Goodman GMH8 Furnace- 6 blinking lights!

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Old 05-15-17, 07:28 AM
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Goodman GMH8 Furnace- 6 blinking lights!

Hi All,

House fan runs, no AC. I'm a novice and don't know anything.

Induction blower starts up, looks good/sounds good, and then shuts down.
6 flashes on board immediately on turning on the furnace.

air inlet grey tubing, and into induction blower, has no blockage since i would have pushed anything out with paper clip and blowing through it.

Wiring together the two yellow leads didn't do anything, i guess it wouldn't.

Impossible i think to have the flue blocked, it goes straight to ceiling.

Possible causes? Possible replacements according to manual include a 3A automotive fuse somewhere i'm not seeing, wiring problem somewhere (couldn't find anything wrong), induction draft blower replacement.

I have replaced the rollout limit before, probably didnt need to, last time it seemed like a piece of dust was caught in the inlet to induction motor..

Suggestions?



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  #2  
Old 05-15-17, 08:38 AM
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The items that you mentioned that you checked (inducer, vacuum switch, etc.) are all related to heating (required to be operational for heat). For A/C, the only part of the furnace that's used is the blower. All the A/C "work" is done by the outside unit (compressor, condenser, and condenser fan). The evaporator coil which is mounted inside the furnace plenum gets cold when the A/C is running, and the blower pushes air past it, exchanging heat from inside the house and sending it outside where it's dissipated into the outside air by the condenser & fan.

The 3A fuse is likely good because otherwise the blower motor wouldn't come on. The 3A fuse is used to protect the 24VAC transformer in case of a short. The 24VAC goes to the thermostat and is used to control the blower and outside A/C unit.

If you have a multimeter, set the thermostat to call for cooling and, at the furnace where the thermostat wires connect to the control board, check for voltage between the Y and C terminals (furnace chassis if no C terminal). With the thermostat calling for cooling, you should measure ~24VAC.

Let us know what you find and we'll tell you what the next step is (depending upon what you measure).
 
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Old 05-15-17, 09:00 AM
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Have you replaced the thermostat lately ? Something is not wired right. Like Bob said on a call for cooling you should not see the combustion blower running.
 
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Old 05-15-17, 06:25 PM
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I didn't even think about that, thanks. on further look, the furnace at the moment is not responding at all to the thermostat, i can't control the fan, or change from heating to cooling, the furnace appears 'stuck' in heating everytime i activate the case switch cover. The induction motor starts with the house fan, the induction motor quits and then the house fan stays on. can't shut it off at thermostat.

The thermostat is 10 years old when i put it in, just replaced batteries with fresh yesterday before i fiddled around with anything else.

I'll use the excuse to get a new thermostat (they did not have the one i wanted in stock at Lowes, going to HD tomorrow), and when i plug the new one in, I'll check how it works and the 24VAC in bridging Y and C at the furnace when i have new one set to cooling.

Thanks!!!
 
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Old 05-16-17, 05:32 AM
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Since it sounds like you have a voltmeter, here's what you ideally should measure. With the thermostat calling for cooling, at the furnace side of the thermostat wires you should measure the following voltages:

R to C: 24VAC (R is the 24VAC coming from the transformer & fuse), C is common or ground
Y to C: 24VAC (Y is the control voltage that turns on the outside unit)
G to C: 24VAC (G is the control voltage that turns on the blower in A/C mode)

24VAC is the nominal voltage from the transformer, however many transformers put out 26-28VAC, so don't be alarmed if the voltage is a little on the high side.
 
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Old 05-16-17, 07:00 PM
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OK a quick update, I discovered several 'gifts' left by the installer tech.

1- Bought a new thermostat on sale, Honeywell Lyric T5. Went to shut off at breaker, no shut off on labeled attic furnace, switch off 'attic fan' breaker, attic fans shut off and furnace. The furnace line terminates in a box in the attic, and furnace line split off from fans!

2- OK, onto the rest. I wire in thermostat, blue C line is nipped inside the wall. Cut some drywall, splice it in with spare. OK, go to attic, blue line nipped at line, tear back more insulation plastic and wire it into the blue line "C" from furnace, that is already tied into a white line that goes i guess outside, along with a yellow line splice wire (so 2 wires go outside, x5 go to thermostat). A brown line is not connected and not on my furnace.

3- in process of wiring up the blue C line, i notice the tech left like an inch of the red line (24V?) exposed outside the wire nut, and this was very close to the metal drip pan, not sure if it was touching the metal pan or not..

4- Wire it all into thermostat, flip breaker, thermostat says no power, and furnace still kicks on induction blower and house fan, with errors.

So basically, i can't even get the thermostat to register it is connected...

Also, my voltmeter can't pick up any voltage in any of the pairs when the fan is running and the 6-blinks are going off, C to anything else (R Y or G). When i have the case-fan tripped so the fan is off, still the same thing, no power i can find (except the AC 120V coming in, i can see that!)


Geez, does that mean my whole board is fried?
 
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Old 05-17-17, 09:54 AM
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I think you mentioned that there is a 3A automotive style fuse on your furnace control board. Check that fuse to see if it's open. It's purpose is to protect the 24VAC transformer should there be a short on the 24V line (e.g. your bare red wire touching metal).

If the fuse is bad, replace it. Otherwise, check to see if there is 24VAC coming out of the transformer. You can do this either of 2 ways. Pull the 3A fuse and check for voltage between the furnace chassis and either side of the fuse (one side is input, other is output). Alternatively, look for the transformer (shouldn't be too hard to find, usually square/rectangular with 4 leads, 2 on each side). One side will have 120VAC (possibly 240) going in, the other side will have 24VAC going out.

You need to get your 24V working, as everything is run off of that.
 
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Old 05-17-17, 02:04 PM
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OK an update:

1- Yes, fuse was blown. Replaced it, now on power-up the thermostat works, and the furnace is off including induction blower and house fan.

2- at 'rest', with the thermostat in the off position, i can measure:
R to C: 35VAC (i double checked this with reset)
Y to C: 0
G to C: 0

3- at call for cooling, the outside unit still does not turn on, no cold air still, but house fan turns on. I can measure:
R to C: 2-9VAC. It stays 2 or 3 for a few seconds, goes to 9, then goes to 1-2, and back up, spends most time at ~3VAC.
Y to C: 0
G to C: 0


Only thing i changed in wiring from before was fixed the red line bare wire issue, and i wired in the blue C line into the wire nut that was connecting from furnace to the 2-wire line that runs outside.

What to do next do you think?

Thanks everyone for helping!!!!


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Old 05-17-17, 04:54 PM
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2- at 'rest', with the thermostat in the off position, i can measure:
R to C: 35VAC (i double checked this with reset)
Y to C: 0
G to C: 0
35vac is a little high. You needed to measure from Y to R and G to R. (with stat removed)

I don't like to see splices in the middle of nowhere.

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There should be only two wires on the 24vac terminals of the transformer. The white wires aren't connected to the 24vac side of the transformer.... are they ?
Measure the 24vac terminals at the transformer.

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The voltage fluctuating from 2-9 volts doesn't make any sense.
 
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Old 05-18-17, 04:35 AM
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well it got beyond my paygrade.

called a tech.

$100 later, He found another short in the AC unit outside, a bare red wire was rubbing against the case. Not sure what the wire was, i guess the 24V line from inside, probably blew the fuse.

Coupled with my new thermostat, we both could not get the Lyric T5 round to call for cooling even though the software and display said it was doing so.

So, a couple lessons learned:

On my furnace, an induction blower and house fan can definitely operate with a blown 3A fuse, they were locked into the 'on' mode, and after that I should have just wired together the Yellow and red lines at the thermostat to see if the AC unit outside would have come on, or try to hunt down the short outside myself.

It is tough dealing with more than 1 problem at once!, especially one when was self-inflicted (new thermostat not working).

Anyhow i put back my old thermostat and life is good

Thanks!
 
 

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