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Modern Trucking Techniques
Wiring maintenance DOs & DON’Ts and more
LL’s technical editor goes to the industry experts to offer valuable tips on care and maintenance of your rig’s wiring system

by Paul Abelson, technical editor

If the engine is the heart of today’s truck, then surely its wiring is the nervous system. Wiring powers lights and accessories, and in modern vehicles, does so much more.

Signals from sensors run through wires. Systems like ABS are activated through wiring. And climate control and driver-comfort accessories get their power and control through wiring. Clearly, the care and maintenance of wiring is essential to the smooth operation of any truck.

So I could get it all right, I turned to two widely respected industry experts. Brad Van Riper is vice president of research and development for Truck-Lite Co., a leading supplier of safety lighting and vision systems. Carl Smith is vice president of sales for Sure Power Industries, a leading behind-the-scenes supplier of wiring devices used by virtually all the truck manufacturers.

Both Van Riper and Smith are heavily involved at The Technology and Maintenance Council, where they helped create all the recent Recommended Practices dealing with wiring. They are both frequent panelists and presenters at the Society of Automotive Engineers annual Truck and Bus meeting.

Wiring’s greatest enemy is corrosion. That’s probably true for the entire electrical system. With today’s snow-removal chemicals and other uses of chlorides, you must protect wiring from these salts. Whenever metals come in contact with moisture and chloride salts, corrosion occurs. Iron-, aluminum- and copper-based metals all oxidize, and oxidized metals raise electrical resistance significantly. Corrosion can create short circuits that result in incorrect signals being sent to processors, causing malfunctions.

The new snow-removal chemicals, magnesium and calcium chlorides have another property. They are deliquescent. That means they have the ability to liquefy by drawing humidity from the air. They may be relatively new to snow removal, but they have been used for years to control dust. You don’t have to drive on dirt roads to be exposed. Just driving near them can be enough to expose your truck to these salts.

Depending on where you operate, chemical corrosion can be a year-round problem, but one that can be solved.

First and foremost, keep the underside of your tractor and trailer clean. Power wash your rig. Then practice good wire maintenance. Keep wiring sealed. Never probe through insulation to determine if current is flowing or to take measurements. Even the tiniest pinhole can be a conduit for salt-laden moisture. Do your diagnostics by removing wires at the connectors and touch your probe to the exposed metal. When you’re done, use dielectric grease if called for and plug the connectors back together.

Truck-Lite’s Van Riper suggested these “DOs” and “DON’Ts” when working with truck wiring:

DO use wiring with hermetically sealed connectors whenever possible. Truck-Lite and Grote provide sealed wiring systems for their lamps that have plugs molded in and internal grounds, so no wiring should be left exposed.

DO use sealed connectors when sealed wiring harnesses are not available. When splicing wire, use weather-protected terminals with heat-shrink covers. After crimping, use solder if space allows. Then heat the terminals to shrink the specially designed plastic tightly around the insulation. The best heat-shrink terminals have a glue that melts inside the sleeve and forms a perfect watertight seal.

DO use drip loops. Instead of running wiring directly into a terminal, use enough extra wire to form a downward-facing loop, so any excess moisture from splash, spray or condensation will flow to the low point and drip off instead of flowing into the terminal.

DO cover all connections whenever possible. If boots are available to cover connections, use them.

DO seal connections with adhesive or cover with dielectric grease. Naturally, Van Riper recommends Truck-Lite’s NYK.

DO NOT mount electrical connectors vertically. That lets chloride-bearing moisture drip inside.

DO NOT allow exposed wires. Any wire exposed to moisture will wick the moisture inside. When chlorides are present, the wires will corrode underneath the insulation.

DO NOT use dissimilar metals. Brass terminals can be used with copper wire, but avoid aluminum terminals unless you’re using aluminum wire, which is very rare on trucks these days.

DO NOT create side loads on connections. Make sure there is enough loose wire so no side stresses are created. They can pull the terminals partially apart, exposing metal to corrosion.

DO NOT route wiring near exhaust components or other sources of high heat. Excessive heat can drive plasticizers out of insulation, leaving it brittle and easily cracked. Higher heat can melt the insulation, leaving wires exposed.

Electrical systems have improved over the years. You should take advantage of the improvements whenever working on your lights or wiring. For example, light emitting diodes (LEDs) have come down in price. They pay for themselves more quickly in longer life and lower maintenance cost. If you are still using bulb-replaceable lamps, at least switch to sealed lamps with modern plugs attached instead of using chassis-ground wires.

The newest wiring systems from Truck-Lite have triple-sealed plugs that do not require dielectric grease to keep moisture out. Their Series 88 harness can be ordered with sealed modules that eliminate the need to wire a trailer nose box. Just plug the modules together and you get a perfect connection from the 7-pin cable through the trailer.

Installing and adding new wiring
There are times when you need to add new wiring to power or control new accessories, such as inverters, refrigerators or any of the comfort and convenience devices now available. Sure Power Industries’ Smith offered some tips on adding new wiring.

First of all, don’t use a single 12-volt power outlet to power half a dozen devices. We’ve all seen cabs with multiplier plugs plugged into multiplier plugs, with huge loads running through just one outlet. Most lighter sockets are designed — and fused — for a maximum of 10 amps, or 120 watts at 12 volts. That’s fine for a CB radio, a small cooler and perhaps a cell phone charger, but there are limits as to what can be run. I’ve seen drivers who kept blowing their 10-amp fuses, so they replaced them with 30-amp ones. That’s pushing the limit of the wiring, and could result in melted insulation or even a cab fire.

Anytime you want to add a major direct current (DC) appliance or accessory, install a dedicated circuit, or else consider getting an inverter and using household alternating current (AC) appliances. When getting an inverter, make sure it is sized (and wired) to handle all the appliances it will power. Keep inverters as close as possible to the batteries, and use very heavy-gauge cable to handle the current. A 1,500-watt inverter with 20 percent inefficiency can draw a steady 150 amps, with surges doubling that. That’s why many manufacturers call for 4/0 cable from the batteries.

According to Smith, here’s the proper procedure to follow when installing and wiring-in add-ons:

  • Make sure you use the proper size wire. You need to total the amperage you’ll draw through the wire, and how long the wiring run will be. If you want to be sophisticated, you can look up how much voltage drop per foot you get with each wire size, but I prefer to use the chart Smith provided that gives wire gauge for various current loads and wire lengths, with no more than a 0.15-volt drop.
  • Use proper circuit protection. Fuses have fast response, are readily available for replacement and are inexpensive, but they are for one-time use. Circuit breakers are more expensive, but can be reset. Some reset automatically when the overload is removed. Wiring and circuit protection should be at least 20 percent greater than the maximum expected load. If you expect your maximum load to be 8 amps, size your wiring and your fuse for 10 amps. For a 25-amp load, wire and protect to 30 amps. For 100 amps, wire and protect to 120 amps.
  • Select the right type of wire. Truck wiring must resist chemicals, abrasion and climate while remaining flexible. Household wire, identified as GPT, is not suitable for truck use. It is insulated with vinyl (PVC) rated to 176 degrees F. Look for SXL or GXL wire. They are insulated with cross-linked polyethylene, rated for 257 degrees. SXL is standard wall thickness, while GXL has thinner insulation.
  • Route wiring for maximum protection. Place wiring inside channels whenever possible. Keep it away from heat. Avoid abrasion. Protect wires with sleeves or tubing. Use rubber or plastic grommets or pass-through fittings whenever going through bulkheads or firewalls. Tie wiring with nylon wire ties every 8 inches to 24 inches depending on size, stiffness and exposure to vibration.
  • Plan for the current source. You can pick up power at the fuse center, from a power port or other wire, or from the battery. Fuse centers usually have ample current for most applications up to 30 amps (360 watts). For higher current draw, consider going directly to the batteries. If you need to control a high-draw device with a switch, consider using a relay.
    It’s often not a good idea to draw power for any but the lowest current devices (phone chargers, navigation devices, etc.) from another wire or a power plug. The additive current may be greater than the circuit was designed to handle.
  • Consider the effect on the rest of the system. Some appliances create electrical noise or produce voltage spikes on a start-up. They can affect sensitive electronics that may be on the same circuit.
  • Always follow manufacturers’ instructions regarding wiring and installing any device.

I’d like to add one more piece of advice. Whenever possible, err on the safe side. Use heavier wiring, never lighter. Use wire with thicker insulation, never thinner. Space your cable ties closer. Keep in mind why wiring fails and how to wire successfully, and you’ll avoid many common problems.

Paul Abelson can be reached at

How to select cable size
Using a voltage drop table
Values inside chart are voltage drop per foot