Bench test power hookup for furnace using 12v automotive battery

[Mark_K5LXP]

If the spark path is shorted to ground, the non contact indicator probably won't indicate even with a good coil.

Mark B.
Albuquerque, NM

 

[Kirk]

When using the spark plug method I connec

Leave the high voltage lead connected to the the circuit board as normal and connect the other end to the contact on the end of the sparkplug. Then put that spark plug where is it making contact with the furnace frame, if you have it grounded. You should be able to see and hear the spark when it fires.

 

[calstar]

Leave the high voltage lead connected to the the circuit board as normal and connect the other end to the contact on the end of the sparkplug. Then put that spark plug where is it making contact with the furnace frame, if you have it grounded. You should be able to see and hear the spark when it fires.

Showing my ignorance here but how do I ground the furnace when it's on the bench? As I said previously this is a learning experience for me, and I am learning a lot. thanks

 

[Mark_K5LXP]

Ground in this case is the chassis of the furnace. For the purposes of the spark test, the body of the spark plug will touch whatever the other side of the electrode is when the furnace is operating normally.

Mark B.
Albuquerque, NM

 

[Kirk]

how do I ground the furnace when it's on the bench?

Run a wire from the frame of the furnace to the negative battery post and that will serve well. Your RV as well as motorized vehicles just connect the negative post of the battery to the frame or the chassis and so supply a return for the 12V system that way. On the workbench you need to supply the return to the negative battery post.

 

[calstar]

I was going to use the spark plug butI tested the igniter by holding the ignition wire about 1/8" from the electrode and got a good continuous arc, I had the ground wire from chassis to - at battery and left it connected. Ok, hooked up the propane and turned on the furnace, it initially ignited but went out after a couple of seconds, it went through the sequence of opening the gas valve to light 2 more times with the same results, ignition and then lost flame after a few seconds. I ran this full test 4 times, always the same, during the testing I could hear the igniter sparking/clicking and smell some propane, but it never stayed running. Does this rule out the circuit board as the problem, might the issue be with the burner assembly or...? pic of test set up below, thanks

 

[Gary RV_Wizard]

I'm not clear on what a faulty igniter circuit refers to other than the igniter wire, its continuity tested good ,

By "circuit" I merely meant the entire thing, i.e. the igniter itself, the wire to the control board, the connector where it mates to the board, and the ground path at the igniter end. All 4 of those things have to function perfectly to get a spark. Continuity tells you the wire isn't broken but nothing about the other three. The ground path for the igniter is the spark jumping from the igniter tip to nearby metal.

 

[calstar]

I continue down the rabbit hole. New dinosaur control board, it did the exact same thing as the old one(see post above), that's fine as I'm ok with installing the one, supposedly better than the oem. Searching the net again, on this site My Rv Works, Inc. he says the correct positioning of the electrode on the burner is important, he describes symptoms of bad alignment as the burner igniting and running for a few seconds and/or it turns on and runs a few seconds then there is a "whoosh" sound as the propane ignites then blows itself out(igniting propane as a very small explosion that blows it out), this second symptom is what I'm experiencing. He then goes shows how to adjust the electrode to the proper position. Ive removed the burner assembly and electrode, no cracks in the ceramic and burner looks ok(what do you think?). Problem for me is it's not possible to check the position of the electrode on the 7900II when it's installed, easy to do on his (Suburban SF-35) so what's my next step? The gas ignites(ignition is good, fuel input must be good) but then it goes out. Possible gas control valve issue, soot/smudge on the burner(maybe from the flame blow out?) an issue, is there a spec for the gap at electrode? I also removed the burner head from assembly (looks good inside, pic below), checked the orofice on the fuel valve, no obstructions. thanks

 

[Henry J Fate]

Have you installed a new ignitor ? I think you mentioned that you tested the existing ignitor? How did you test the ignitor ?

Ignitor gap will be in the service manual. Usually around 1/8 inch but look at the specification in the manual.

 

[Kirk]

it initially ignited but went out after a couple of seconds, it went through the sequence of opening the gas valve to light 2 more times with the same results, ignition and then lost flame after a few seconds.

That sounds like you are not getting the return signal from the ignition probe that tells the circuit board that the flame is burning. Have you replaced the ignition probe? That is the most common failure of them in my experience.

 

[calstar]

Have you installed a new ignitor ? I think you mentioned that you tested the existing ignitor? How did you test the ignitor ?

Ignitor gap will be in the service manual. Usually around 1/8 inch but look at the specification in the manual.

Not sure what you mean by the ignitor. Do you mean the ignitor coil on the board? I put a new board on and it ignited the fuel like same as the old one for a few seconds before going out, that tells me the spark is reaching the electrode tip(I think), any reason I should test the electrode further? I thought the ignitor gap of 1/8" was the distance to the burner head, not the distance between the electrode tips, regardless the tips are 1/8 apart(pic with 1/8 hex).

 

[calstar]

That sounds like you are not getting the return signal from the ignition probe that tells the circuit board that the flame is burning. Have you replaced the ignition probe? That is the most common failure of them in my experience.

Is the probe part of the electrode? I don't see what it is if not an electrode part. I just looked at the 7900II
diagram and went through the parts list, no ignition probe shown or listed. Is the probe the thin rod in the pic? Took another look at the electrode, carbon build-up on one side. First pic of it before I cleaned it(center area is where I scrapped it with my fingernail) then enlarged pics as cleaned, don't know its a crack but something is going on, you or someone else mentioned carbon collects in cracks in the ceramic so maybe a crack but I couldn't feel it with my fingernail which supposedly can detect a .005 irregularity. Possible if cracked the spark is going to ground at the area of smudged/soot on the burner plate? At this point I may as well buy a new one, only $20 so worth a try.

 

[Kirk]

Is the probe part of the electrode?

It is the electrode. Between sparks the heat for the burning propane creates a signal back to the circuit board that is too small for most meters to detect but it is there, if working properly.

 

[Henry J Fate]

Best to replace the ignitor/electrode. It is dual function. Not easy to test it for flame verification. Just buy a new one they are cheap. It seems that this may have been the problem all along. Lets hope that's the case. You will still need proper alignment and gap.

 

[RayIN]

As Kirk said, the return signal to the gas valve is quite small, 4-6 milliamps. Often just unscrewing the metal line to the gas valve and polishing the bulb on the end, then the contact in the gas valve opening will clean off corrosion to reestablish good contact. Do not use abrasives, a pencil eraser works great. Exercise care when tightening the line back to the gas valve, crushing the bulb ruins it, requiring a new igniter/flame sensor.

 

[calstar]

7920 & 80012 used the same part. That was electrode # 31380 which is now NLA. The specific electrode # in that kit was 37517. Unfortunately both are now NLA. We have no replacement part.

The above is from RVupgrades.com , they say those parts are no longer available, hard to believe there's not some aftermarket electrode. I believe mine is a 7912 II to 7916 II, both use the same electrode.
Two part numbers for the electrode are listed on the Atwood parts manual, 37517 CHF 2(NLA) and 36998 CHF 4 , what's the difference, if any, between the CHF 2 and CHF4? Fingers crossed there's something available.

 

[Kirk]

I can't say for sure but I believe that one of these Amazon ignition probes should fit. If the dimensions are the same what you have now, it should work. Since the alternative is a new furnace, it would probably be worth a try.

 

[calstar]

^^^ Definitely worth a try, I'll order one today.

 

[Henry J Fate]

I suggest to go directly to Dometic/Atwood. Dometic merged with Atwood many years ago. Have your model number and serial number handy when calling or when using their customer service email.

Contact Us | Support for You

 

[Kirk]

Dometic merged with Atwood many years ago

In October 2014 Dometic Group AB acquired Atwood Investment Holdings LLC (and its subsidiary Aircommand) as part of a global transaction. He has said that the part shows as not available.