Goodman GMT045-3b cycling.

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Old 12-04-10, 01:44 AM
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Goodman GMT045-3b cycling.

This furnace was cycling on and off. I cleaned the flame sensor with 220 sandpaper and 400. It didn't really look that dirty prior to cleaning. Replaced the flame sensor and it fires up for about 45 seconds when cold. Then the flame goes out and it cycles on again. The next times the flame is only on for about 8-10 seconds. It just does this over and over. I saw in a service instruction PDF to not use sandpaper when cleaning the flame sensor.. said use steel-wool or Emory paper to clean the flame sensor. Could there be other problems? Thanks!

http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf
 
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Old 12-04-10, 08:57 AM
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Well, of course there is something else wrong if the furnace doesn't operate properly. I personally don't recommend using anything but a Scotchbrite pad for cleaning flame rods and anything more abrasive (or coarser) will leave many little ridges that will quickly become oxidized and result in no flame signal being sent.

Now that you have used sandpaper go back and use the finest grit you have, then check ALL the wire connections from the flame sensor to the logic board to be certain they are clean and tight. If you have any equipment grounding jumpers between the burner assembly and the case of the furnace and/or the logic board make sure these are also clean and tight.
 
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Old 12-04-10, 09:03 AM
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I'd check the pressure switch and see if you have 24 VAC on both sides. The pressure switch or something else is probably shutting the furnace off.
 
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Old 12-07-10, 08:01 PM
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Initially this furnace would cycle on and off a good number of times and wouldn't lock out. I never had to reset it. I checked grounding several places and everything checked out fine with very low resistance. I had the pressure switch out a few days and was looking for a new one around town but couldn't locate it. I did check that it was open with the fan off and closed with the fan on using resistance. Then it also seemed to have voltage (26ish) on both sides moments after the fan powered up.

I tried to start up the furnace after it was all in place and got a lockout (which was the first time)... I discovered then that I had actually disconnected the flame sensor at the control board the day before because I had planned on testing it in series for uamps after the flame ignited. Anyway... I reset the lockout and plugged the flame sensor back in..

After everything was in place the last time the heater seemed to work fine. Will see how it goes for a few days..

I have a new flame sensor arriving tomorrow ($9) and will likely buy a OEM pressure switch (ebay) to have on hand. Was interesting that my neighbor told me about a Goodman dealer and appliance supply store just 4 blocks away... When I went there they had a fresh sign that said out of business for good..

I had planned on having a furnace shop come out and install a new pressure switch and/or flame sensor and check things over...

If the pressure switch had a fault it seems the furnace would not have fired all those times... isn't that correct?

Maybe there was some strange problem that got reset when the lockout happened.. Or maybe there was a loose connection somewhere that got adjusted. hmmmm....

Thanks for your replies...
 

Last edited by kalo93406; 12-07-10 at 08:20 PM.
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Old 12-07-10, 08:25 PM
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Replacing the flame sensor is a waste of money and there is a 95% probability that replacing the pressure switch is a waste of money too. It very unlikely either will ever need to be replaced.
 
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