Goodman GMT Furnace open limit switch problem


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Old 11-03-14, 03:14 PM
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Goodman GMT Furnace open limit switch problem

Hello, Ive been having problems with my goodman furnace.
Heres what happens:

When i turn the thermostat on, the blower will turn on and shortly thereafter the flames will ignite. They will run for three or four mins and then go out and the blower will continue to run for awhile before the process repeats itself.

Last year i had a problem with my flame sensor so i replaced it. However, that was not the culprit. I then noticed i was getting 4 blinks from the diagnostic light; which correlates to an open limit switch. I replaced the limit switch and still have the same problem.

It's starting to get cold and the old lady is just about running out of patience for my do it yourself fix. I really think i'm capable of fixing it, but i'm just not sure what to do next.

Any help would be greatly appreciated!!!
 
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Old 11-03-14, 03:31 PM
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Welcome to the forums!! An open limit switch is indicative of an over temperature condition and is usually caused by a dirty/clogged air filter, a blocked/restricted vent pipe, an obstructed return air duct, or a dirty/clogged a/c evaporator coil (if equipped). The most common cause is an air filter that has not been changed in awhile or is too restrictive.
 
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Old 11-04-14, 06:07 AM
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thanks for the reply!

I have a new air filter in, and tried running it a few times without a filter, and still encounter the same problem.

none of the cold air returns are blocked or anything like that...
 
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Old 11-04-14, 06:35 AM
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Here is a link to the service manual: http://www.hvacpartsshop.com/Gmph075-4%20Series.Pdf

Since you replaced the main limit switch and it did not correct your issue, I suggest you check the auxiliary limit switch to see if it is tripping during operation.

A 4-flash code can be for an open main limit or an auxiliary limit. The auxiliary limit switch is usually mounted on the blower housing or deck (page 38). The wiring diagram also shows a stack-over limit switch that may be tripping. These safety devices are wired in series and the best way to locate them is to follow the purple wires.

Have you checked the vent pipe to make sure there are no blockages? If you have an a/c coil make sure that is clean also. The coil is an often over-looked item that will block the flow of air and cause over-heating and tripping of the limit switches. especially the auxiliary limit switch.. Also, make sure all of the room vents are open. Do you have strong air flow coming from the registers?
 
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Old 11-04-14, 08:31 AM
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i'm having trouble located the aux limit switch... but i will head down there again in just a moment,

No air conditioning.

I was able to remove a panel of the duct work about the blower and shine a flashlight to see in. i wasn't sure what i was looking at haha, some sort of coil?But everything looks relatively clean in there.

Air flow was has never been great coming from vents even when the furnace was working properly last year? You can definitely feel heat coming from the vents when it first starts but its not like its going to blow your skirt up or anything...

I've posted a picture of the inside of the housing
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Old 11-04-14, 08:53 AM
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It is possible you don't have an auxiliary limit switch. Service manuals and wiring diagrams cover many models of furnaces and changes are often made by the manufacturer. The best thing to do is to check the wiring diagram on the furnace to see if it shows an auxiliary limit. If you have an auxiliary limit it will probably be on the side of the blower. If your air flow is not that great it could be the cause of the limit switch tripping. Sometimes, homeowners install air filters that are too restrictive or forget to change the filter which will restrict the air flow and cause overheating. As stated earlier, if an a/c coil is used it can also be clogged up and restrict air flow.

Some Goodman furnace blowers have the auxiliary limit as shown in the attached photo (circled in white).

To better assist, can you post the entire model number of the furnace?
 
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Old 11-04-14, 09:57 AM
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Hello, thanks for the reply!

The model # is GMT090-3A
 
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Old 11-04-14, 10:37 AM
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after further inspection it appears what i've been referring to as the "blower" is actually the draft inducer. The blower which is below the compartment i poster a picture of doesn't seem to be coming on at all? Is this possible that i would still get a bit of hit through my vents without the blower coming on at all?
 
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Old 11-04-14, 11:07 AM
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If your blower is not coming on that will explain why your limit switch trips. Your blower motor has a run capacitor that looks like the attached photo. Hopefully, that is all it is. The capacitor can be tested with a voltmeter that has a capacitance test feature to see if it is still within specs. The capacitor is relatively cheap and most DIYers replace it as a first step. With power shut off to the furnace you can inspect the capacitor for a bulged top which is often (but not always) an indicator of a bad capacitor. The run capacitor should be 12.5uf/370VAC.
 
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Old 11-04-14, 01:21 PM
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I appreciate all the help!

looks like the run capacitor was bad. Luckily i was able to find a shop in town that had one on hand. I replaced the capacitor and its blowing out heat like never before!

We would have gotten to the bottom of this much sooner had I diagnosed the problem properly! Thanks for the patience!

So in the end I spent about 30 bucks on parts (half of that amount I didn't need) but i still feel I made out a lot better than calling a repair man.

Looks like i'll have to celebrate my first furnace repair with a beer!


Thanks again!
 
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Old 11-04-14, 02:06 PM
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Awesome! Glad to hear you are back up and running and thanks for the update. You definitely saved some money and got the satisfaction of DIY.
 
 

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