FINDING AN ELECTRICAL SHORT --
MAKE YOUR OWN TESTER
By David Eidell
A really bad short circuit will smoke wire insulation and might cause a fire if the wire doesn't have a fuse or circuit breaker protecting it. Lesser shorts can be labeled “drains” and they are just as frustrating. A drain is a sneak thief; it'll rob the charge out of a battery sooner or later. Drains account for a high percentage of battery failures, and aspirin consumption. Electricity is frustrating --- you can't see or hear it (sometimes you can feel it but let's not go there). All kinds of elaborate meters can be purchased, but then you have to learn how to use them. Even when you learn how to use a meter, they're a pain to use --- you have to read them and your brain has to translate what's on the face of the meter into “What the heck does that mean?” Ever try to read a meter when you got your head upside down staring at a fuse panel?
Soon after my introduction into an electrical career I dealt with this frustration. There had to be a faster and easier way to tell if whatever repair technique I was trying was having any effect on the short.
In a nutshell the light bulb that went off over my head is a literal translation of what actually took place. “A Light Bulb!”. I could use one of those old-fashioned 12 volt light bulbs that look identical to those you use in a house. Trouble lights for cars have been using these bulbs for years, and most auto parts stores have them in stock. They come in various wattages but let's stick with the most common size: 50 watts. Next thing to ask for is a pair of spring loaded alligator clips. When the counterman asks “How big?” reply “Big enough to clamp on a battery post. Don't skimp on the size --- one of the clamps has to open wide enough to fit the lead post that sticks out of the battery case. The last thing will be a pair of electrical terminals. Ask for “spade (forked)” terminals to fit a number eight stud, for 14 gauge wire. Don't buy ring terminals because the lamp base that you're going to buy at the hardware store has terminal screws that cannot be unscrewed all the way out --- you'll have to slip the fork beneath the screw head then tighten it.
Head for the hardware store. In the electrical department pick up a porcelain or plastic light bulb base, the kind that your new 12 volt bulb will screw into. Plastic is a heck of a lot lighter than ceramic, and doesn't break as easily.
Go to the bulk electrical wire rack and yell for help. You're going to need to buy enough black extension cord to comfortably reach from your house battery bank all the way around into the door of your coach. Continue on inside until you can set the lamp somewhere near the fuse panels. The house battery fuses may be further yet than your chassis battery fuses. I use a cord that's 25 feet in length.
It doesn't matter which terminal is connected to what. Go ahead and strip the wires and crimp the terminals on one end of the cord (Cut back the outer sheath to allow the wires an ability to spread the distance between the battery terminals).
You could get fancy and smear silicone sealer on the screw terminals underneath the lamp base, or if you're in a rush apply a swatch of duct tape to insulate them. Exposed terminals could short out which would add no small amount of irony to what you're trying to fix. I like the gob of silicone sealer way because it will gluewires to the base and make it harder to pull the wires loose.
Test the lamp by connecting the two alligator clamps across a good battery. The light output will look similar to that of a 60 watt household bulb.
CONNECTING THE LAMP TO DETECT A SHORT OR DRAIN
Unless you are somewhat electrically inclined, the following connection directions may make no sense, and rather than try and puzzle it out, I recommend that you take things at faith.
Leave the positive battery cable connected to the coach battery. The negative cable should be disconnected from the battery post and laid at least a foot away from the battery post.
Connect one (it doesn't matter which one) of the lamp cord alligator clips to the negative battery post.
Connect the other alligator clip to the disconnected negative battery cable. You read right; if you need to, go ahead and re-read the connecting instructions again. Connect the lamp exactly as described and worry about figuring it out later.
Even though a system may have a catastrophic dead short, that could explode a battery, and start a fire, this tester can only draw 4 amps of power. That's about as much current as an interior light. The lamp acts as a safety limiter, a resistor.
• If the bulb lights up bright, you have a severe drain or a short, greater than 4 amps. Go around and shut everything off including lights, fridge, water heater, water pump. Remember you are dealing with the 12 volt system and not the shore power 120 volt hookup, as far as shutting off switches. Disconnect a stereo amplifier, and power inverter if you have one. Don't forget to switch off the shore power cord as it may power a high potential charger inside the coach.
• Carry the lamp into the coach and set it next to the 12-volt fuse panel. Start removing the fuses from the panel one-at-a-time, but do not replace them as you go. Leave them out. If the light goes out or suddenly dims after you pull a fuse, you've found the drain or short. Identify what the fuse protects and go straight to that device. A malfunctioning heater circuit or refrigerator can draw power that isn't high enough to blow a fuse.
• If the lamp stays lit with all the fuses out, you could use a separate 12-volt test light and touch the probe tip to the power inlet side of the fuses. If the test light lights up you don't have a catastrophic short (you could do this test with the fuses in or out). If you have no light at the fuse panel then you may have a shorted main power wire which would be located between the battery and the fuse panel.
• The short tester light bulb will dim more and more as the severity of the problem goes down. A dimmer than normal light could mean however a discharged battery. Take your time and make sure the battery is charged. Part of the reason why I like to check the short tester light outside at the battery first is because if it is bright then, the battery is charged enough for the test,
• A “true” dimming of the light bulb is progress although things won't be right until you cannot see the faintest glimmer from the filament. After you have located and fixed all the blatant drains and shorts, throw a blanket over your head and check the bulb out while you're in the pitch black.
• Try to shoot for a thoroughly “out” bulb filament. Sometimes it isn't practical but it's a goal worth seeking. You will be surprised at how faint a glow will still show up in the dark. Look a bit away from where the bulb is as your eyes are more sensitive on the side.
• I like to see a total battery drain level of less than one-twentieth of an amp. That's small but necessary so. If the drain is larger than that I recommend the installation of a battery cutoff switch or leave the rig plugged in to maintain the state of charge. You can barely see one twentieth of an amp lighting up the filament --- the filament would glow a faint rose color.
TIPS
The test light is pretty handy for other tasks. I have used it as a general area trouble light. While camping in bug country, I'll stretch out the cord and place the lamp as far away from me as possible. This cuts down on the number of bugs. I unscrew the bulb when it's not in use. A dab of red paint may help to identify the bulb and keep it out of a regular light socket. I can tell you from slightly embarrassing experience that the low voltage bulb can physically blow up and send glass fragments flying if it is connected to household power.