Bench test power hookup for furnace using 12v automotive battery

[calstar]

I have my camper furnace(Atwood 7900 series) out to replace the sail switch and want to bench test it before reinstalling it. Just double checking here, I believe the red wire(+) will connect to the auto + battery terminal and the white to the negative and the terminal correct?

More questions:
1) If the delay relay is bad the fan motor won't turn on at all, right? The fan was running before I removed the furnace but wouldn't ignite, I have a switch to replace the relay but since the fan runs maybe I won't do that
2)although the fan turned on I could not hear the click of the gas valve or the ignitor, If the sail switch was bad it wouldn't send a signal to them, right?
3) blue wires are to thermostat which is basically an on/off switch, if I connect the 2 blue wires that will signal the relay to start, correct?
thanks, Brian

2nd pic below it the delay relay

 

[Lou Schneider]

The fan should start turning as soon as you short the thermostat leads. The delay relay waits several seconds after the air flow switch trips to let the combustion chamber purge itself of any residual gas before opening the gas valve and activating the ignitor. If you use a clip lead to bypass the sail switch can you hear the valve or ignitor click several seconds after you short the thermostat leads?

The sail switch is located on the house (inside air) side of the fan. A common problem is the blower not turning fast enough to move sufficient air to trip the switch. Your blower may not be receiving enough voltage or the bearings may be dragging. Or the blower turns fast enough to let the airflow switch trip on the workbench but there's too much restriction in the furnace ducts or in the return air intake to provide sufficient airflow when the furnace is mounted in the RV.

 

[Kirk]

Lou just covered everything that I was about to say, and very well. With it on the bench you should be able to hear the snapping of the ignitor if it fires but it will only try 3 or 4 times each time that you connect the blue wires.

The fan was running before I removed the furnace but wouldn't ignite,

That pretty much eliminates the blower and related circuitry. You could lift the leads from the sail switch and put your ohm meter across it to see if the blower is closing it for sure.

 

[Gary RV_Wizard]

Find the Atwood service training manual https://myrvworks.com/wp-content/uploads/2019/04/Atwood-Furnace-Training-Manual.pdf

Page 14 shows external wiring for a 7900-1 and the color code. Red is 12v+, Black is 12v-, and White is the thermostat trigger, which will also be 12v+ when the thermostat calls for heat. The 7900-2 is the same.

 

[calstar]

The fan should start turning as soon as you short the thermostat leads. The delay relay waits several seconds after the air flow switch trips to let the combustion chamber purge itself of any residual gas before opening the gas valve and activating the ignitor. If you use a clip lead to bypass the sail switch can you hear the valve or ignitor click several seconds after you short the thermostat leads?

The sail switch is located on the house (inside air) side of the fan. A common problem is the blower not turning fast enough to move sufficient air to trip the switch. Your blower may not be receiving enough voltage or the bearings may be dragging. Or the blower turns fast enough to let the airflow switch trip on the workbench but there's too much restriction in the furnace ducts or in the return air intake to provide sufficient airflow when the furnace is mounted in the RV.

The fan spins freely, no bearing noise at all. The return air intake is clean, I've removed the ducts to the bathroom and bed area(covered the duct outlets at the furnace, second pid), I've read where they didn't distribute much heat and the space is so small in the camper I don't think they're necessary, that should remove restriction as a problem.
Bypassing the sail switch, just with alligator clip lead right?

Very usable info from both of you, the blower control ignition board diagram is helpful.

 

[Gary RV_Wizard]

Bypassing the sail switch, just with alligator clip lead right?

Or a piece of tape to hold the sail in the closed position. Or just hold your finger on it...

 

[calstar]

Find the Atwood service training manual https://myrvworks.com/wp-content/uploads/2019/04/Atwood-Furnace-Training-Manual.pdf

Page 14 shows external wiring for a 7900-1 and the color code. Red is 12v+, Black is 12v-, and White is the thermostat trigger, which will also be 12v+ when the thermostat calls for heat. The 7900-2 is the same.

Great source, thanks Gary. I believe the larger white wire is 12 - as the pic shows its spliced to a black wire (-) going into the furnace, does this seem correct? I think the blue wires go to the thermostat(the pic doesn't show the cut ends) as one is spliced to red + and other black - (white wire taped black to designate -) , is this right?

 

[Kirk]

Here is a link to the 58 page service manual used by technicians. If you look at the schematics in it you will see that the 7900 series drawings show white wires for the thermostat and blue wires for the 8500 & 8900 series.

 

[Gary RV_Wizard]

Great source, thanks Gary. I believe the larger white wire is 12 - as the pic shows its spliced to a black wire (-) going into the furnace, does this seem correct? I think the blue wires go to the thermostat(the pic doesn't show the cut ends) as one is spliced to red + and other black - (white wire taped black to designate -) , is this right?

I can't imagine why a 12 gauge wire would be needed to go to a thermostat. The amp load on that circuit is much smaller than that. But extra thick wires don't hurt anything - it's just overkill.

https://myrvworks.com/wp-content/uploads/2019/04/Atwood-Furnace-Training-Manual.pdf

 

[calstar]

I can't imagine why a 12 gauge wire would be needed to go to a thermostat. The amp load on that circuit is much smaller than that. But extra thick wires don't hurt anything - it's just overkill.

https://myrvworks.com/wp-content/uploads/2019/04/Atwood-Furnace-Training-Manual.pdf

Yes, I mistakenly added the 12 before the negative(12 -) when it should have been only - in that post.


Also regarding color of the wires I'll check the blue at the thermostat, thanks for the link:

"Here is a link to the 58 page service manual used by technicians. If you look at the schematics in it you will see that the 7900 series drawings show white wires for the thermostat and blue wires for the 8500 & 8900 series."

Pics below show the blue(16 awg) is the thermostat, red+(12awg), must be the white(12awg) is -. In the 2nd pic red spliced to red(and blue) and white spliced to black going into furnace(to ground) and then black(white marked as black with black tape) connected to blue, does this make the furnace an 8000 series? Question about terminals at thermostat, can either blue wire be terminated at either terminal? thanks

 

[Kirk]

Wire color to the thermostat doesn't matter as long as it is connected to the proper place in the furnace. If you have the thermostat that I think you have, all that it does is to provide a set of contacts that close to start the furnace when below set temperature and open them when set temperature is reached. However, if the same thermostat is shared with an air conditioner, the way that things are connected changes as the thermostat then goes to a control board in the air conditioner and that connects to the furnace's thermostat leads. It might be helpful if you would post a picture of the thermostat that we are dealing with.

 

[calstar]

If you have the thermostat that I think you have, all that it does is to provide a set of contacts that close to start the furnace when below set temperature and open them when set temperature is reached. However, if the same thermostat is shared with an air conditioner,.... It might be helpful if you would post a picture of the thermostat that we are dealing with.

No air conditioner, here's pics. When I test on the bench do I just connect the wires that go to and from the thermostat to each other to start blower, I'm not clear if that's how I do it. Also in the last pic are the wires going to the thermostat(I cut them to get the furnace out), does it matter which wire goes to which side of the terminals in the thermostat, ? I realize these are pretty basic questions but this is a learning experience for me. The last pic is the board I have, it's the only one I saw in the 7900
II / 8000II manual wiring diagrams with the 3 pin connecter to the board. Thanks again to all who are helping me on the forum.

 

[Kirk]

does it matter which wire goes to which side of the terminals in the thermostat,

It does not. The thermostat that you have is the most basic one with a simple bimetallic coil that opens and closes a set of contacts, much like a switch but based on temperature.

Also in the last pic are the wires going to the thermostat(I cut them to get the furnace out), does it matter which wire goes to which side of the terminals in the thermostat, ?

In all of my RV furnace experience, blue wires like yours are the thermostat leads and so shorting them together should turn the blower on and start the process, separating them again turns the furnace off.

 

[calstar]

Checked sail switch which I didn't do before, continuity ok, hooked up to 12v, delay relay worked as it should, fan turned on, held sail switch closed, after short time the gas valve clicked but no ignition rapid clicking, kept holding the sail switch and the gas valve clicked two more times at 20+- second intervals(total 3) as I believe it should, maybe my board is bad. I'm waiting on a propane hose with regulator that should arrive tomorrow, I'lI connect the propane then and test but I don't see it why the ignitor would act any different. I've been watching many vids as tutorials, utuber John Daniel shows a 12v relay that is not an uncommon fail , iust above the 3 pin connector on the left side of the pic, he also point to a diode that may be burned but mine isn't. He says that relay can be replaced with a common 12v relay but I don't know how to do that, is there a way to test it? What do you guys think, bad board? thanks

 

[Gary RV_Wizard]

With those symptoms, the likely causes are a bad board or a faulty igniter circuit (including the igniter wire and ground).

 

[Lou Schneider]

With those symptoms, the likely causes are a bad board or a faulty igniter circuit (including the igniter wire and ground).

I've also seen the ceramic insulator on the ignitor crack, shunting the spark voltage to ground before it can jump the gap. Do you see a spark if you temporarily remove the high voltage wire from the spark coil and replace it with a wire whose end is the proper distance from ground? Or use a grounded spark plug to ensure the proper gap for the spark to jump across.

 

[calstar]

With those symptoms, the likely causes are a bad board or a faulty igniter circuit (including the igniter wire and ground).

I'm not clear on what a faulty igniter circuit refers to other than the igniter wire, its continuity tested good , also not sure how it's grounded(connection at electrode?) or how to check that. Would it help to remove the burner assembly and electrode and look at those or is that not likely the issue?

edit: Gary, I just saw your post(below) immediately after I posted the above.

Do you see a spark if you temporarily remove the high voltage wire from the spark coil and replace it with a wire whose end is the proper distance from ground

I assume you mean remove the wire from the electrode and hold it about 1/8" away and see if a spark jumps the gap, can I use the igniter wire for this? Since the continuity of the wire is good if there is no spark the issue is in the board?

thanks

 

[Lou Schneider]

I'm not clear on what a faulty igniter circuit refers to other than the igniter wire, its continuity tested good , also not sure how it's grounded(connection at electrode?) or how to check that. Would it help to remove the burner assembly and electrode and look at those or is that not likely the issue?

edit: Gary, I just saw your post(below) immediately after I posted the above.

Do you see a spark if you temporarily remove the high voltage wire from the spark coil and replace it with a wire whose end is the proper distance from ground

I assume you mean remove the wire from the electrode and hold it about 1/8" away and see if a spark jumps the gap, can I use the igniter wire for this? Since the continuity of the wire is good if there is no spark the issue is in the board?

thanks

Yes, but I would not hold the wire in my hand unless the insulation is rated for several thousand volts. Also, the spark coil on the circuit board can be damaged if there's not a proper gap for it to spark across. That's why I suggested using a spark plug to observe the spark - if the metal shell is grounded you'll always have a spark gap.

 

[Kirk]

I like the spark plug test, as I never thought of that one. I have held the high voltage lead next to a ground source but used a wooden holder that I made with a piece of board and a pinch type clothes pin to hold the wire. I have seen far more failures of the ignition probe than of the circuit board, but they do occasionally fail. The cracked ceramic can also be intermittent and sometimes go away when cleaned well for a short time. That crack will collect carbon from the combustion and carbon is a conductor. If you do choose to replace the circuit board, I highly recommend you use one from Dinosaur Electronics as they have a better warranty and cost less than the OEM boards.

 

[calstar]

I like the spark plug test, as I never thought of that one. I have held the high voltage lead next to a ground source but used a wooden holder that I made with a piece of board and a pinch type clothes pin to hold the wire. I have seen far more failures of the ignition probe than of the circuit board, but they do occasionally fail. The cracked ceramic can also be intermittent and sometimes go away when cleaned well for a short time. That crack will collect carbon from the combustion and carbon is a conductor. If you do choose to replace the circuit board, I highly recommend you use one from Dinosaur Electronics as they have a better warranty and cost less than the OEM boards.

When using the spark plug method I connect one end of a wire(12awg) to the coil and the other end to the plug, then touch the plug to the metal cage of the furnace(ground), correct?

Searching around the net for coil testing I saw this, would this work? Its testing an engine coil but maybe using a non-contact tester like this(I have one) might work for the circuit board coil, they're both 12v, if it flashes the coil is good.