clothes dryer not running long enough to dry the clothes when using the "Moisture Sensor"
drying option rather than just a timed selection.

This dryer uses a "moisture sensor" installed in the area where the clothes are being tumbled around.
The sensor is just two strips of metal parallel and close to each other on a base of plastic. Apparently the moisture
still in the clothes creates a low resistance to ground across the "sensor" which tells the control circuitry to
continue running the timer. Once there is little or no moisture, the circuitry sees an open and triggers the cycle
to complete.
My question is.... is there any special property to the metal strips or are they simply two metal pieces closely spaced
to provide a low resistance when enough moisture is still being across them? The new part is scarce and rather expensive.
I think it was from $30-40 .... for a couple of pieces of narrow metal on top of a piece of plastic about 2-3 inches long.

I'm trying to understand if it is justifiable to invest in this part. If it is simply a couple of pieces of tin on plastic of proper
length and spacing... it should be very usable as is... other than some simple cleaning with soap and water and cloth.

Anyone familiar with this? Would really appreciate your help. Thanks very much !

 

It would help if you post the make and model of the dryer and also a photo of the moisture sensor showing any identification markings.

 

The only moisture sensors I am familiar with use a plastic film that is metalized on both sides. Humidity causes the plastic to expand which changes the distance between the metal layers. The thinkening causes the capacitance between the metal layers to decrease. Both the initial capacitance and the capacitance change areqite small -- In the pF range.

p.s. I have some of this type of sensor and never got them to work.

 

My question is.... is there any special property to the metal strips or are they simply two metal pieces closely spaced to provide a low resistance when enough moisture is still being across them?

Your logic is good. It's just 2 pieces of metal designed to be resistant to (water born, ionic) corrosion and polished to avoid snagging cloth. 301 series stainless steel or brass would be a good option. All the magic happens in the circuitry behind the metal strips, and I've never seen one that worked correctly...probably because they were just becoming available as my appliance repair days were ending.

You have the right to make your own out of whatever you have laying around. You have the right to decrease the distance between the edges so they keep sensing low impedance as the clothes get dryer, thus extending the run time before the sensor goes, "open". You have the right to buy jewelers rouge and a buffing wheel to get the finish right.

That's all I have right now because I never bothered to autopsy one of these circuits. I just use the timed cycle because I have the intelligence to judge the correct drying time for a standard load without a nanny circuit designed for stupid or lazy people.

 

These are from Farnell at under £5.00.

 

These are from Farnell at under £5.00.

That might work in the exhaust air stream, but they won't install very well in the tumbling drum.

 

I think it could just be a thermal switch. While the clothes are still damp and the water is still evaporating from them the latent heat of vaporisation of the water will be extracting heat from the hot air so it will be cooled down. When the clothes are dry there will be much less heat extracted from the hot air so the temperature of the exhaust air will be higher. I think one or both of the strips are bimetal which bends as the temperature changes.

Les.

 

Thanks everyone for your help... the make and model: Maytag MDE3000AYW
Here is a link to Amazon showing the sensor bar:

https://www.amazon.com/dp/B002BUWZ4...d=kwd-302741170205&ref=pd_sl_5d4hnwv1dn_e_p37

Here is wiring diagram:
https://www.easyapplianceparts.com/...51310&Position=8&mfg=Maytag&Type=Dryer&Mark=8

See the "electronic control" board and note the sensor connected at lower left. One end grounded.

 

..... sorry, should be noting the sensor connected at LOWER RIGHT of electronic control board.

 

My samsung dryer had a similar problem. I found that the connectors to the bars were oxidizing and making poor contact.

 

If a fabric softener is used, it may build up a wax like coating on the sensor bars. Use a cloth soaked in house cleaning ammonia to clean them. Our dryer had a similar problem and that trick fixed it.

 

Thanks for the tips on cleaning the sensor. I see nothing visibly on it and no feeling
of a residue either. I did use a bit of soap and water and gentle wiping with fine
sandpaper but so far haven't tried it out after this.

 

Oh yeah, one other thing to consider - is your dryer vent clogged? Even a partial clog will cause similar symptoms.

 

I'll add advice to check that the drum is rotating normally and that air is blowing. My dryer is similar to yours and I've had poor drying a few times as you've described, but it wasn't the sensor. I've had to replace the squirrel-cage blower twice over the ~30 years I've had it. It's plastic and the center cam that holds it to the steel motor shaft eventually wears out so there is no purchase on the shaft. Everything seems fine except it's not spinning and moving air as effectively as it should. It will eventually start making an awful sound.

 

Well I had initially totally cleaned out the vent piping from just inside dryer and all the
way up and out the wall....was quite a bit in there but cleaning that out made no difference.
I tried the "cycling thermostat" and checked other thermostat and limit devices. I also
confirmed both legs of the 240 present at the machine and can see that the heating coils
are glowing bright red from end to end.
The blower motor squirrel cage has been cleaned and the portion where the shaft connects
is just fine.. solid, plus it and the drum are rotating fine.
So down to this moisture sensor business. You guys have been of greater help than an
"appliance forum" I tried !!! When the next wash and dry time comes along, we will check out
what "cleaning" the sensor and it's spade connectors bears out.
Thanks again everyone.

 

I think it could just be a thermal switch.

I respectfully disagree. The usual clothes dryer has 2 or 3 (Klixon Brand) temperature switches controlling max (delivered) air temperature for regular and gentle cycles and sometimes a panic temp sensor right on the nichrome heater compartment in case the usual safeties fail. If all it required was a temperature sensor on the exhaust pipe, that's where it would be. Therefore, attaching anything to the drum must be looking for something besides temperature, i.e. humidity.

 

In very round numbers, it is a retriggerable monostable or missing pulse detector, very much like a fan fail monitor except for the impedances involved.

ak

 

I'm back... no real change resulted from the steps taken, cleaning the moisture sensor
and it's terminals.

If you take a look at the schematic I gave a link to in post #8 you can see the circuit involved
in the sensor monitor board. I don't know enough about circuit design to know exactly what
takes place here. What is the PTC on lower left of board...looks like an adjustable resistor
control. Just wondering if there would be a way to force the drying cycle to run just a bit
longer to solve the problem. Would changing the value of the two resistors of 510K at the
blue terminal (that the sensor is connected to) be a way to effect a change?

I just don't want to throw money into this since it will probably have to be replaced before
too long but right now I've got too many bigger obligations so would like to get it running
for least money right now... any further thoughts?

 

@izon

Off topic

When you are typing, do not hit the Enter key when the letters reach the end of the typing window. Just keep on typing. Allow the browser to automatically wrap the lines depending on the viewer's window width.

Hit the Enter key twice when you want a new paragraph.

 

Ok sorry for not being aware. Will try to remember. Best regards.