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#1 Posted : Friday, August 25, 2017 8:09:43 AM(UTC)

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I was hoping to find an expert as this problem is not going to be easy to solve. I have a coin-op dryer the initial complaint was the dryer would never shut off unless you opened the door or unplugged it. However to restart it all one had to do is plug it back in, close door, push the start button - no coins required! I am pretty handy and tit sounded like a timer issue. It turns out the timer was actually mechanically broken, it was a small grey plastic tab that latched the white plastic timer mechanism to advance to the next cycle. I replaced the timer.
However it seems there is another problem, not sure if it was caused by the dryer being left to run for hours on end due to above issue or that it truly is a secondary problem in it's own right. Here comes the hard part - the dryer will start normally, igniter gets hot, gas valves open, flame gets hot, then after only 10 seconds flame goes out, 90 seconds later it comes back on again. Those steps repeat in an endless loop. I checked the cycling thermostats, the thermistor, and the high limit T-stat all are working all show continuity at room temp. I put my Volt meter across the high limit switch while flame is burning and saw that it goes open when flame stops. I suspected that perhaps the high limit switch was kicking open too soon, so I tested it on a hot plate, -it works great. Shows continuity below 230F and then went open above 230F, on the cool down side the switch closed at 205F. So I checked the airflow to the dryer I did have a lot of lint but cleaned it all out - even removed the exhaust tube from the back, and there seemed to be a pretty forceful column of air coming out.
So I'm lost as to what could be going on here. Additional clues are:
1) While testing dryer thru these fast cycles, the high limit tstat kicks open but the cycling thermosts upstream are not tripping. Interesting to me because only 18" downstream at the high limit I know we are going above 230F and the cycling t-stats operate at much lower temps. So will investigate further airflow inside the machine
2) I haven't checked to see if the igniter is shorted to the case - read somewhere to check that
3) I haven't measured the exhaust temp coming out of the dryer
4) I am testing without clothes in the dryer - could this cause high temp at the limit switch?
5) I have checked the flame sensor early on in the process it shows continuity. Also can hold it up to my hot flashlight bulb for about 15 seconds and it trips it open, so I assume it's good. Also flame does ignite so wasn't thinking of flame sensor as the problem.
If anyone has logical suggestions based on the info provided please I could use the help.
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#2 Posted : Sunday, August 27, 2017 8:33:20 PM(UTC)

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If the high limit is opening obviously it will kill power to the coils and the valve closes, flame stops and it resets when high limit closes again. You've properly IDed airflow as the most likely suspect by far.

Any odd noises or vibrations from blower area that might indicate something in there? The blower cover comes off easily enough to check for a sock or other item stuck or a loose blower wheel as these can tumble normally but have a broken or stripped out blower wheel that isn't spinning well.

Since you can observe the burner while it runs on these look for evidence in the burner tube that the flame is actually leaking around the flame spreader and directly at high limit so it opens early, low likelihood but I've seen it enough times to be worth looking at. You may be able to see in real time or just infer from color of metal at high limit.

And as to your numbered items,
1) Definitely a good idea
2) Probably not, never seen an ignitor operate and be shorted at the same time, but easily checked.
3) Manuals usually spec exhaust temp at outlet but few techs will look at it there, won't hurt though. Checking in drum while running would be better if you have a wire temp probe with your multimeter.
4)Testing empty is best, as that is the best airflow it can have so the operating thermostat should be controlling it. High limit should be just what it's name implies, a high temp safety at the burner if the operating thermos haven't opened at their set temps.
5)Flame sensor is probably fine if you have any ignition.
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