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Old 01-07-2022, 08:24 AM   #1
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Ground on 12v positive line blowing fuse

I'm troubleshooting a ground I have on a 12v DC positive line. It blows the F3 fuse which is labeled "bedroom lights" on the panel.
Here is a drawing I made from my troubleshooting efforts. You may notice I have four question marks. All these component (light) connectors have 2 white wires (grey in the drawing) and 2 green wires. The white wires are ground/negative. The green wires are/should be positive. All the green wires at the top have the ground present. The green wires past the wall switch and connected to the ceiling light do NOT have the ground present. This is with the light fixtures unplugged. So it's not a light bulb or fixture problem. It is a wiring problem somewhere else I can't find.
I've done continuity checks on all wires to get this drawing. However, there are still some unknowns.
Does anyone have any ideas?
Does anyone have a specific drawing for this circuit? I have an electronic copy of the drawing book for my rig. however, the 12v house drawing only has page 1 of 17. I really could use the other 16 pages.
Thanks for the help.
I can't find any other components that are not working (i.e. can't find any lights or fans that are not working). That makes it very confusing to figure out where those other lines go.
2006 HR Endeavor 40PDQ.
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Old 01-07-2022, 08:33 AM   #2
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If you separate the connection of the hot wire between the two closet lights will the fuse hold?
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Old 01-07-2022, 08:40 AM   #3
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I haven't tried that. I was hoping to avoid cutting any wires. The wires are crimped together inside a connector.
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Old 01-07-2022, 08:46 AM   #4
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Did it ever work for you (is this a new situation)? If so, then have you driven any screws to mount anything lately?
Other than those ideas, in order to fully troubleshoot this, you'll need to isolate wire runs even more than you have to narrow down the source of the fault.
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Old 01-07-2022, 08:53 AM   #5
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The fastest way to locate a short is to keep dividing the circuit to see where it is and where it isn't. Also bear in mind that a load such as a lamp filament can show on your meter as continuity to ground from the power side. I know you have all the loads you can find disconnected but possibly missed a fan or outside light?
From your drawing it looks like the issue is after the first closet light. At the first closet light one green wire brings power in and the other brings power out to the next light but apparently that section got shorted somewhere. Separate the connection between those lights and if the fuse holds, if possible, separate the green wire after the small closet light. If the fuse blows the problem is between those two lights. If the fuse holds the issue is beyond the small closet light. In this case leave it disconnected and if it leads to other loads you will soon discover what they are. Maybe it just got abandoned in the wall/ceiling and has now shorted.
I once had a Lance camper that had an intermittent short in the ceiling lights, only happened occasionally at first but as it got older it happened more often till finally it was a dead short. Turned out to be a roof rack screw had barely nicked a wire and took years to finally make solid contact.
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Old 01-07-2022, 09:00 AM   #6
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It is a long story filled with jerry rigs and such I've found from the previous owner. I've owned the rig for about 2 months. My goal is to fix stuff correctly. I have my work cut out for me...not just in this 12v issue.
I haven't screwed any screws in and can't find any that look out of place. A knicked wire was my first thought. Just got to find it.
Oh...and yes it worked...but only because the previous owner jumpered a positive wire from another circuit (probably because of the ground). I'm trying to fix the source of the original problem instead of bypassing it and pulling more power from another circuit.

Quote:
Originally Posted by Injuneer View Post
Did it ever work for you (is this a new situation)? If so, then have you driven any screws to mount anything lately?
Other than those ideas, in order to fully troubleshoot this, you'll need to isolate wire runs even more than you have to narrow down the source of the fault.
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Old 01-07-2022, 09:04 AM   #7
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Yeah. Half-splitting is my jam. I was an Electronics Tech in the navy for 20 years. But, troubleshooting without a drawing is really reverse engineering. Not what I had hoped to be doing.

Do you have any thoughts on rewiring cut connections? Is a wire nut stuffed back in the hole an okay idea in an RV? Or should I use a crimp connection with shrink-wrap? or some other method of reconnecting?
I was hoping to avoid cutting...but not sure I have any other options with the crimped plug pins on the light fixture connectors.


Quote:
Originally Posted by bigb56 View Post
The fastest way to locate a short is to keep dividing the circuit to see where it is and where it isn't. Also bear in mind that a load such as a lamp filament can show on your meter as continuity to ground from the power side. I know you have all the loads you can find disconnected but possibly missed a fan or outside light?
From your drawing it looks like the issue is after the first closet light. At the first closet light one green wire brings power in and the other brings power out to the next light but apparently that section got shorted somewhere. Separate the connection between those lights and if the fuse holds, if possible, separate the green wire after the small closet light. If the fuse blows the problem is between those two lights. If the fuse holds the issue is beyond the small closet light. In this case leave it disconnected and if it leads to other loads you will soon discover what they are.
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Old 01-07-2022, 09:08 AM   #8
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Voltmeters and ohmmeters can lead you astray even if you are very seasoned at using them.

Grab a 12 volt test light and run a ground for it.

With everything still unplugged install a new fuse. Check to be sure it does not blow immediately.

Use your test light on all of the molex plugs that are disconnected.

If the light works on all of them and the fuse does not blow you can begin plugging fixtures back in.

If the fuse blows immediately then of course you have a hard short on the line and can now use your test light in another way as an indicator.

It will take some effort but put the test light in place of the blown fuse.

The test light should come on which indicates the short is there of course.

Now as you work to isolate the wires by snipping them wherever you have to to isolate the string the light should go out when you disconnect the shorted one.

Make sure you leave plenty of wire so you can reconnect the cut wires. You might consider a blade connector so you can disconnect the wire if needed.

These can be a real pain to follow and a signal tracer can help you follow the path of the wiring.

A signal tracer is not an expensive tool these days and should be in every do it yourselfers tool box.

You would inject the signal on the load side of the fuse location for example. Then using the sniffer you can follow the path of the wiring. It is not perfect but sure is helpful.
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Old 01-07-2022, 09:15 AM   #9
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I like where you're going. Some stuff I've considered but have been hoping for a less evasive approach to finding the problem (i.e. I was hoping to avoid cutting any wires).

The fuse blows as soon as I plug the Molex back in with all light fixtures unplugged. One thing I don't know is if I have all the fixtures. Those question marks on my drawing are for wires that go somewhere I have no idea. There are no other light fixtures in my bedroom...I've looked high and low. I've thought about under storage lights, but they are on a different circuit.

Questions:
Will a signal tracer work through the metal framing of the coach? I've never used one before so I'll have to YouTube it.
Also, do you have any recommendations for reconnecting any wires I cut? Wire nut? Crimp & heat-shrink? Something else?

Thanks.

Quote:
Originally Posted by YC1 View Post
Voltmeters and ohmmeters can lead you astray even if you are very seasoned at using them.

Grab a 12 volt test light and run a ground for it.

With everything still unplugged install a new fuse. Check to be sure it does not blow immediately.

Use your test light on all of the molex plugs that are disconnected.

If the light works on all of them and the fuse does not blow you can begin plugging fixtures back in.

If the fuse blows immediately then of course you have a hard short on the line and can now use your test light in another way as an indicator.

It will take some effort but put the test light in place of the blown fuse.

The test light should come on which indicates the short is there of course.

Now as you work to isolate the wires by snipping them wherever you have to to isolate the string the light should go out when you disconnect the shorted one.

Make sure you leave plenty of wire so you can reconnect the cut wires. You might consider a blade connector so you can disconnect the wire if needed.

These can be a real pain to follow and a signal tracer can help you follow the path of the wiring.

A signal tracer is not an expensive tool these days and should be in every do it yourselfers tool box.

You would inject the signal on the load side of the fuse location for example. Then using the sniffer you can follow the path of the wiring. It is not perfect but sure is helpful.
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Old 01-07-2022, 09:15 AM   #10
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Quote [ The green wires past the wall switch and connected to the ceiling light do NOT have the ground present. This is with the light fixtures unplugged. ]

Are you saying when you unplug the ceiling light the fuse doesn't blow?
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Old 01-07-2022, 09:17 AM   #11
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Quote:
Originally Posted by HammerGeek View Post
Yeah. Half-splitting is my jam. I was an Electronics Tech in the navy for 20 years. But, troubleshooting without a drawing is really reverse engineering. Not what I had hoped to be doing.

Do you have any thoughts on rewiring cut connections? Is a wire nut stuffed back in the hole an okay idea in an RV? Or should I use a crimp connection with shrink-wrap? or some other method of reconnecting?
I was hoping to avoid cutting...but not sure I have any other options with the crimped plug pins on the light fixture connectors.
If you use wire nuts put some tape on them to keep from vibrating loose, otherwise use crimps. Heat shrink if you like but not really necessary in dry locations.
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Old 01-07-2022, 09:21 AM   #12
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Quote:
Originally Posted by Persistent View Post
I presume you have done more testing than you describe in your post to determine the wiring diagram.

I guess the two white and two green are input from previous fixture and output to next fixture.

I recommend you disconnect the greens from the two closet lights, both input and output.

Disconnect the green ceiling light at the wall switch and pull the F3 fuse.

Test to see if green is still shorted to ground. You must test at the now missing fuse, at both greens at each of the two closet lights, and at the ceiling light switch.

Normally daisy chained lights lead from one to the other. This may isolate each block of wire. Hopefully only one is shorted to ground.

If it is the green leading off to the left in your diagram, you can just leave that one disconnected until you find what it leads too.
Same as I said in post #5
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Old 01-07-2022, 09:36 AM   #13
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No. The fuse blows with all light fixtures unplugged.

Quote:
Originally Posted by RamiDav View Post
Quote [ The green wires past the wall switch and connected to the ceiling light do NOT have the ground present. This is with the light fixtures unplugged. ]

Are you saying when you unplug the ceiling light the fuse doesn't blow?
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Old 01-07-2022, 09:58 AM   #14
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I would not use wirenuts on a MH (as a FYI they are not allowed per ABYC stds on boats and MHs are similar conditions IMO).
If you are trouble shooting you may want to use bullet connectors instead of double crimps. This would allow disconnect / reconnect easily if you need to and they are pretty secure. If in doubt you can always add shrink wrap over them once problem solved.
Above would help isolate the ? Wire at the end of the last light. I assume that just means you don't know what it serves & where it goes.
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