Gas Dryer Ignitor won't burn

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[genoapr]

The ignitor (P/N 279311) on my gas dryer had gone bad (no continuity Ohms) so I purchased a new one and replaced it.  The new one still does not glow.  I checked the input voltage to the ignitor and it reads 30 volts (clearly not the 110 volts needed to energize the ignitor.  I checked the thermal fuse (P/N 3390719) and high limit thermostat (P/N 3403140) and have continuity across both of them.  I checked the thermistor (P/N 8577274) and have no continuity so I know this component is bad and needs to be replaced.  

My main question is that I also checked the radiant sensor (P/N 338906) and have no continuity on this component.  Is this sensor normally open and closes when it senses heat from the ignitor or is it normally closed? This will help me determine whether the part needs to be replaced or not.

Is there anything I may have missed that would account for the low voltage reading at the input to the ignitor?  I did the diagnostic test on the control panel and everything seems to function properly.  Also, I checked the resistance on the gas valves and they all check out within specifications.

I need to order the parts and just want to make sure I am getting the right ones this time around.

[jumptrout51]

The flame sensor is normally closed.

[genoapr]

Thanks.  I will order both the Thermistor and Flame Sensor.

[RegUS_PatOff]

The Thermistor won't have continuity.

It's a variable resistor, and should be about 10,000 OHMs (10k) at ambient.

[genoapr]

Not sure I completely understand.  The specs for the thermistor read 10K Ohms which I interpret to mean that "in addition to continuity" it must have that resistance or it is probably no good.  I get an open circuit reading on the thermistor so cannot determine resistance.  Where am I going wrong in my logic.

[RegUS_PatOff]

set your OHM meter for 20k and try reading again.

[genoapr]

Thanks.  I set the ohm meter to 20k as suggested and got a normal reading of 10.2k Ohms which is well within specification.

For the record, I replaced the ignitor and the flame sensor and the dryer is back up and running. 

I am not sure if it was on this forum or another but there was a posting that strongly implied to the original writer that if the ignitor would not burn then looking at the flame sensor was absurd because it was not part of the "pre" control circuit and would only effect whether the gas values would open once ignitor burn was detected.  I want to let any future reader know that this is not true and very misleading.  A faulty flame sensor apparently limits the power supplied to the ignitor; in my case the voltage getting to the ignitor with the bad flame sensor was only 30v.

Thanks for all your help

[JWWebster]

OK so you are saying the flame sensor/detector had continuity but was still bad?

[genoapr]

Let me start by saying I am way in over my head on this one.  I'm a chemical engineer by training...really no nothing about electrical.

When the bad flame sensor (no continuity reading) I still got a voltage reading to the ignitor of around 30 volts.  When I installed the new flame sensor the voltage reading to the ignitor went back to normal (115 volts). 

My comments in the previous post simply had to do with the fact that when I was searching for clues as to what I should look at to trouble shoot the problem I ran across a posting were the technician was berating the diy'er about the fact that the flame sensor could not possibly have anything to do with the ignitor glowing since it was designed to detect it once it was glowing.  Unless I am wrong, it looks like it must have some feedback to prevent the ignitor from activating.

[JWWebster]

This is my understanding and if anybody knows better let em speak up. You got 30 volts because the ignitor's cold resistance is competing with the operator coils and their is no flame present. Therefore the the voltage reading is gonna naturally be low because the ignitor has to have a lower resistance in order for them solenoid coils to work properly. The only way to do it is change resistance in the circuit. The only thing that changes in resistance is the ignitor as it gets hotter. If the flame sensor has continuity full power is applied to the ignitor by it. When the temp reaches 1800f the bimetal inside the flame detector opens the circuit to the ignitor and kills power to the ignitor. Since it is blazing hot the raw gas comes out hits it , ignites, washing the flames through the ignitor. The ignitor stays toasty, becomes a thermocouple type safety, and allows power to them solenoids because its resistance has changed. If for some reason the ignitor did not ignite, once it cooled down it kills the circuit, shutting down the whole gas assembly.

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