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Amana Dryer Blowing Motor Thermostat
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Model: NED7200TW10 Brand: Amana Age: 1 - 5 years
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horrocks  
#1 Posted : Wednesday, July 18, 2012 5:42:56 AM(UTC)
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horrocks

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First failure was motor would not start. Problem was open TH1 Thermostat Cut off. 4 loads later, TH1 opened again. I decided to check and clean everything I could. The dryer exhaust fan was quite dirty. I am torn now whether to continue to troubleshoot or just try it and hope I fixed it. I am unable to determine how the heating circuit works. The root cause could be that the heater never turns off. Is TH3 (thermostat cut off in heater circuit) a temperature cycling device? Would TH2 thermostat (Hi-Limit) open before TH1 (motor ciricuit) would open? What does the thermistor do in this dryer? Does it control the heater relay which could also cycle the heater?
Thanks in advance,
Mike Horrocks

Since this posting, I took some measurements. I checked to see if the heating coil was shorted to ground and it was not. The resistance of the heating coil itself was about 9.5 ohms. I ran the dryer empty on regular temperature (I think defaulted anyway), timed dry, removed the exhaust connection and measured the temperature. It was steadily climbing and I stopped measuring at about 155 F. I believe it can go as high as 175 C or 347 F. The exhaust is quite forceful in air flow after I cleaned out the machine yesterday, but, I did not check it before cleaning and do not have an airflow measurement device to compare before and after. After I ran it for about 3 minutes on timed dry, I shut it down and measured the thermistor resistance in the exhaust. It measured about 9.45K ohms. I did not measure it cold,k which I believe should be about 10K ohms. It did have a few minutes to cool down while I disassembled the front to access the thermistor. So, that reading probably does not tell us much other than it is not open.
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denman  
#2 Posted : Thursday, July 19, 2012 2:41:29 AM(UTC)
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denman

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Here are your parts
Replacement parts for Maytag NED7200TW10 29" ELECTRIC DRYER | AppliancePartsPros.com

See the attachment for the tech sheet, just in case you do not have it.

[COLOR="Blue"] I am torn now whether to continue to troubleshoot or just try it and hope I fixed it.[/COLOR]
I would just try it as a dirty vent system is the most common cause of this cut-off blowing. Also check that the outside vents open fully.

[COLOR="Blue"]The root cause could be that the heater never turns off.[/COLOR]
Not sure why you mention this.
Have you seen the dryer do this and if yes under what circumstances?
[COLOR="Blue"]
Is TH3 (thermostat cut off in heater circuit) a temperature cycling device?[/COLOR]
No, it is also a thermal fuse like TH1

[COLOR="Blue"]Would TH2 thermostat (Hi-Limit) open before TH1 (motor ciricuit) would open?[/COLOR]
Not necessarily. They are protecting different areas in the air flow path.
So TH1 usually blows when there is a problem with the venting.
TH2 is limiting the overheating of the heater. So if you have a thermal run away due to a blown control board, a bad thermistor, a grounded element etc., it opens and cycles. What you would see is that the exhaust/drum temperature stays the same no matter what temperature you select (always runs on high). It is a safety device so is not designed to run a lot and eventually it's contacts will fail (can weld together). TH3 then blows as it is the backup for the hi-limit.

[COLOR="Blue"]What does the thermistor do in this dryer? Does it control the heater relay which could also cycle the heater?[/COLOR]
Yes, it's resistance changes with temperature. The control board reads the thermistor resistance and then uses that info to turn the heater relay on/off to regulate the temperature. This is the temperature control system.
The thermostats are all safety devices and should not open unless there is a prob;lem with the unit.
[COLOR="Blue"]
The resistance of the heating coil itself was about 9.5 ohms. [/COLOR]
This is good.

[COLOR="Blue"]So, that reading probably does not tell us much other than it is not open.[/COLOR]
I could not find out if the thermistor is a PTC or a NTC. One increases resistance as the temperature rises. The other decreases the resistance with temperature increase.
File Attachment(s):
NED7200.pdf (1,696kb) downloaded 3 time(s).
THIS FORUM IS DEAD!!!!!!!
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horrocks  
#3 Posted : Thursday, July 19, 2012 3:41:55 AM(UTC)
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horrocks

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Thank you very much for the information Denman. I have looked at numerous posts and usually only get contradicting or just bad info. According to the technical document you attached, the thermistor is a PTC device:

Component Electrical Testing (with ohmmeter)
• Thermistor resistance 10K Ω @ 25°C 77°F PTC
(2P-Blue & Red wire)

Therefore, if after running for 3-5 minutes on timed dry, and if the thermistor measured only 9.5K ohms, it appears it may be defective. It should be well over the 10K ohm spec. Do you agree?

Update:
I decided to remove the thermistor and measure it at room temp. It measured 27.5K ohms. Definitely not to the 10K ohm spec. I then heated it with a hair dryer and it reduced to about 12K ohms. Contrary to the component test spec, it appears to be an NTC device and not a PTC device. Also, in the diagnostic code information in the spec you sent me, it says at very low temperatures the resistance will be higher, also signifying a NTC device.

I will replace the thermistor, thanks again Denman for the info and spec sheet.
Mike
Mike
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denman  
#4 Posted : Friday, July 20, 2012 1:48:00 AM(UTC)
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denman

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You are welcome.

Sounds like you have found the culprit.
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