Old Carrier Furnace Short-Cycling----Not Spark Electrode

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rhino1

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I've got an old Carrier gas furnace---394GAW model (40 years old?); it's similar to 58GS models. The furnace pilot lights up, but my burners will only stay lit for a few seconds (although last night the burners stayed lit one time for a minute or so, then the blower came on and the burners went out). So it's short-cycling. But I have already replaced the spark wire, spark ignitor, pilot assembly and ignition control module, and I still have the same problem. We also tried a new control board, but that didn't help either.

I have seen online posts from people with a similar problem who replaced their inducer motor and/or pressure switch. But I don't see these parts on the parts list or diagram for my model.

A few other pieces of info----Just before I noticed that burners stopped working, we heard a loud bang from the furnace---perhaps a bad air/gas mixture? There was also a burnt electrical smell. I hear a humming sound now, when the pilot is on (but burners are not).

So not sure what to try next----

Thanks in advance.
 
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John Gayewski

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This sounds like gas valve although I'm not familiar with your furnace.

What senses the flame on yours? Something is telling the gas valve to shut. It'll generally shut on its own without a signal to stay on.

I can usually only fix furnaces if I'm in front of them.
 

rhino1

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I think the hot surface ignitor acts as the flame sensor. Could it be the solenoid in the valve that isn't functioning?

The furnace in this video is similar to mine.

Btw, do i have to remove one of the wires on thermostat before jumping r to w, to avoid blowing out anticipator?
 

rhino1

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This sounds like gas valve although I'm not familiar with your furnace.

What senses the flame on yours? Something is telling the gas valve to shut. It'll generally shut on its own without a signal to stay on.

I can usually only fix furnaces if I'm in front of them.
reply is above.
 

fitter30

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I think the hot surface ignitor acts as the flame sensor. Could it be the solenoid in the valve that isn't functioning?

The furnace in this video is similar to mine.

Btw, do i have to remove one of the wires on thermostat before jumping r to w, to avoid blowing out anticipator?
No jumping R to W takes stat out of the picture
 

John Gayewski

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Apparently a common failure for that furnace is in the board. The relays in the board won't allow the fan to come on.


4:48
 

fitter30

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When u installed pilot assembly did u pull and clean the burners? If a limit or vsss would open R and 6 on the spark igniter would lose power to gas valve and would close. Fan time delay seem like it working from the first post. Run a wire from the burners to ground bar on spark igniter just in case theres bad connection under one of the mounting screws that holds the burners. Take a pic of the wiring diagram.
 

John Gayewski

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Roll out switch is shutting the gas valve because the fan isn't coming on. The fan came on once but right as it did the roll out switch shut it off.

That's my guess.

Try tapping on the relays on the board. That can at times make them (relays) work. If tapping on them makes them work they they are bad and you need a board. If it doesn't work you'd need to see if you can find a way to test them.
 
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rhino1

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1642140329217.jpeg
 

rhino1

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swapped out boards at outset; that didn't help.

blower only came on the one time that burners stayed on for a minute. then burners went out, but blower stayed on for a while.
 

fitter30

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Run the blower with the thermostat fan on switch on see if the burner stays fired. Still have the original thermostat? Is there a jumper between R and W h ? Looks like when 6c1 printed circuit board gas2 terminal gets powered with main valve which sends a signal to fan timer delay.
 
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rhino1

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Run the blower with the thermostat fan on switch on see if the burner stays fired. Still have the original thermostat? Is there a jumper between R and W h ? Looks like when 6c1 printed circuit board gas2 terminal gets powered with main valve which sends a signal to fan timer delay.
Thanks. I will check on that.
 

rhino1

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I just installed a new valve, and now nothing works----just dead silence when i turn on the thermostat. How can I check the valve to confirm that it's bad? I think that I got the wiring right. It's a EF32CW233 (White-Rodgers). Thanks.
 

Reach4

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I just installed a new valve, and now nothing works----just dead silence when i turn on the thermostat. How can I check the valve to confirm that it's bad? I think that I got the wiring right. It's a EF32CW233 (White-Rodgers). Thanks.

If the cover is off, the interlock switch would probably de-energize everything.
 
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rhino1

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looks like there are some burnt out components on the control board. so i guess that will need to be replaced. but i thought the board just controls the blower. would burnt out pcb cause furnace not to power up at all?
 
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