Old furnace wont ignite main burner. Bad valve?

Hi there,
I hoping to get some help regarding my gas forced air furnace. Here are some details.

Make: Johnson
Model:Air Ease has105bd

Symptoms: Not Responding to Thermostat / Burners not recieving gas.

-Pilot light is slightly weak, but stays lit.
-Cleaned pilot and thermocouple.
-Tested power coming from thermostat ~25v
-checked and cleaned wire connections.

There seems to be no response when temperature on thermostat is increased, however the voltage drops to 0 when there is no call for heat. In the past, it has sometimes done this, but by switching the power off via the switch and back on (sometimes several times), it would always give a click or a clunk and respond. This is no longer working, however again the current drops when switched off and jumps to 25 when turned on.

My understanding is the the thermocouple only controls the first valve, and it would be pointless to replace. Can someone clarify this for me? Any chance its the thermocouple?
I have one in hand and am going to change it regardless.

Any other ideas? Should I order the gas valve? The one in it looks relatively new and I dont know the history of the unit, however the is a humidifier connected that has been unplugged and neglected.

I can upload photos or give any new information in real time. Thanks in advance. Getting cold in Colorado!

Cheers
Andrew

 

No.....don't just randomly order gas valve.

You have a volt meter. Have you checked for 24vac on the gas valve ?
I couldn't find any wiring diagrams for your unit.
There should be one on the back of the blower door or main access door.

 

If the pilot stays lit it is not a thermocouple problem UNLESS the pilot valve is stuck open. If the latter is true it is a BIG safety problem and the gas valve(s) need to be replaced.

 

I have indeed and i have 24vac on the valve, only when the thermostat is calling for heat, unless i am misunderstanding but i dont think so, its the only leads that attach to the gas valve.

I dont believe the pilot valve is stuck open because I cant light the pilot withough using the lever / button.

thanks for the responses so far.

 

Presumably you have a good gas supply since the pilot is operating.

The 24 VAC ought to open the gas valve for the main burners.

So you apparently have a gas valve that needs to be replaced. I'd double check that by doing a continuity check through the gas valve to see if the gas valve operator is burned out, which would give you infinite resistance across the gas valve electrical connection when you disconnect one existing wire.

I recommend against DIY replacement of gas valves, since they need to be adjusted to match the furnace input.

If you shop around for a repair service to replace a 24 volt gas valve with a thermocouple safety and specify the BTU input from the furnace rating plate, you should be able to find someone to give you a good price.