Weird voltage spike at gas valve?

scribb

Hi everyone—

I've been banging my head against an old Carrier furnace for days, and I'm failing to keep my family warm. :(

When heat is called, the fan starts, the spark ignites, but the gas valve isn't opening the pilot valve. I've been troubleshooting and have found some unusual voltage behavior that I can't pinpoint. Here’s what I’ve done and where I’m stuck:

• The pilot valve on the gas valve doesn’t open during ignition.
• The spark ignites as expected, but no gas flow occurs.
• The transformer tests fine: 24v at the circuit board.
• The thermocouple is putting out 8mv, which seems normal.

I tested voltage at the gas valve, and this is where it gets odd. If both the blue leads for pilot/gas, and the green lead that connects to the thermocouple and igniter are connected to the gas valve, the brown-wire terminal and the gas valve that connects to the pressure valve shows 35v, even when the brown wire isn't connected.

If I test the brown disconnected wire and com, it's at 24v.

If either the blue pilot/gas wires or green wire to the thermocouple are disconnected, the spike goes away. Is this what's causing the gas pilot not to open when the heat is called? Or is this a red herring, and the cause is likely something else?

Thanks for your help!

pedmec

if you want help you need model number. and wiring diagram which is normally on the backside of the cover

HomerJSmith

So, it is an intermittent pilot assembly. Check the ignition board, the gas valve doesn't open the pilot valve, the ignition board does after all the safety checks has been done.

8mv is not normal, it should be about 30 mv. Replace the thermocouple, clean the pilot assembly and make sure of the ground. Check the solder joint on the back of the board.

What kind of fuel? Are you sure you don't have limited trials for ignition? Unplug the unit, wait a few minutes and the re-plug it back in.

Then post back the results.

mattmia2

Sounds like the solenoid for the pilot valve is inducing current in the main valve coil which has no load on it, doesn't mean anything.

Also sounds like your pressure switch isn't closing.

HVACNUT

25-30 mv for a good thermocouple.

RPK

I'm confused a bit by the original post. If there's a spark igniter then the burner is intermittent ignition type and there wouldn't be a thermocouple. @scribb, can you post the wiring diagram and furnace make/model as @pedmec suggested? I think @mattmia2 is probably correct about the pressure switch.

scribb

Thank you for the help!

It's an old Carrier 395baw024, natural gas. The gas valve is a SX345NSX-12.

What I called the thermocouple is actually the pilot burner assembly, which was just replaced.

scribb

Welp, I unplugged it for awhile, plugged it back in ignoring the 35v spike on the gas valve. Fan ran for about two minutes, and then the transformer melted. :(

109A_5

Hello scribb,

I'm assuming the furnace previously ran for the two minute trial period before it was noticed that the furnace did not work properly. Are you sure you put all the wires back where they originally were ? The transformer can be replaced and you may be back to the original defect and possibly the one that took out the transformer. Transformers can just fail for no apparent reason, but that is rare.

I'm guessing you don't have the Canadian version with the fuses on the transformer's secondary.

Assuming an overload took out the transformer that seems odd since previously you had apparently higher than normal Voltages in some places. Overloads on the transformer Secondary usually lower the Voltage out of the transformer.

If the transformer had an internal defect (Primary to Secondary electrical leakage) you may see odd Voltages since one side of the secondary is grounded at the Spark Generator.

Does this unit actually run on 230 VAC ? Or is that another 'When Used' option.

Anyway if you can get it back to the point before the transformer failure the control logic should not be that hard to chase out to find out why the gas valve is not opening.

scribb

It's on 110, and yes, it's American, without the fuses. I tried chasing down the problem—testing the pressure switch, high limit switch, testing for 24v coming from the circuit board, etc. It's just eluded me.

109A_5

Hello scribb,

Now I not saying you haven't but when you use a logical troubleshooting structure, flowing the wiring diagram the defect usually becomes obvious fairly quick. Folks often use a random approach that is just confusing and does not eliminate the good part of the circuit. The 35 Volts you were seeing may have just been stray AC due to a lack of a proper load at that time.

What actually happened with the transformer, I can only guess, I was not there, however I always think it is odd when the transformer dies when the 'whatever' piece of equipment is being messed with, you are not the first time that has happened.

I would get a another transformer and move on with either with a fuse or two and/or a Incandescent lamp in series with the primary or the secondary of the transformer to avoid another transformer failure.

pedmec

The problem with these old 395 carrier units is that they didn't use push pins on the molex connectors to the board like they do now. They are more like fingers folded over. The problem is that when they crease the metal to fold it creates a weak spot in the metal. when you reattached the molex to the control board the fingers will snap in half and wont make contact with the control board. Take the molex off the board and look down into the molex plug. you might find it broken off. You have to look real closely to see it.