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Thursday, September 29, 2011

Locating 12-Volt DC Shorts in an RV

Can my 1986 Yellowstone travel trailer have a positive grounded electrical system? When I wire it as a negative ground, I keep blowing 40-amp fuses. Also, would I be better off to use a standard battery charger than the converter? Hank H., (Yuma, AZ)


Hank, I've never heard of any recreation vehicle using a positively grounded DC system. Those even disappeared from the auto industry many years ago. It's apparent, however, that you have a direct short to ground somewhere within the 12-volt DC battery system. Equipped with a digital volt/ohm meter (VOM), it is relatively easy to "ring out" the circuits to find which contains the direct short. Here's how....


The first thing to do is to ensure all 12-volt loads are turned off. Then remove all the fuses from the DC fuse block. Next install a new 40-amp main fuse. If the fuse still blows, the short is between the battery and the fuse box and not likely within an individual DC load circuit. With all the fuses still removed, set the VOM to the ohm's scale and measure for continuity (zero ohms), between the load side of each fuse position of each circuit, to ground. If you measure some resistance, but not continuity, you're probably reading the resistance through that load so make sure it is turned off. You'll be looking for a direct short to ground. 




Some digital meters will have a diode test function which is also good to use. In the diode test setting, the meter will emit a audible beep when there is direct continuity. The circuit that indicates continuity is the problem circuit to focus on.




Next, begin eliminating each device on that circuit one at a time. Obtain an automatic resetting 12-volt circuit breaker and attach two short leads with alligator clips to the two terminals. Use the breaker instead of continuing to blow fuses. Attach the circuit breaker to each side of the fuse holder. This test breaker replaces the individual fuse and will automatically reset after it cools down. This eliminates burning through a pile of fuses during the troubleshooting procedure.

If, for instance, the short is in the lamp circuit on the left side of the RV, turn on each lamp, one at a time until the circuit breaker trips. When the breaker trips, that's the device on that circuit that contains the short. By process of elimination, it will be possible to determine the problem circuit and the problem device on that circuit in most cases.

Once the short is located, in most cases, it's probably easier to run a new wire from point A to point B rather than trying to actually located the exact section of damaged wire. Just cut the wire and tape each end, then run a new conductor.

Standard, automotive battery chargers are not recommended as a 12-volt power supply or as a replacement for a 12-volt converter/charger. I do favor a dedicated smart charger, however. Read my review of the new Xantrex TrueCharge 2 battery charger. The key to correct battery charging is to make sure charger uses algorhythms designed for the type of batteries it will be charging. Shop battery charges simply won't be equipped with that sophistication. And most typical converter/chargers won't as well. The serious RVer should consider upgrading their charging system designed around the premise of "what is best for my battery bank."


##RVT805

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