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Old 11-03-2011, 05:09 PM   #1
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Goodman furnace dead?


I have a Goodman GM100-3 furnace that had been giving me some trouble operating when I wanted it to for the past few days. I had originally thought that it may be a corroded or bad flame sensor. I wanted to replace the old mercury t-stat with a new digital so I changed that today then cleaned the flame sensor with some emery paper. when all that was done, the furnace would not turn on at all. I call for heat at the t-stat and the furnace does nothing.
I checked the power to make sure I had remembered to turn it back on and it was. I checked the wiring to make sure I didn't pull any off while cleaning the flame sensor. I double checked that the new t-stat was wired correctly. I checked the voltage across the red and white wires and it read 26.6 VAC.
The LED on the circuit board is steady, no fault codes. The fuse on the board was fine but I changed it anyway. I checked the roll-out switches and the door interlock switch and all worked fine.
I jumped the red and white wires from the t-stat and that did nothing. I even jumped the terminals on the limit switch and that did nothing.

I am out of ideas and I still have a furnace that does not run... any suggestions?

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Old 11-03-2011, 05:27 PM   #2
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Goodman furnace dead?


Check the voltage between c and r at the furnace. Should be 24. Then check the voltage between c and w at the furnace with a call for heat. Should be 24v.
Let us know what you come up with.

Also during all of these tests make sure the door switch is made.

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Old 11-03-2011, 05:28 PM   #3
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I would double check your stat wiring. At the furnace board you should see ~24VAC across red to common all the time and across white to common on a call from heat. Not sure why you have 24V across red and white?
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Old 11-03-2011, 05:52 PM   #4
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If the thermostat was calling for heat, and you read 24 volts across R and W, then either a wire is broke, or the thermostat is bad, or not set up right.

As said above, with the thermostat calling for heat, check for voltage between W and C/common.
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Old 11-03-2011, 06:08 PM   #5
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Goodman furnace dead?


you can unwire stat wires off control board..and jump between r-w see if starts this will by pass thermostat.....ben
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Old 11-03-2011, 10:03 PM   #6
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Quote:
Originally Posted by hvac122 View Post
Check the voltage between c and r at the furnace. Should be 24. Then check the voltage between c and w at the furnace with a call for heat. Should be 24v.
Let us know what you come up with.
Also during all of these tests make sure the door switch is made.
I have 4 wires coming from the furnace. Red (t-former)= 24vac, White (control/heat), Green (fan), Blue (used as yellow to AC)
Red to Ground at the t-stat is 26.6vac and the same at the furnace.
White to Ground with a call for heat is also 26.6vac
AC settings work as expected as does the fan only setting.

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I would double check your stat wiring. At the furnace board you should see ~24VAC across red to common all the time and across white to common on a call from heat. Not sure why you have 24V across red and white?
Double and triple checked. tried running nothing but the Red White and Green wires without the blue to AC condenser just in case. I even put the original mercury dial t-stat back on with no better results.
I did not type exactly what I wanted to say in my original post. I meant to say that I have 26.6vac at the Red wire as expected therefore confirmation that the t-former is operating correctly and when jumped to the White wire, I still get nothing.

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Originally Posted by beenthere View Post
If the thermostat was calling for heat, and you read 24 volts across R and W, then either a wire is broke, or the thermostat is bad, or not set up right.
As said above, with the thermostat calling for heat, check for voltage between W and C/common.
See above two answers. I knew what I was thinking but by time it reached my fingers it was convoluted.

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Originally Posted by ben's plumbing View Post
you can unwire stat wires off control board..and jump between r-w see if starts this will by pass thermostat.....ben
Tried this also. When I jump them all I get is a click, then nothing but the faint hum of electricity flowing


A bit of new info though... in my original post I had said that I had checked the roll-out switches but I had only checked that none of the four had tripped, and they hadn't. I went back to check continuity across the switches and found nothing. I get nothing in or out of any of the four switches with the t-stat calling for heat or not.

With all else functioning as expected, 26.6vac, continuity everywhere I have checked except for the rollout switches, AC and fan functions working, steady LED with no fault codes... is the next assumption going to be that the board is shot?
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Old 11-03-2011, 10:11 PM   #7
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Goodman furnace dead?


Roll out switches and temperature limit switches are normally closed meaning you SHOULD have continuity through them if they are NOT tripped. When a furnace is running and you pull a wire off of a roll out or limit the furnace will stop producing heat, signalling a tripped switch.

Without any continuity through them then you will not have heat, but you should have a fault code for those.


Do you have cooling? Fan? You can jump r to y or connect the stat back and put it in cool and find out, and fan ON.
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Old 11-03-2011, 10:24 PM   #8
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Roll out switches and temperature limit switches are normally closed meaning you SHOULD have continuity through them if they are NOT tripped. When a furnace is running and you pull a wire off of a roll out or limit the furnace will stop producing heat, signalling a tripped switch.
Without any continuity through them then you will not have heat, but you should have a fault code for those.
Do you have cooling? Fan? You can jump r to y or connect the stat back and put it in cool and find out, and fan ON.
Yes, Exactly. A tripped switch will have no output but should still read something in... or does that logic not apply in the HVAC world? Thats not really sarcasm, its a real question because I dont want to sound confident then be informed of some reason I did not expect.. lol
None of the switches were tripped, they were closed as expected yet showed nothing in or out of the switch and no fault codes.
I do have AC and fan only functions both with the t-stat connected and by jumping the wires.
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Old 11-03-2011, 10:34 PM   #9
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Goodman furnace dead?


No inducer fan motor starting? That's the exhaust fan that, well, exhausts the burnt gases out through the metal pipe through your roof.

If nothing while on call for heat than on the control board, trace the two wires from the inducer back to the board and remove those wires. Check for VOLTAGE between those terminals. You should have 115/12o-ish volts to the inducer motor between those two terminals which one terminal on the board will be labeled DI for Draft Inducer (or similarly labeled) and the other will be on the neutral "bar" next to about three or four other (white) neutrals. The inducer neutral may or may not be white but one wire from the inducer will be on a neutral.

Check that and if you do have voltage to those terminals than more than likely the draft inducer motor is bad. You can hard wire that motor to see if it comes on, bypassing the board. You hard wire the inducer (using the main power to the furnace, the hot and the neutral) wire nutted straight to the two inducer wires and then turn the power (back) on to the furnace. If the inducer is hard wired with power on to it but doesn't come on than that's your problem.

Please turn off the power to the furnace before doing any hard wiring work.

Last edited by Doc Holliday; 11-03-2011 at 10:37 PM.
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Old 11-03-2011, 10:36 PM   #10
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Goodman furnace dead?


Btw, the inducer fan is THE FIRST thing to come on while on call for heat, regardless of any switches. If that is not coming on than that is where you check first.
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Old 11-03-2011, 10:49 PM   #11
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Quote:
Originally Posted by jtobias View Post
Yes, Exactly. A tripped switch will have no output but should still read something in....
they are wired in series, if one is tripped than the entire line is out.
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Old 11-03-2011, 11:33 PM   #12
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Btw, the inducer fan is THE FIRST thing to come on while on call for heat, regardless of any switches. If that is not coming on than that is where you check first.
Ok, the inducer does not kick on at all. I pulled the wires and read 120vac across them. Just to be sure though, I went right to the switched power coming in to the furnace and jumped it directly from there. It still did not kick on when i flipped the switch.
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Old 11-04-2011, 07:34 AM   #13
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Goodman furnace dead?


if your reading 120V going to the ID and have no run the motor is shot ohm the 2 wires from the motor should see something and check to the frame of the motor..the ID motor is the first thing to run on a stat call for heat from the control board
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Old 11-04-2011, 07:46 AM   #14
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Goodman furnace dead?


I win.
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Old 11-04-2011, 03:34 PM   #15
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Goodman furnace dead?


All Fixed!!
Thank you to everyone who replied and moved me in the right direction.

Turns out that the motor itself was not bad. Somehow one of the wires had broke free from the terminal.
Last night I had verified that the motor was getting the 120vac it needs so this morning I went back to check for resistance across the motor itself and there was none. I must have wiggled it just enough because one of the terminals broke off and revealed the problem. there was just enough of the wire available to make a new connection to the terminal, then a big scoop of high-temp epoxy and I am back in business!




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