How to Find an Electrical Short in an RV

by William Collins
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Short circuits range from direct shorts that blow the fuse immediately, to high-resistance types that take several minutes to blow the fuse, to "phantom loads" that never blow the fuse, but can drain your RV batteries within a few days. Although a 12-volt electrical short circuit in your RV can be elusive, a systematic approach to finding and eliminating possible trouble spots, one at a time, puts this repair within reach of the typical RV owner.

Direct Short

Step 1

Remove the fuse from a circuit that blows instantly.

Step 2

Attach the male spade AWG disconnects to the ends of the buzzer wire leads, so that you can plug the buzzer into the fuse socket.

Step 3

Insert the male spade AWG disconnects into the fuse sockets and listen for the buzzer. If you are using a piezo buzzer and hear no sound, reverse the polarity of the wires and you should hear the buzzer. Leave the buzzer on; this will let you know when you find the short.

Step 4

Identify all the appliances and lights that this circuit powers, and disconnect them one by one. If the buzzer turns off, then you have found the appliance that has the short circuit in it.

Step 5

Remove the male spade AWG disconnects from the fuse plug, and plug in a new fuse.

High Resistance

Step 1

Clamp the ammeter to the positive (+) terminal of the RV battery.

Step 2

Identify the appliances this circuit powers. Make sure all the appliances are turned off.

Step 3

Check the amp rating of each appliance, and write it down on a piece of paper.

Step 4

Turn on the first appliance, and check the ammeter to see how many amps it's using. Check the appliance's amp rating list to see if it's close to the listed amount.

Step 5

Continue checking each appliance. If you see one that draws more amps than it's rated for, you have found the appliance that is slowly blowing the fuse.

Phantom Loads

Step 1

Clamp the ammeter to the battery's positive (+) battery cable. This will show you how much current (amps) the RV is draining from your battery.

Step 2

Set the ammeter to the "Milliamp" scale.

Step 3

Remove fuses. To isolate which circuit is causing the slow electrical leak, remove each fuse, one at a time, while checking the ammeter. When you see the ammeter reading decrease, or go down to zero milliamps, note which fuse caused this. If the amp draw decreased, but did not go down to zero, leave the fuse out and continue removing the other fuses one at a time until the amp draw is at zero.

Step 4

Check the circuit. Once you've found the specific circuit that is draining the battery, determine which appliances or lights are on that circuit. Plug the fuse back in and disconnect each appliance in that circuit, one at a time. Look at the ammeter each time you disconnect an appliance from the circuit, until you find the amp draw.

Common phantom loads include:

· Electronic refrigerator circuit

· Carbon monoxide detector

· LP gas detector

· Television and stereo memories

· Illuminated switches

· Clocks

· Fans left on

· Cycling water pump

· Antenna amplifiers

· Automatic cabinet lights left on

· Power converter

· Inverter

· Electronic thermostat

Checking Wiring

Step 1

Disconnect all power sources and disconnect all appliances and lights on the shorted circuit.

Step 2

Remove the fuse from the shorted circuit. Plug a male spade AWG disconnect into the load side of the fuse plug (i.e., the wire that goes to the appliances and lights). Connect the ohmmeter to the spade connector and to a ground source such as the frame or sheet metal of your RV. If the circuit is still shorted, the meter will register continuity---the needle will go to zero.

Step 3

Trace where the wire from the fuse goes. Follow the wire loom and see if there is any mechanical damage to the wires. Look for screws that penetrate the wire loom, sharp metal edges that might cut the wires, compressed wires caught between two plates, and loose wires. Make sure all the wires are secure, to prevent them from being damaged by road vibration.

Step 4

Strip the insulation tape off the wire loom and find the color-coded wire that comes from the faulty fuse.

Step 5

Cut the faulty wire with a wire cutter. Use the wire strippers to strip 1/4-inch of insulation from each end of the wire you just cut, and use the wire crimpers to connect a male and female AWG disconnect. Write down the location of the cut you made on the wire.

Step 6

Check the ohmmeter to see if the needle returns to infinity. If it's still at zero, the short is between the cut and the fuse. If the needle goes to infinity after you cut the wire, the short is further down the line. Connect and tape the AWG disconnect.

Step 7

Continue to cut the shorted circuit wire every few feet, write down the location, and crimp disconnects on the wire. When the ohmmeter stays at zero after you cut the wire, you have found the section of wire with the short circuit (i.e., the section between your last cut and the previous cut).

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