Troubleshooting a Gas Control Valve

16 years ago

My furnace (Sears NTP6) has not been working for over a week. Its had a "Soft Lockout error". I finally got a Sears service guy to look at the furnace and he tells me my Gas Control Valve (SV9540Q2456) needs to be replaced at $700 for parts and labor.

I did some investigation and found a troubleshooting guide for this controller:

http://www.fwwebb.com/pdf/htgcat03/2003htgcat_p207-208.pdf

After the service man left I started going through the troubleshooting guide.

Step one Â Checked the pilot burner element - ohms to be less then 10. Everything OK there.

Step two Â Turn off gas supply, Make sure the smartvalve is set to "On", Disconnect the control harness, and set the thermostat to "Call for heat". Â OK

Step three Â Check for the proper voltage at the control harness. This should be 24V from Common to Thermostat (or pressure switch) and 24V from Common to "Hot". = THIS WAS NOT OK. I got 7.5V from Common to thermostat and 0.075V from Common to "Hot". The troubleshooting guide calls for something other then the gas control valve as the problem (Most likely the power board Â which is under warranty).

Strange thing here is that the service person had the power board or the Fan controller board (208907) as needing to be replaced under warranty but told me that was a mistake and the gas valve was what needed to be replaced.

Can someone instruct me on the voltage issue and tell me if I am on the right track given the situation.

Regards,

Evan

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Comments (6)

syrinek
16 years ago
Can't help with the voltage, but you may want to call a appliance parts place and see how much the control board and the gas value would cost. First I had to replace the board because the furnance would not fire up, which cost about a $100.00 and then about a year later I had to replace the gas value, which was about a $100.00. I ordered the parts and replaced both myself. I order parts through a place called Appliance Warehouse where you can get parts for all appliances and Heating and Air. Sometimes they have to order the parts as in my case which took about a week. That SV number does sound familiar.
syrinek
16 years ago
You also need to make sure the circulation fan is coming on. I also had a problem with that which prevented the unit from firing up. The board problem turned out to be a bad solder joint, determined after changing it out.

There also should be a 24V Transformer that puts out the 24 volts needed. Perhaps the Transformer is not putting out the 24 volts.
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evbrowne
Original Author
16 years ago
The circulation fan is coming on.

Last night I went to re-connect the wiring harness connections and found that the pilot ignitor was working. I turned the gas back on and then the furnace was working.

This morning the furnace was not working again - having the same problems. It worked for a few hours then it was not working.

I will also try checking the transformer.
tmajor
16 years ago
If you feel that the problem may be heat related, you can get some "Freeze-It" spray at you local electronics store. This will help troubleshoot a heat sensitive area of the board or a component. This can be used during operation.
jca1
16 years ago
Ok you have a smart valve with pilot right? If the pilot gets dirty the rectification process will not take place and the board will get no signal to open the valve which would explain the voltage problem I guess. Is the pilot flame lazy and yellow tipped? I can't tell you how many times I see this very thing happen.

If it is yellow tipped and lazy looking, I'll help you from there.
evbrowne
Original Author
16 years ago
I checked the transformer secondary and I get 29.8 VAC. This is the same at the board common terminal to 24V terminal.

I checked the common off of the transformer (yellow wire - plugged into COM on board) to each pin of the four pins powering the gas valve. I get 9 VAC on what is shown as 24V Thermostat or pressure switch) pin and also 9VAC at the 24V Hot pin. The 24V common pin is 0V (probably should be). and the EFT pin measures 123V.

The ignitor and temp sensor are new but don't glow. If I add a flame on a call to heat the furnace ignites and runs properly until the next cycle.

Evan