Suburban heater woes in a '78 International LY

[sladew]

I am presently experiencing an issue with my forced air heater (Suburban NT30). When it is switched on, the blower comes on, the electrode starts snapping, but the furnace does not start. The furnace will eventually light if the unit is wrapped on by a mallet or a fist. Ocasionally the furnace will start if the whole unit is switched off and then on at the thermostat. I took the unit into an RV repair shop. They could find nothing wrong with the unit. My inital thoughts are a faulty electrode, electrode wire or electrode control board or possibly an issue external to the unit. Any ideas?

Thanks, Slade Weaver

[TomW]

The snapping sound means the igniter is sparking somewhere, so you can probably rule out the control board. The electrode itself is the easiest place to spark, so unless there is obviously something wrong with the wire going to the electrode, the spark should occur close to the pilot jet where you want it to.

My gut feeling is that you have a dirty pilot light jet.

Remove the pilot light assembly, soak in solvent (carb cleaner works well), blow out with compressed air, and reassemble.

Much more detail is available depending on how much of this work you want to do yourself.

Tom

[AirConditioner]

If you're willing to spend a little time, and are comfortable using basic circuit tools, check the following:
(1) Thermostat calls for heat and starts the heater.
(2) 12vdc is sent to Fan relay through the normally closed Heat Limit Switch.
(3) There is a 10 second delay then the Fan relay sends power to the fan motor.
(4) Fan air flow closes the sail switch, which sends power to the gas valve solenoid and air temperature limit switch.
(5) The ignition system sparks and lights the pilot and main burner.
If you have to rap the unit with a hammer, one of the solenoids or safety devices might be sticking. Could be dangerous to allow to run if a safety device is not working right! It seems likely that if the blower is running the gas solenoid valve might not be opening, or the sail switch isn't operating.

You'll have to do some detective work to determine if the problem is with the wiring, Heat Limit Switch, Fan Switch, or thermostat..

If you are comfortable, you can use a multimeter to follow the progression of the voltage, observe the working during start up, and part-by-part, find the failure(s).

Hope this is helpful.

[TomW]

I do not believe steps 2 - 4 will occur unless the pilot light is lit, but I could very easily be mistaken. Why spin the the recirc fan until the pilot is lit? Discussion is welcome.

The thermostat can be ruled out since the igniter is snapping.

The heat limit switch can be ruled out since the furnace is trying to light.

If nothing else, it sounds like Slade is trying to get past step 5 at this point. Since I still light a pilot light by hand as a first step, I am obviously not an authority on late model furnaces. But the issue still sounds like a clogged pilot jet.

Tom

[sladew]

Folks, Thank you for your suggestions and troubleshooting details. I will start troubleshooting the unit when I get a moment. If I have any additonal questions, I will post them to this thread. Again, Thanks

Slade