The dryer has been taking longer than normal to dry clothes. I've disassembled it and checked for lint cloggage but didn't find any. Sensors are clear, fan is clear, ducts all the way out are clear. The burner comes on and ignites. I haven't watched it to tell if it's shutting down prematurely. And if I DID watch it - I wouldn't know how long is normal.

Asking: Can a bi-metallic thermal switch lose its rating? I mean when new it was rated for (oh, say) 200˚F. Is it possible it's no longer breaking at 200˚F but rather at some lower point (oh, say) 160˚F or lower? I'd find it hard to believe that such a switch could change its rating. But I'm not sure.

Diagnostics: When set to high heat the igniter glows and the gas ignites. Air coming out of the vent is obviously warm. The flame looks normal, though I wouldn't know what that should look like. Mid range and high range thermal cutoff switches have continuity when cold. I don't have a way of checking for temperature. Somewhere in my vast hay stack I have a small one, but haven't seen it in years.

So can a switch degrade its break point to some lower setting?

 

Hi Tony,
Has the 'plastic' support mounting for the actual bi-metal strips degraded due to being exposed to hot air and/or switch heating due to 'arcing' for some time?
Most general purpose plastics seem to degrade over time.

E

 

Stop buying that cheap Chinese gas.

But seriously have you noticed if possibly the spin cycle in the washer is doing its job.

 

have you noticed if possibly the spin cycle in the washer is doing its job.

Excellent point. I shall look into that.

Has the 'plastic' support mounting for the actual bi-metal strips degraded

No. No plastic parts here. The gas burner is below the drum. It's hot air is pulled into the tub via a chute that brings the hot gasses into the drum while an exhaust fan pulls the heated and moist air out of the drum. It is on that chute that the sensors are located. They do get hot. Without pulling the machine apart again I'd guess that the high temp BM Switch is probably around 160˚F - but I don't know that at present for a certain. The second sensor used for low heat is probably around 120˚F. Again, that's just a guess.

The selector selects the temp as either "High/Cottons", "Medium/Casual" and "Low/Delicates". That last one is likely just "Air Dry" - no heat.

In the past it has taken about 45 minutes for a normal sized load to dry on high. I've never used any other settings, but the wife has. In the past week she has complained that it's taking much longer to dry the clothes. That's why I pulled it all apart, to clear any air passage ways that might be choked with lint. I've pulled older dryers apart and have found LOTS of lint not only in the ducts but inside the cabinet as well. I was imagining that I'd find the same thing. Instead I found a dryer whose internals were very clean. The fan blade had very very little lint stuck on it. The filter was also clean. The filter you pull out after every load. I could find no reason for my wife's complaint about how long it takes to dry clothes. The dryer has a moisture sensor that consists of two smooth insulated from each other bars that sense how much moisture is in the clothing that is tumbling against it. But we never use the moisture sense cycle. Just the manual timed dry circuit and high heat.

Which is why I'm asking about the BM switches, if they can change their value over time.

 

What model dryer is this? I had a similar problem, a gas valve coil fails (opens) but only when hot. So on a
cold start the dryer lights normally but when the gas valve coil heats up it opens and the gas is shut off.
The dryer keeps trying to relight, eventually the coil cools enough to internally reconnect, the dryer relights.

This, of course, heats the coil again, repeat over and over. Takes forever to dry...

This was a Maytag DG410 dryer series 01. I replaced the failing coil carefully following the color code and
"nothing". The hot surface would glow, the detector/interlock which watches the hot surface would click
but no gas.

It turned out that the dryer is all 120 VAC and the gas valve has two coils which must be in phase. The replacement
coil wires were color coded but backwards from what the dryer needed so that the coils didn't aid each other
(part of the safety interlock). Switching two wires fixed it... <voice over from trek: reverse the polarity>

I sent this message to the parts supplier:

It would help if it contained a notice that the polarity of the boost and holding coils did not matched
the original Maytag coils -- the polarity is backwards requiring one of the coils be connected in reverse
to enable the gas valve. Wiring matching wire colors results in no gas flow, I reversed the boost coil
(red to white/white to red) and it works fine.

They replied:

The coil kit is normally a color coded connection. The fact that you had to reverse the polarity for
the coil would suggest that the polarity of the outlet the dryer is connected to is reversed. You may
want to test the outlet for the fault.

They have no clue...

 

I have a gas dryer, it has a few sensors buried in plasic.

There's likely two temp sensors (snap switches) on the heater side, one other temp sensor, and one more over-temp sensor on the exhaust side of the machine (inside/under the machine, etc).

It could also be the igniter, they sometimes fail to get hot enough to re-ignite when the dryer is running. It's a draft style gas heater, etc.

I have heard in past the valve failing to work because bad coil, but it's usually one of the other parts.

Instead of trying to figure out which item, just replace all the temp switches and sensors using OEM parts, not the "works with your dryer" china stuff from amazon, but rather the OEM made-in-china stuff.

Over the years I fell into the trap of just replacing the failed item, then a few weeks later some other sensor failed. Best to just do all of them as a set.

Past that, if it's a newer dryer with electronic controls, a line spike can damage the controls in ways that make the thing act funny.

 

It could also be the igniter, they sometimes fail to get hot enough to re-ignite when the dryer is running. It's a draft style gas heater, etc.

Thanks. I'll have to watch for that.

It turned out that the dryer is all 120 VAC and the gas valve has two coils which must be in phase. The replacement
coil wires were color coded but backwards from what the dryer needed so that the coils didn't aid each other
(part of the safety interlock). Switching two wires fixed it... <voice over from trek: reverse the polarity>

Thanks for the advice but my problem isn't with the burner igniting. As DC_Kid suggests it might be the igniter.

What model dryer is this? I had a similar problem, a gas valve coil fails (opens) but only when hot. So on a
cold start the dryer lights normally but when the gas valve coil heats up it opens and the gas is shut off.
The dryer keeps trying to relight, eventually the coil cools enough to internally reconnect, the dryer relights

This might be an issue as well.

I wouldn't have thought of the foregoing possibilities mentioned. Will spend some time on the floor watching through the inspection window and see what happens.

 

What model dryer is this?

Kenmore series 500 dryer isn't a model number.

 

For a diagnostic check it is simple to punch a small hole in the external vent pipe and insert a cooking type dial thermometer to see if the discharge air is hot enough.
As for the flame in the burner, it should be clear and Blue, with no yellow.
And those thermostats are typically snap disks that stay calibrated if they do not rust. But contacts fail some times, not always closing properly. AND on occasion , the valve in the discharge vent sticks partly closed. That makes it take a lot longer.

 

What model dryer is this?

Kenmore series 500 dryer isn't a model number.

Don't want to go through all the hassle of pulling it back out just to check for that information.

As for the flame in the burner, it should be clear and Blue, with no yellow.

Now there's something helpful. I have a lot of yellow in the flame. Will take another look and see if I can get a picture of it.

Be back in five minutes.

 

Trouble uploading a video. May have to post it on YouTube then post a link here.

 

Don't want to go through all the hassle of pulling it back out just to check for that information.
Now there's something helpful. I have a lot of yellow in the flame. Will take another look and see if I can get a picture of it.

Be back in five minutes.

If the flame is yellow than there is excessive gas to the amount of air flow. And reduced air flow will also make the drying take longer. A dryer problem that I have seen several times is that the blower wheel becomes loose on the motor shaft and so it does not provide enough air flow. It may require replacement, which s simple and easy, or it may just need a better clamp to hold it on the shaft. No need to post a picture, really. The flame should be all shades of blue, not yellow.

 

The dryer has been taking longer than normal to dry clothes. I've disassembled it and checked for lint cloggage but didn't find any.

When you say no lint clogging did you mean just in the dryer? Years ago my Mom had a similar problem. When I looked at it the dryer itself was pretty clean. So then I looked at the discharge pipe, where it went out side. The discharge pipe was very blocked. The pipe was longer than normal and made from tin furnace duct pipe. I disconnected the flex pipe from the tin pipe and used a bamboo/cane fishing pole to push the lint clog through and that fixed the problem. That hot air and moisture has to be able to escape for the dryer to work fast.

 

Long way around: Here's my YouTube of the dryer flame. Mostly blue but definitely some yellow going on as well.
https://www.youtube.com/shorts/Q9gH77ETqqw

 

A dryer problem that I have seen several times is that the blower wheel becomes loose on the motor shaft and so it does not provide enough air flow.

Bingo.

I'd first check again for airflow outdoors at the exit vent. If it's functioning properly there should be quite a good flow there, enough to open little louvers if that's the kind of exit you have. Any restriction of airflow will cause the temperature to get too high, which in turn will trip the cycling thermostat and the
gas to turn off "prematurely". (It's actually functioning exactly as designed.)

If the vent is clear and airflow is still inadequate, that points to the blower wheel. On my Maytag the wheel is plastic and attaches to a metal shaft. The torque is too much for the plastic and that attachment point fails every decade or so. I think they've improved the aftermarket parts, so they last longer now. There's usually a change in the way it sounds when it fails but not always. You have to take off the shroud and observe the wheel, and with the power off try wiggling the wheel on the shaft to check for a solid connection.

You can test your cycling thermostat on a temperature-controlled griddle, if you have one of those. My guess is that it's fine. They seem to be one of those things that either work properly or fail completely. You could also test it in-service if you're careful. It'll have continuity when cold, and you can watch to see if it opens when the dryer contents are "too cool". I think (I haven't done this) you could watch for AC voltage on the pole of the thermostat opposite the high-cutout fuse. When cold, you'll have full power on both poles. If it opens, one side should go cold. Using a small AC bulb (like a nightlight) might be easier and more definitive than a meter to determine if power is present.

 

"Mostly blue" should not be a problem. I thought I heard YELLOW, which means not enough air.. Just like SB in post #13, there has to be enough air flow.

Bingo.

I'd first check again for airflow outdoors at the exit vent. If it's functioning properly there should be quite a good flow there, enough to open little louvers if that's the kind of exit you have. Any restriction of airflow will cause the temperature to get too high, which in turn will trip the cycling thermostat and the
gas to turn off "prematurely". (It's actually functioning exactly as designed.)

If the vent is clear and airflow is still inadequate, that points to the blower wheel. On my Maytag the wheel is plastic and attaches to a metal shaft. The torque is too much for the plastic and that attachment point fails every decade or so. I think they've improved the aftermarket parts, so they last longer now. There's usually a change in the way it sounds when it fails but not always. You have to take off the shroud and observe the wheel, and with the power off try wiggling the wheel on the shaft to check for a solid connection.

You can test your cycling thermostat on a temperature-controlled griddle, if you have one of those. My guess is that it's fine. They seem to be one of those things that either work properly or fail completely. You could also test it in-service if you're careful. It'll have continuity when cold, and you can watch to see if it opens when the dryer contents are "too cool". I think (I haven't done this) you could watch for AC voltage on the pole of the thermostat opposite the high-cutout fuse. When cold, you'll have full power on both poles. If it opens, one side should go cold. Using a small AC bulb (like a nightlight) might be easier and more definitive than a meter to determine if power is present.

The simple way to verify a thermostat switch is read the voltage across it. Of course the circuit must be powered for the test to work, so that you read voltage when it opens. The test is not totally conclusive because the circuit mus be live. It is hard to tell the difference between zero and nothing.

 

I'd first check again for airflow outdoors at the exit vent.

Have good airflow. Pulled down the vent pipe going to the outdoors, checked the clean-out tee and made sure there was no buildup of lint on the critter guard. Airflow is good in all places. The only thing I haven't done is check for clogging of the mixing plate that adjusts the amount of air that mixes in with the gas flow. It may have moved or there may be lint clogging it. I'll have to get around to that as well. But mostly the flame looks good. Some yellow, but I've always seen yellow in all the gas dryers I've ever messed with.

The simple way to verify a thermostat switch is read the voltage across it.

If the switch has failed then it would likely fail in the open position. If for whatever possible reason it could fail closed then the fusible link, which protects the machine from massive overheating would have blown. And that would have shut down the whole machine.

On my Maytag the wheel is plastic and attaches to a metal shaft. The torque is too much for the plastic and that attachment point fails every decade or so.

Found no slippage of the fan blade and motor.

Using a small AC bulb (like a nightlight) might be easier and more definitive than a meter to determine if power is present.

I'd fear that the bulb might draw enough current to hold the gas valve open. I doubt it would open the valve, but once the valve is open it might not close. That could cause an overheating condition. Then I'd be replacing the fusible link.

For anyone who doesn't know what a fusible link is - it's a thermal fuse that blows at a given temperature. Once blown it's blown for life. It can not be reset. It's not the same as a thermal switch, those self reset. If I bypass the fusible link I could have a full blown fire and my wife would get to meet the firemen with the broad shoulders and hairy chests. (bit of a cliche)

 

The blower wheel on my Kenmore has failed on that connection to the drive shaft several times. The fix that has lasted the longest so far is a lightweight, stainless steel, band type hose clamp. The adjustable ones come in light and heavy duty, this one is the 3/8 wide, thin, adjustable kind, to minimize the unbalance.
Getting to that fan wheel requires removing the front of the machine, which is easy, once the top is popped up a bit.
One caution is that the blower housing is plastic so never use a power screwdriver to put the screws back after replacing the blower wheel. Stripping out the screw holes is not fixable for most people.

 

The blower wheel on my Kenmore has failed on that connection to the drive shaft several times. The fix that has lasted the longest so far is a lightweight, stainless steel, band type hose clamp. The adjustable ones come in light and heavy duty, this one is the 3/8 wide, thin, adjustable kind, to minimize the unbalance.
Getting to that fan wheel requires removing the front of the machine, which is easy, once the top is popped up a bit.
One caution is that the blower housing is plastic so never use a power screwdriver to put the screws back after replacing the blower wheel. Stripping out the screw holes is not fixable for most people.

Fan is in back and fan is not the issue. No slippage.

 

So your current hypothesis is that the cycling thermostat has degraded to where it shuts off the gas at too low an internal temperature? I guess that's possible. You can spend time testing it as I mentioned earlier or simply replace it - they're not very expensive.