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Old 07-19-2013, 07:18 PM   #1
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positive to ground short

When I went to periodically start the engine in my parked 95 land yacht, I found a dead battery. In replacing the battery, after hooking up the positive terminal, the ground cable sparked against the ground terminal. Test showed 14+ volts.Everything was fine up to that point. Although the rv is parked in town, I think a pack rat chewed a wire.
Can anyone suggest the procedure and/or test equipment that will lead to the location of the chewed wire?

Thank you for any help. rspivey7919
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Old 07-19-2013, 07:37 PM   #2
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If the spark was at the ground terminal it may just indicate a poor connection there. If you hooked up the ground last that was just completing a circuit. Most everything has phantom loads, capacitors that need to charge etc. So it is not uncommon to get a spark . It is quite possible that nothing is wrong. I would watch how the battery performs first before looking for trouble
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Old 07-20-2013, 08:41 PM   #3
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positive to ground short

I consistently measure 14+ volts between negative terminal and ground wire. I wish it were a phantom! ray
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Old 07-20-2013, 09:56 PM   #4
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I would start with an ohmmeter at the main 12 volt distribution panel and remove all fuses and then start looking for the short. I would also run a test with an ammeter installed at the battery. Once you get the short located down to a specific circuit, you have to start taking things apart until you see the short has disappeared. Hopefully you will not have to take the interior panels out to find the exact location.
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Old 07-20-2013, 11:05 PM   #5
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Finding short?

Hello rspivey7919. If you are reading 14 VDC between the disconnected battery ground cable and the negative terminal of the battery then that is what you should be reading. Put the ground cable back on and read the volts across the battery. If you have a short the volt meter will show the battery volts dropping. Just because it sparked doesn't mean a short it just means that there is something drawing some current at that time.
Happy camping nm1oqrz

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I consistently measure 14+ volts between negative terminal and ground wire. I wish it were a phantom! ray
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Old 07-21-2013, 04:49 PM   #6
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Originally Posted by nm1oqrz View Post
Hello rspivey7919. If you are reading 14 VDC between the disconnected battery ground cable and the negative terminal of the battery then that is what you should be reading. Put the ground cable back on and read the volts across the battery. If you have a short the volt meter will show the battery volts dropping. Just because it sparked doesn't mean a short it just means that there is something drawing some current at that time.
Happy camping nm1oqrz

Maybe it will help if I withdraw the word "spark" and substitute " major electrical explosion". Any contact between the negative battery post and the negative ground cable destroys the wire used to make it.
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Old 07-21-2013, 10:50 PM   #7
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Re: Troubleshooting Electrical Problems

Yikes! It sounds like you have more than a pack rat chewing through a wire, causing an OPEN. You probably have a +12 VDC short to ground. Just guessing, but probably somewhere, a +12 volt line is laying across a metal pass-through hole, and the insulation has rubbed through or been cut so that the center conductor is shorted to the chassis (ground).

First, disconnect your battery completely, both positive and negative cables. Also, disconnect from shore power so that your converter is not providing power to the +12 volt and DC ground lines.

If you have solar or any other 12 volt DC sources (e.g., a battery charger), make sure those are also disconnected. Remove fuses, switch circuit breakers OFF, etc., so that the positive and negative 12 volt lines are completely dead.

Next, I would start with the easiest things first. Visually inspect the +12 volt battery cable, beginning at the battery terminal connector (which should NOT be connected to the battery during testing), and trace the cable back through the battery box and the through-hole where the positive cable enters your Airstream. Look at the insulation at cable clamps, pass-through holes, etc., where the insulation may be damaged and bare conductor wire is visible through the cut. This cable should go to a buss panel where wires start fanning out to the rest of your Airstream. In addition, look for sheet metal screws and other metal objects that look out of place, which may be stabbing through the insulation and connecting the inner conductor wire to chassis ground.

Note: Think back, since the last time your batteries were OK, have you drilled or cut any holes or screwed sheet metal or wood screws into the walls, floor, etc.? Have you done anything that could have cut or stabbed through hidden wires? Make sure to check these locations first.

Make sure all of the connections on the buss panel are clean and tight. Insure that stray wire strands, extra lengths of bare conductor, mounting hardware, metal twist ties or parts, set screws, etc., don't protrude and touch other bare wires and connections. Also insure connections are secure and none are free to move around, causing an electrical short to adjacent connectors.

Follow the biggest, thickest +12 volt wire (probably has red insulation) from the BUSS panel to the FUSE panel, which is most likely located near your converter. Check the cable for cuts and scrapes, etc., as above. Then, check the routing and insulation on any other +12 volt wires that come off of the buss bar, and follow them all the way to where they terminate at a light fixture or appliance. (Probably, there are only a couple of +12 volt wires; and the biggest one will go to the fuse panel.)

If you haven't found anything yet, remove all of the 12 volt fuses from the fuse panel (make sure to keep track of where they go for later reinstallation). One end of each fuse goes to a light fixture (or several lights/appliances connected in parallel on that line), and ultimately ends up connected to chassis ground. The other end of the fuse goes back to the +12 volt connector on the buss panel.

Using an Ohmmeter set to low resistance (e.g., Rx1, Rx10, etc.) with one test lead connected to CHASSIS GROUND, probe the electrical connections at both ends of each fuse holder.

Note: A continuity "buzzer" would also work; and it might even be better, if you are working alone.

One end of each fuse holder goes to the +12 volt BUSS PANEL; and this end should NOT indicate a low resistance SHORT to chassis ground. If any of these do, THAT WIRE OR SOMETHING ON THAT CIRCUIT IS DAMAGED, AND THIS LINE SHOULD BE THOROUGHLY INSPECTED.

The end that goes to the light fixture or appliance may indicate a direct short to ground, which is expected. However, it may also read as an OPEN circuit; because the ON/OFF switch may be in the OFF position.

If no short to chassis ground is found on the +12 volt side of each fuse, I would begin checking the wires that go FROM the FUSE PANEL to light fixtures, appliances, etc. Use your Ohmmeter to check the second side of each fuse for a SHORT to CHASSIS GROUND, and start with the circuit that has the highest rated fuse, since you indicated that the negative cable "exploded and burned". (The circuits with smaller fuses would probably have destroyed the fuse before burning up your cable.)

All of the above assumes that test readings are not affected by Ohmmeter test voltage leakage through your converter. Without actually trying this, I'm unsure if this will be a significant factor. However, even if all readings indicate some resistance, you may be able to make a good guess at which circuit is shorted by the Ohmmeter reading closest to ZERO Ohms (the lowest resistance). Mostly likely, any leakage back through the converter will have significantly higher resistance. If not, you may need to disconnect the +12 volt cable that runs between the fuse panel and your converter's +12 VDC output.

Once you isolate the circuit that has the short, you just need to carefully inspect those wires for damage. Also, look for burned wires, bad splices, overheated switches, etc. in that circuit.

The above is a best guess at how to approach troubleshooting this problem, as it's hard to do this without actually being there.

Please post your inspection and test results, especially if you don't find the problem source. Your results may help indicate what to look at next.

Good luck!
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Old 07-21-2013, 11:14 PM   #8
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Hi, Phoenix gave you the long version of what I would say. He has one paragraph where he is asking basically what was the last thing you did before this problem occurred. I did automotive electronics for many years and things like flashlight brackets screwed to the floor or dash board with these screws going right through a wire loom and causing a short. My first step would be to disconnect one battery cable, then connect a 12 volt test light between the cable end and the battery post. A good [actually bad] DRAW will have a very bright light and a normal DRAW will have a dim light. Now with the bright test light on, remove one fuse at a time. When the light goes out or dims, check what circuits are run with that fuse. This usually finds the problem. Not fused or fused with a fusible link, disconnect the main wire to your alternator; Rare, but these can DRAW also. Starter solenoids can stick too; Also rare.

Note: A DRAW is not always a short, but mostly a load, like lights left on somewhere and they seem to be in a place not easily seen from where you are at. Could be a bathroom light or fan. [in cars it was usually a hood, trunk, or glove box light that was staying on and draining the battery over night]
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Old 07-21-2013, 11:19 PM   #9
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Quote:
Originally Posted by Phoenix View Post

Note: Think back, since the last time your batteries were OK, have you drilled or cut any holes or screwed sheet metal or wood screws into the walls, floor, etc.? Have you done anything that could have cut or stabbed through hidden wires? Make sure to check these locations first.


Good luck!
Hi, This has helped me diagnose many electrical problems in cars. I simply asked the customer, "What was the last thing done to your car before this problem occurred?"
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Old 07-22-2013, 04:12 AM   #10
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Re: Troubleshooting Electrical Problems

Sorry, just realized we are talking about a motorhome. I should have been paying more attention. Robertsunrus mentioned the starter solenoid, and I noticed your Land Yacht description indicates "Diesel". Does your starter system use two 12-volt batteries to create 24 volts DC? If so, that might explain why you read 14+ volts with your voltmeter. (This could be the result of measuring two discharged batteries wired in series, or one good battery in series with one that's dead).

Also, the starter (and solenoid) circuit can draw over 100 amps during normal cranking; and without seeing your vehicle, I suspect it is not fused. Usually, a heavy duty (10 gauge?) starter cable runs directly to the starter/solenoid from the battery. Thus, if the solenoid is stuck, or if the starter motor is frozen and won't turn, this circuit could be drawing arc-welding-type current through the battery cable when you connect it.

What gauge wire burned in your original post? Specifically, did the negative battery cable throw off arc-welding-type sparks, glow red, burn off the insulation and melt? Or, did you see sparks, then use a jumper wire like a 120 volt extension cord (16 or 18 gauge) to connect between the negative battery post and the negative battery cable; and that smaller wire burned?

You might try bumping the starter solenoid using a jumper wire or remote battery switch to see if the solenoid works and isn't stuck closed (i.e., not internally shorted to ground). Also, you might try touching a live wire to the starter motor terminal to see if it spins. If the solenoid and starter motor work OK, these are most likely NOT the problem. -- Just trying to eliminate possible items...

IMPORTANT NOTE: If you are not familiar with your engine electrical system, you may wish to get assistance with this. Incorrectly jumpering wires to the starter circuit can possibly damage the solenoid and/or starter, throw a lot of sparks, burn cables and cause personal injury due to cranking the engine while your hands are in close proximity to moving engine parts. If you strongly suspect these parts are bad, you may wish to have a mechanic perform these tests.

In fact, if you are inexperienced or uncomfortable working with electricity, and/or do not have a lot of time to devote to troubleshooting, you may wish to weigh the cost of paying an RV tech to troubleshoot this problem versus possibly spending a lot of frustrating hours looking for one bad wire or connection.

Personally, I'd check the easy things. And, if I couldn't find anything wrong, I'd pay an expert a limited amount to just find the problem. Then, I'd fix it myself.

Do you have a friend or relative that works on automotive electrical systems that could help you with troubleshooting? It might be worth a few refreshing beverages (after you're done) or a dinner out to have them help you find and fix this problem.

==========

Example of remote starter switch (not necessarily the exact switch you may need): KD Tools Remote Control Starter Switch - Tools - Mechanics & Auto Tools - Automotive Specialty Tools
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