All systems go

By Paul Abelson
senior technical editor


If your truck’s chassis is like your spine, the cab like your head, and its powertrain like your heart, then its wiring is your central nervous system.

It carries the signals from sensors to its brains, its computer systems. Just as nerves signal your muscles to act, wires signal safety and control systems.

If wires are cut, scraped or corroded, electrical signals and power won’t get where it is needed. Batteries won’t charge, turn lamps won’t flash, markers will flicker, or engines will die. In the worst case, wiring causes fires that can burn your truck to the ground. But wiring problems can be avoided.

Tractor wires are weather protected, so cab corrosion isn’t often a problem. But they can be overloaded. When adding accessories like a DVD or refrigerator, use a separate circuit from the battery and place an appropriate fuse on the hot wire, as close to the battery as possible.

When adding devices to a circuit, you need to remember that 12-volt power points are fused for either 10 or 20 amps. Don’t use plug splitters to add circuits unless you know your total amps will be within its limits. For example, you may have three low-amperage electronic devices on a 1-into-3 fused and switched plug. You need to know your total draw will not exceed the 10-amp limit. That means you should never plug in a refrigerator, for example. The overload would blow fuses and heat wires. With the wrong wiring, insulation could melt away and cause a short circuit or fire.

With more household appliances, computers, CPAP machines and entertainment systems in use, inverters are now commonplace. When wiring an inverter, add the wattage of all the devices it will power. Multiply by 1.3 to allow for internal resistance losses. Divide by 12 (for the volts), and that will tell you how many amps your inverter will need. You may need 1/0 or 2/0 gauge cable from the  battery to the inverter. Remember to fuse it close to the battery.

Now let’s look at the trailer, which is where most problems occur.

Trailer builders often use sealed harnesses. They assemble quickly, are durable and eliminate warranty claims. Older or specialty trailers often have discrete wiring, individually strung from the switch to the lamp or device.

All major lighting companies make modular wiring harnesses with sealed plugs and connectors. If your trailer is relatively new or you plan to keep it for a while, look into replacing discrete wiring. Harnesses are always the right size, and connections are sealed against corrosion-causing moisture.

When stringing individual wires, make sure they’re the right gauge, type and size. If too small, resistance will be too high and voltage will drop; the longer the wire, the greater the drop.

Insulation and outside diameter are important, too. Type GPT (automotive) melts at 175 degrees, more than 75 degrees lower than the SLX or XLP used by OEMs. In any gauge, GPT can be 20 percent smaller than OEM wiring. Because of that, it won’t fill connectors or clamp properly, allowing moisture in.

When crimping, use the right tool. The best tool is a ratcheting crimper with dual jaws, for wire and insulation. You need to make two crimps, one on the bare wire and one on the insulation.

If you crimp too hard with a single jaw tool, you can damage the insulation, allowing moisture in. If you don’t crimp hard enough, the connector will loosen. As you squeeze, the jaws grab the wire and connector, then continue until just the right amount of force is applied. Keep squeezing and the jaws release, leaving a perfectly crimped connector.

The next step is to seal the wire. It’s best to use connectors with sealant inside. After the wire is stripped and the crimp is made, heat the connector to release the sealant. Use heat shrink tubing over any splices.

Before splicing wire, put heat shrink tubing on one of the wires you’re joining. After crimping and sealing, slide the tube over the splice and heat it evenly with a heat gun until the tube conforms, sealing it tightly. When using ring connectors on screw posts or making connections where sealing and tubes won’t work, another option is a can of liquid electrical tape. This vinyl paste-like compound brushes on and dries in about 10 minutes, making a waterproof seal. It’s flexible and can be slit and peeled off if necessary.

Never cut into a wire’s insulation. If you need power for a new accessory, make a proper splice using connectors or string a new wire from a power point. Connectors that push in are for household use and are not subject to salt spray and vibration. Even they should be well-insulated with electrical tape. 

Loose wires chafe. If too tight, wires can pull out of connectors. Don’t coil up long wires. Splice them to proper length. Cut wire with about 3 feet more than the direct length, then form a drip loop about 1 foot in diameter. Fasten it with wire ties so the loop faces down. This will take any collected road spray away from connections that are most vulnerable to corrosion and relieve tension on connectors.

Place wires inside split plastic conduit. It protects from impact, spray and abrasion. Use wire ties liberally. They keep wires together and prevent excessive vibration. When stringing wire through holes in chassis, cabs or brackets, always use grommets to protect wire.

Sealed harnesses take the ground back to the battery, while most discrete wiring uses chassis grounds, often a major problem area. When lamps don’t work on the truck but do with a tester, check the ground. If it is corroded or damaged, remove all signs of rust or corrosion with a wire brush or emery cloth. Use baking soda paste to neutralize any remainder. Flush thoroughly and dry with a heat gun. Splice a new terminal to clean wire.

Don’t buy a replacement lamp until you test yours. A lamp tester should be in every tool box. All have standard terminals and a 9-volt battery, enough to light a 12-volt lamp. Manufacturers report that most bulbs returned under warranty, including LEDs, operate just fine. They didn’t light because of wiring problems, not burnout.

You should also keep a multimeter in your toolbox, too. It can be set to read amperage, voltage and ohms. With it, you can determine the current available, voltage drop and other wiring conditions.

A continuity tester identifies current flow. With an alligator clip on one wire and a pointed probe on the other, the tool quickly identifies if there’s current between two points. Clip the jaws to an exposed wire or terminal, then use the point to touch exposed metal. If the tester lamp lights, current is flowing.

It can also determine whether current is flowing through a long length of wire. Disconnect the wire and connect the tester clip to a ground. Touch the probe to the wire strands or the metal part of the connector.

Never, ever put the point of the probe through a wire’s insulation. It will leave a pinhole that will allow moisture to wick through the wire. Within a month, a foot or more of wire will be internally corroded, often cutting current flow completely.

If corroded internally, replace the entire ground wire. After attaching, paint over it to prevent more problems.

If you size wire properly, protect it from damage and corrosion, and secure it, then wiring problems should be behind you – and I don’t mean in the trailer.