Suburban Furnace Problems

[JeffRil]

So my dad gave me his old rv that he just replaced the main board in the furnace its a Suburban model NT34 S and it seems to run fine about 3 time, then it goes to ignite the 4th time and will not ignite, fan runs, and there is spark but no fire, I have taken it apart and cleaned it all and re-thread taped all the gas lines, it is getting propane because it runs the other 3 times just fine but then wont start unless I turn the thermostat completely off and then back on again.

If anyone has any ideas or any way to point me I would really appreciate it, Thank you in advance.

[enblethen]

OP say: "It seems to work now, just in time too it started snowing tonight, thank you for the info, ground contacts where corroded."

[dougrainer]

JeffRil wrote:
So I replaced the valve and still a no go so I got creative, it's run by a 12v line and the valve is 12v so I got some roach clip wires and wired the power to the cable and it opens but if connected to the furnace it won't open, any ideas on THIS problem?

IF the valve opens with direct 12 volts, but does NOT when connected to the furnace harness, then you are having a drop in voltage to the Gas Valve. I would be testing on the work Bench with the furnace still inside the furnace case with a jumper wire from the gas valve to see what the voltage is thru the furnace harness. I would lean towards a lack of full 12 volts TO the furnace and not a internal furnace harness problem. My recollection of the NT34 S is, there is a small metal relay(instead of the standard time delay relay) that runs the 12 volt power for the motor and to the control board. Possible that relay the points are pitted and not supplying full 12 volt power thru the Control board to the Gas valve. Doug

[enblethen]

OP said he found a corroded ground on the gas valve wiring. Last posts says it is working.
He did apply 12 volts directly to coil and it did not work.

[wa8yxm]

IS not the gas valve visible when you open the exterior compartment?

All I'm recommending is this
Check for power to the solenoid (NOTE you can use long leads on that test light and hook 'em up when you have it out, He said he cleaned and re-tapped all valves, jets and such so he's had it out) The testing for magnetic field is optional if the coil and the solenoid are only solid as a combined unit then if it has power and is not letting gas pass. it needs replacement

But if it's not getting power.. Problem is elsewhere.

NOTE: if you use a test LED setup. You need not remove it, you can leave it in there forever.

[JeffRil]

enblethen wrote:
There may not be enough do to higher resistance in the circuit.
Verify the ground is connected to the furnace. Make sure it is clean and tight. See if the ground is connected to the same location as the valve ground. Add a jumper if they are not. Do not rely on the ground through the circuit board mounting.

It seems to work now, just in time too it started snowing tonight, thank you for the info, ground contacts where corroded.

[enblethen]

There may not be enough do to higher resistance in the circuit.
Verify the ground is connected to the furnace. Make sure it is clean and tight. See if the ground is connected to the same location as the valve ground. Add a jumper if they are not. Do not rely on the ground through the circuit board mounting.

[JeffRil]

enblethen wrote:
No output from the circuit board. No power to the circuit board to go to valve.

That's the thing there is power but I have to tap on the valve to get it to open, every time it's like there isn't enough power to open the valve. If I turn it upside down it opens but I can't install it like that.

[enblethen]

No output from the circuit board. No power to the circuit board to go to valve.

[JeffRil]

So I replaced the valve and still a no go so I got creative, it's run by a 12v line and the valve is 12v so I got some roach clip wires and wired the power to the cable and it opens but if connected to the furnace it won't open, any ideas on THIS problem?

[wildmanbaker]

I'm sitting back and laughing at some of the replies, after Doug, an RV Tech for 30 some years, has given the answer to the problem.
It seems we all get fixated on our own experiences and kind of go blind on what the OP has posted, or misread it. I know, I do this myself. An old saying in my former job was, "The mark of a good Reactor Operator is, the ability to recognize the correct answer when you hear it." You know, this applies to everything in life.

Thanks Doug, and good for Jeff finding the problem.

PS: This post is not meant to poke fun at anyone, more a review my own personal life.

[dougrainer]

enblethen wrote:
The increase in resistance in the thermostat will allow the time delay relay to energize but not provide sufficient power to get the motor up to speed.

NOT even close. The Time delay has the full 12 volt power on one side of the relay. Once the tstat sends the signal to the delay, it energizes the FULL 12 volt power to the Motor thru the relay. Please google search the operation of a Tstat Anticipator. Doug

[JeffRil]

I tried Doug's idea of tapping on the propane supply line going into the valve when the lighting issue happened again and the next time it sparked it lit so I believe I'm looking at a bad valve. The control board isnt even a year old yet so I do not see how it can be bad and the fan and everything else works so I will see about changing out the valve and see where that leads.

[D_E_Bishop]

There is such a thing as Infant Mortality where electronic are concerned. Being a new board does not mean that it could not fail. What might the problem be if not a bad valve or a bad board.

Speculation on resistance of the bi-metal coil and all are very hard to determine. Why not suggest a length of zip cord, 4 insulated mini alligators clips and connect them to the output of the board and the terminals of the solenoid and try measuring the voltages. Take it apart and put I back together with the zip cord running out of the case.

I'm reading a lot of it might be this or it might be that but no real info on doing the tests needed to see if this or that might be the problem. Checking the output of the board and if the power is getting to the solenoid would either identify or eliminate the board and the solenoid as problems.

[enblethen]

The increase in resistance in the thermostat will allow the time delay relay to energize but not provide sufficient power to get the motor up to speed.