Down to the Wire

By on January 1, 2010
RoadKing Mag

Except for physical damage that can wreck a rig in an instant, corrosion is the primary destroyer of trucks and trailers. Nowhere is this more apparent than in a truck’s electrical system. Progress has been dramatic in the past decade, but many truckers still find, as the TMC of American Trucking Associations reported in the 1990s and early 2000s, lighting and electrical problems are the greatest contributors to maintenance costs, in both dollars and numbers of incidents.

Why do lighting and wiring have such a significant effect? Because most problems stem from corrosion, and corrosion is an electrical phenomenon.

The mechanism of corrosion is very similar to the workings of a storage battery. When dissimilar metals are together in an electrolyte fluid that conducts electricity, the metals react chemically and become transformed. Metals containing iron react to form iron oxide, a compound of iron and oxygen from the air. Salt spray is the electrolyte. Many are used for snow and ice control. Copper wiring reacts to form various compounds, but a sure sign of corrosion is the green powder that becomes part of the wire.

While many trucking operations still experience a great number of wiring problems, especially in trailers, those that specify sealed wiring harnesses, light-emitting diode (LED) safety lamps and modular plugs seem to have overcome the problems. They report virtually no wiring and electrical problems.

Sealed harnesses encase multiple wires in protective, insulating covers, with connections between modules that keep moisture from pins. With only solid pins exposed and dielectric grease protecting the pins, sealed harnesses are the most effective way to keep water and chemicals from the wire. I believe sealed harnesses are worth the added cost. But not every owner can afford to re-wire trucks.

Make good connections
Most owner-operators have trailers with discrete wiring (individual wires that run from a battery, a terminal or switch to the point of use, or to a chassis ground). With discrete wiring, sealing connections is critical. Corrosive spray can enter exposed wire ends and, over time, wick through the strands for 20 feet or more.

The green powder that we see as corrosion in wires is not a coating. It was wire, but the corroded portion no longer conducts. When doing electrical testing, use a continuity tester on metal terminals only, never through insulation. Moisture will enter the pinprick and wick through the wire. The resulting corrosion can turn 12-gauge wire into the equivalent of 18- or even 22-gauge. In one week or less, a wire can become useless. Narrowing the copper increases electrical resistance, causing localized heating that can soften common insulation. Any rubbing can then wipe away the insulation, causing short circuits and even starting fires.

Replacement wiring prices vary widely by wire gauge, amount and type of insulation. If I didn’t know about differences, I would buy the cheapest. But the differences are important. Common automotive primary wire, Type GPT, uses vinyl insulation, good to about 175 degrees F. If the internal temperature of the wiring gets any hotter, the insulation will soften and melt. Truck, trailer and body builders use more expensive SXL or XLP wires. They remain stable beyond 250 degrees, providing a greater safety margin.

There are differences in overall diameter, too. Fourteen-gauge SXL wire, for example, has a 20 percent greater overall diameter, including the insulation, than the same gauge common wire. Wire should fit connectors as tightly as possible for a good seal, so insulation diameter is important. Connectors fit the largest diameter. Smaller wire can leave openings that let salt spray enter the connector.

Sealing and protecting
When working on wiring, always seal connections so corrosive spray is kept out. Use the same grade wiring that manufacturers use. The type should be identified on the wire. If not, bring a sample to the parts store so you can match gauge, type and diameter. To get the best possible seal against salt-carrying moisture and other chemicals in the air, place heat shrink tubing over one wire to be joined.

Use connectors that can be heat-sealed. Crimp them with ratcheting tools that have one die set for the wire and another for the insulation. If possible, solder exposed wire strands after crimping. The best connectors have adhesive sealant inside the insulation. Use a hair dryer or heat gun to melt the sealant and get it to flow around the wires.

After sealing the connectors, slide the tubing over the splice and use the heat gun to mold it around the connector and wire. Whenever possible, use lamps with sockets rather than wire pigtails. Use dielectric grease in sockets to prevent corrosion and keep out moisture.

When laying out wire, allow about a foot extra so the wire is not pulling on plugs or connectors. Place downward-facing drip loops in the wiring so spray will run from connections, not into them. If routing through holes, use grommets or conduit to protect the wire.

Don’t pressure-wash the undersides of tractors and trailers to remove corrosive salts. Pressure forces chemicals deeper into crevices. Gently flood and brush around wiring to remove salts.

If you take care of your wiring, you will eliminate a major cost in maintaining trucks and trailers, and will help them provide many years of trouble-free service.

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