Electrical circuit issue furnace 12v

[RVr-WO]

2021 adventurer 27n bought from original owner at 2000 miles.
Within 100 miles the 12v circuit for suburban gas furnace, thermostats (2), and bath exhaust fan pops breaker.
Turned off furnace switch that has a single wire to front thermostat (thermostat also controls roof a/c heat pump unit. Rear thermostat (and rear a/c unit) and fan now work. Front thermostat does not power up.
Disconnected front thermostat with same results.
Changed furnace control board with same results.
Checked continuity from furnace wire to thermostat plug and have 2000ohms, but probably due to circuit fuse.
Also when I connect furnace wire and turn on, write gets very hot and circuit pops before digital meter shows 8v.
Must be a short but not sure where to quickly look. Wires are all hidden and diagrams are sketchy.

Thanks for any suggestions.

 

[creativepart]

Welcome!

Sorry to hear of your problem. I try to fix all of my RV’s problems myself, but this issue you have would have me seeking outside help. I have a good independent repair shop, but if you don’t have one of those in your area I’d look for a good mobile RV repair service that would come to you.

I looked at your Profile here and it’s difficult to tell where you’re located, many of the registration details you skipped completing, but if you are in the Phoenix area you have many choices for RV repair and troubleshooting.

 

[Morich]

I might be able to lend a bit of help but then we do have to admit that Winnebago has gone into what I call a "bait and switch" setup on electrical info as they do not give us the info we need but lots of "Show" junk that makes it look like they are trying!

I assume you have found your drawing in the set and gone to the 12Volt:
https://www.winnebago.com/Files/Files/Winnebago/Resources/Diagram/2020/000172590.pdf

Not sure which serial number you have, so this is from the first group with 7th digit of 1! But it will do as example for you to choose the correct set?

A bit of place to start?
Click these snips to get a better view or go direct to drawings.
On the breaker we can get an ID for the circuit here!



This should fit with an ID that should be stamped on each wire:


There is a chart here giving the "from" and "to"of each wire ID:

https://www.winnebago.com/Files/Files/Winnebago/Resources/Diagram/electrical_guide.pdf

This chart can let us dig some info out of the bits they give us.

Looking at the drawing, we get circuit ID of JB.
Looking at ID chart, we can hit "ctrl" and "F" at the same time to bring a dropdown where we can input "JB". This will bring a list of every place where JB shows in the list. In this case it gives us what wire JB does!
That verifies that we have got the right ID!

Another way to use this chart is by putting in the item we want info on, like "furnace" and we may get it to spit out wire ID's involved in the furnace. Small help but if looking at a bundle of wires, we can often find which is battery and ground! Big help, there!

There is a bit of info on the drawing for wire ID at the thermostat which may help?

But I might have thoughts on part of your thinking. Much of what you have done sounds correct but I might question the part about high resistance from a fuse as I do not find that correct in many cases. I think what you are saying means the 2000 ohms is a problem, not a fuse?? High resistance open, perhaps? Something damaged enough to not be totally open but not totally good either! High enough resitance to make it heat instead of passing the current and voltage as it should? I might recommend checking more closely there.

But I may also be missing what you are saying as it is not specific enough for me to follow close enough to feel that I'm reading you right!
I think you are saying you have it isolated to working when an item is disconnected but blows fuse when that item is connected? Sounds like you are close to the right spot, so maybe dig deeper into what that one item does and which wire then is the problem.

Thoughts on possible problem but based on guesses?
I might look for that high resistance and wire that gets hot as something like a staple through a set of wires or maybe where it has scraped along a metal edge? Going into the roof top unit is one potential that comes to mind. Maybe where the wires go through a metalclamp into a box?

If you can get it down to one specific wire, it may be possible to run a new wire between two points and just abandon the damaged wire.

But repair thoughts are getting way out in front of finding the trouble!

 

[Bones2003]

If you do not have it, search online and download the Suburban Furnace Master Service and Training Manual. It provides some good information that may be helpful.

I think the way it works is that when there is a demand for heat, the thermostat sends 12V to the furnace. I know you checked continuity on that wire but did you check to see if that wire is shorted to ground? If possible, check it with the wire both connected at the furnace and disconnected at the furnace. If there is a short, that will tell you if it is inside the furnace or in the wiring between the thermostat and the furnace.

Also, I'm not sure what thermostat you have but some of them have an internal fuse that may have blown. For normal operation, only the front thermostat controls the furnace so there should be no furnace connections at the rear thermostat.