Goodman GMP075-3 problem: HELP!!!


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Old 02-02-09, 05:29 PM
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Unhappy Goodman GMP075-3 problem: HELP!!!

My GMP075-3 was blowing fuses on the mother board and I traced the problem to the limit switch behind the gas valve. Now the furnace's cold air fan comes on but nothing else happens. The ignition system does not engage at all nor does the draft induction fan. I'm getting 4 flashes from the light on the motherboard for a bad limit switch. I will replace the switch tomorrow but my question is:

Can a bad limit switch cause the cold air fan to run and prevent the rest of the systems to engage?

I want to know so I can check for any further problems before it get any colder than it already is. It's supposed to be 15 degrees F here in the next few days so PLEASE respond as soon as possible. Thanks!
 
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Old 02-02-09, 05:52 PM
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Limit

Yes, when a limit is open the board "thinks" it is due to a high temperature condition. Because of this the board keeps the fan on & the burners off to cool the furnace.
 
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Old 02-02-09, 06:03 PM
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Thanks, Grady!
Now all I have to do is FIND a limit switch in South Bend, IN.

By the way, you wouldn't happen to know the correct part number for it? The numbers on mine are so light, they can't be read. I was just going to take it with me and try to match it up.
 
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Old 02-02-09, 06:11 PM
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Can your limit switch be reset?

And regardless, you may want to find cause of it limiting, like clogged furnace filter, closed off registers in the house, closed off duct dampers in the basement, blocked off cold air returns -and could also be from plugged up a/c coils above the furnace if you have that, or possibly problems in drafting of furnace if the limit for some reason is tripping sooner than the pressure switch would.
 
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Old 02-02-09, 06:16 PM
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Limit

If it is the oval rollout switch which mounts on the burner box, the original # is:B13701-08 & is described as "Rollout Switch, 300". If it is the one mounted on a piece of plastic & has two arms which hold the actual sensor & sticks into the furnace about 7", the original # is: B13700-08 & is described as "Limit Switch, 160, 7 inch".
 
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Old 02-04-09, 08:50 AM
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After a sleepless night waiting to see if the furnace acted up again, I can report it worked like a charm. I had a dead limit switch and one dead rollout switch. After replacing them (and a cursory cleaning of the draft end of the heat exchanger) the system fired up perfectly.

THIS FORUM IS GREAT!!

Thanks for the info, guys. And a "shout out" to you, Grady!Beer 4U2
 
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Old 02-04-09, 04:48 PM
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Rollout

Thanks, whozat. I must urge you to have the furnace inspected by a professional. It is entirely possible the heat exchanger has failed & causing a dangerous condition.
 
 

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