Bottom Line
Maintenance Q&A
Combatting corrosion

By Paul Abelson
Senior technical editor


Q: Two years ago, I bought a 1998 van trailer. A few weeks later, the lights started flickering. The wires and terminals were very corroded with that green powder all over some of the connectors. I rewired all the light circuits and used new connectors. Now the problem is back. Friends of mine with older trailers have gone more than 10 years without problems. Why? And what can I do to stop the corrosion?

A: Your question raises a number of points about corrosion and how it affects wiring. First, your friend may have gone 10 years without problems, and may go 10 years more if the trailer came with a modular sealed wiring harness.

The best way to fight corrosion is to keep corrosive salt spray away from the copper wiring. Sealed harnesses encase wires in a protective, insulating cover, with connections between modules that keep moisture from pins. With only solid pins exposed, sealed harnesses are the most effective way to keep water and chemicals from the wire. I believe sealed harnesses are worth the added cost.

With discrete wiring, sealing connections is critical. 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 is not a coating. It was once wire, but the corroded portion no longer conducts. Enough wire can be destroyed to turn 12-gauge wire into the equivalent of 18- or even 22-gauge. That increases resistance inside the wire, causing localized heating. Enough heat can be generated to soften common PVC (vinyl) insulation. Then, any rubbing from vibration can wipe away the insulation, causing short circuits and possibly starting fires.

Never probe through insulation. Use the point of a continuity tester on metal terminals only. Moisture will enter the pinprick and wick through the wire.

Replacement wires range from $3 to $15 per spool. People buy the cheapest because they never learned why prices differ. Gauge size and the amount of copper, affects price. But wiring also differs according to the type and diameter of the insulation. The most common wire, Type GPT Automotive Primary Wire, uses PVC insulation rated to about 175 degrees F. SXL or XLP wire, used by OEMs, is stable to more than 250 degrees.

There are differences in wire diameter, too. For example, 14-gauge SXL wire has a 20 percent greater overall diameter than 14-gauge GPT. You want the tightest possible connection without damaging the insulation. Connectors are made to accommodate the largest diameter in their range, and smaller wire may leave gaps that create a path for salt spray to enter the wire through the connector.

Seal moisture out to prevent corrosion from starting and growing in your wiring. Use OEM grade wiring. If you want extra protection, buy marine grade tinned copper wire, but I personally think that if you seal your connections properly, you won’t need to spend the extra money.

Use crimp connectors with adhesive sealant in the insulation. When you warm the connector with a heat gun, melted sealant will flow around the wires. If possible, solder exposed wire strands after crimping but before using the heat gun on the connectors so you don’t remelt and lose the sealant.

Put drip loops in the wiring so spray flows away from sockets, not into them.

Use heat-shrink tubing over the entire wire splice and use the heat gun to mold the tubing around the connector and wire. When making connections, whether terminal-to-terminal or plug-into-socket, use dielectric grease to prevent corrosion and keep out moisture.

Conventional wisdom had us pressure washing the undersides of tractors and trailers to remove corrosive salts. At September’s Technology & Maintenance Council meeting, we were advised that pressure forces chemicals deeper into crevices. Gently flood and brush the undercarriage to loosen and flush away salts. LL


Paul Abelson can be reached at