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Old 07-31-2018, 09:36 PM   #1
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Suburban Furnace Ground Fault

I installed a battery power meter in my Kodiak Cub TT. The power consumed by the installed Suburban Furnace model NT-20SEQ does not register on the power meter. All other 12 volt appliances do register.
The furnace negative wire carries no current. Furnace current appears to run to ground. Since there is no ground wire to the furnace, it must be running through the copper propane pipe to the frame.
Does anyone know if it is safe to separate the ground in the furnace from the furnace 12 volt system and how it can be done? This would cause the negative current to flow through the negative wire to the power shunt and show up on my power meter.
I contacted Suburban. They gave me a boilerplate answer. They will not talk to a customer.
Paul Bristol
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Old 07-31-2018, 10:12 PM   #2
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I installed a battery power meter in my Kodiak Cub TT. The power consumed by the installed Suburban Furnace model NT-20SEQ does not register on the power meter. All other 12 volt appliances do register.
The furnace negative wire carries no current. Furnace current appears to run to ground. Since there is no ground wire to the furnace, it must be running through the copper propane pipe to the frame.
Does anyone know if it is safe to separate the ground in the furnace from the furnace 12 volt system and how it can be done? This would cause the negative current to flow through the negative wire to the power shunt and show up on my power meter.
I contacted Suburban. They gave me a boilerplate answer. They will not talk to a customer.
Paul Bristol
I have never seen a furnace bonded to ground through a propane or copper feed pipe. I would look elsewhere. Iím pretty sure that is illegal.
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Old 08-01-2018, 11:35 PM   #3
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These furnaces are 12vdc negative ground type systems. You should not attempt to separate ground in any way.

Since you stated it is not registering current flow though aftermarket device, I would suggest (for testing/troubleshooting) remove the 12 v positive side, temp wire into good quality DMM (such as Fluke 77/79 or 87) in series through meter to verify actual current flow. Wire up, then turn furnace on to confirm current flow.

Installation instructions suggest these be wired in parallel with direct battery and converter. You will have to confirm if in fact this was how it was installed. Basically 2 sources of supply.

Direct current systems while "called" negative or positive ground types; are really a misnomer. D/C systems are using voltage difference of potential between 2 conductors to "transfer" electrons (energy). What truly distinguishes a pos/neg system is the power supply and how the D/C is generated. As an example if you reversed all of the plates in the battery it be a positive system. Since the battery is the source of the chemical to electrical transfer of energy.

As a solution to your original problem, you may be able to source a low voltage current transformer (typically found in commercial HVAC systems) that will "isolate" the furnace from from your aftermarket system.
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Old 08-05-2018, 06:30 PM   #4
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Clearification

I agree. Ground must be maintained.
The issue here is that there is more than one ground.
The battery negative terminal is grounded. That maintains the necessary system ground.
The furnace is independently grounded which is also not the issue. The housing is conductive and may need to be grounded.
The issue is the fan and control circuits are grounded and conducting current through that ground connection. The furnace negative wire carries no current.
The positive side of the furnace is connected to a fuse block built into the 12 volt converter. The 12 volt converter is connected to the battery.
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Old 08-05-2018, 06:38 PM   #5
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After market system

The "after market system" is very simple. It consists of a 100 amp shunt connected to the negative battery terminal. The negative common buss is connected to the shunt. Theoretically, all negative side current should flow through that shunt. All current except the furnace current flows through that shunt.
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Old 08-05-2018, 10:37 PM   #6
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The "after market system" is very simple. It consists of a 100 amp shunt connected to the negative battery terminal. The negative common buss is connected to the shunt. Theoretically, all negative side current should flow through that shunt. All current except the furnace current flows through that shunt.

Not and RV expert. But if your connected to the battery, it seems that it is getting 12 volts from somewhere else?
Is the tow vehicle connected? Or is the trailer hooked up to 120 VAC?
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Old 08-06-2018, 02:47 PM   #7
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Not and RV expert. But if your connected to the battery, it seems that it is getting 12 volts from somewhere else?
Is the tow vehicle connected? Or is the trailer hooked up to 120 VAC?
Thank you for your service ArmyVeteran. We were all novice RV'ers at one time. Many of us have found out the hard way what not to do. I have killed a few batteries and generated some spectacular sparks in my time.

There is only one source of 12v power to the furnace. The positive wire is connected to the fuse block that is part of the 12 volt converter. The negative wire is connected to the 12v common buss. The ground is connected to the trailer frame.
The tow vehicle connecter is wired directly to the battery and does not pass through the shunt or the converter.
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Old 08-06-2018, 05:45 PM   #8
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Thank you for your service ArmyVeteran. We were all novice RV'ers at one time. Many of us have found out the hard way what not to do. I have killed a few batteries and generated some spectacular sparks in my time.

There is only one source of 12v power to the furnace. The positive wire is connected to the fuse block that is part of the 12 volt converter. The negative wire is connected to the 12v common buss. The ground is connected to the trailer frame.
The tow vehicle connecter is wired directly to the battery and does not pass through the shunt or the converter.

Your quite welcome, appreciate it.


My electronics training is ancient and rarely used professionally. I will have to mull this over on paper. Getting too old to think on the run haha.


Hopefully, someone will jump in here.
Good Luck!
Bill
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Old 08-06-2018, 08:13 PM   #9
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Wiring Diagram

Here is a wiring diagram of the changes I made to the Kodiak Cub original wiring.
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Old 08-06-2018, 08:28 PM   #10
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Here is a wiring diagram of the changes I made to the Kodiak Cub original wiring.

Thanks, looking for my glasses and thinking cap
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Old 08-06-2018, 10:39 PM   #11
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Look at attached page from Service Manual. If you "trace" the flow through the entire circuit the following conclusions can be made.

The module control board internal board level circuitry is unknown.

If you look at the yellow wire from MCB to DC Negative, you will note that current will pass "through" the gas control valve. However it does NOT pass through the coils of the gas valve. There is no apparent current flow between the RED positive supply and the YELLOW DC return. This is "why" the metering shunt system doesn't "read" current from this device. The MCB is likely power conditioning and switching (transistors, relays, etc) the DC supply. Or from the stand point of the metering device there is no discernible current flow.

I'm sure someone may point out "why doesnt the fan motor read current flow?", simple it's a restive load, and the Yellow conductor that passes through the gas valve is path of least resistance.

The short simple answer is the Furnace Module Control Board (MCB) is preventing you from metering the current flow. The Furnace is not "one" device, it is at least 4 devices all essentially using a different return paths.

The MCB will also have multiple timer circuits within itself, the initial firing sequence, the re-fire sequence (I think manual said it trys 3 times on re-fire). The safety circuits appear to be hardwired- Valve switch, Sail Switch (confirms actual air flow from blower), Limit switch (high temp most likely).

I'm guessing on this part, but if your major concern is killing a battery. I would suggest a dedicated kill-switch (disconnect) that is convenient and easy for you to access. If you are determined to meter the furnace, then you will need to source individual current transformers for each of the major components (gas valve, blower, electrode, etc). You may be able to get 1 CT to work if it is place in the right place of the circuit. It will be difficult to find a 12v CT, most controls are 24v. And their outputs will be 4-20ma, 1-10 v, 0-5 v, etc. The proportional scaling takes place "usually" within the control software (plc) to register the proper current reading.

As to your last post, you are correct in that the DC is returning through the equipment (chassis) ground side. Thats pretty much what the diagram shows.

I have approx 35 years of industrial controls experience. From 800 sq ft Libraries, MLB ballparks, chiller plants (for whole city blocks), military electronics, etc, etc.
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Old 08-06-2018, 11:28 PM   #12
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Look at attached page from Service Manual. If you "trace" the flow through the entire circuit the following conclusions can be made.

What he said.......haha.


I am glad you got in there, I was getting a headache tracing and well beyond my capabilities,



73s to you Doug....


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Old 08-07-2018, 01:19 AM   #13
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All these years I thought a motor was an inductive load and things with resistance were resistive loads like heating elements and incandescent lamps.
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Old 08-08-2018, 01:07 AM   #14
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All these years I thought a motor was an inductive load and things with resistance were resistive loads like heating elements and incandescent lamps.
Sundancer87 is correct if the circuit is purely A/C, then you would have to look at reactance, is it leading or lagging (this usually comes into play with power plant generation equipment). My comment was relative to how the DC circuit would actually view/respond to the fan blower motor as shown in furnace diagram and specficially why the metering didn't register. Again we don't know exactly what internal circuitry is associated with MCB. I know on surface this goes against theory, however I have had to troubleshoot existing systems piece by piece with an o-scope before. Another area I learned this the hard way was board level (radio) component repair. Radio circuits can do really funny things that cause other equipment to appear to be failure point. Swap-Tronics does not work. The really technical answer would be "as viewed on an o-scope this particular part of the circuit: The voltage and current would be the same wave form with equipment in operation, thereby making it "resistive" in nature. If the voltage and current waveform were out of sync (either lead or lag) it could be classified as inductive or capacitance load."
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