Maintenance Q&A
Fighting the flicker

By Paul Abelson, senior technical editor

Q. Last winter, I had lots of problems with my trailer lights. It’s a 1998 Great Dane with ordinary lighting. I was told that LED lights last longer, but the problem doesn’t seem to be that they burn out any quicker. Actually, they flicker. If you jiggle them the right way, they come back on.

I know the trailer is getting old, but I can’t afford a new one yet. What can I do in the meantime?

A. More and more, modern trailers use sealed wiring harnesses. They have all the trailer wiring enclosed in a molded protective shell. Connections to individual lamps or accessories are made using self-sealing plugs that connect the harness to the lamp or accessory to be powered.

Many operators further protect from corrosion by applying small amounts of dielectric grease to the connections. On a side note, if you use dielectric grease, apply only small amounts. Too much will prevent the self-sealing connections from being properly seated.

Your trailer, on the other hand, has what is called discrete wiring. Each lamp draws power from a junction at the 7-pin connector through an individual wire. Some wires go individually to a single lamp, but most often – as in the case of clearance/marker lamps, brake lights and turn signals – a larger wire carries current to smaller wires spliced to it. The smaller ones power the individual lamps.

In sealed harnesses with molded connectors, the ground current goes back to the battery through a ground wire in the harness. With discrete wiring, the ground is often to the chassis or other metal part through a fastener right at the lamp. These are targets for corrosion.

Corrosion is the greatest enemy of trailer wiring, especially with the increased use of aggressive snow-fighting chemicals. The road spray containing the dissolved chemicals penetrates every nook and cranny on a truck. There, it attacks the wires and the truck’s structure. The secret to avoiding problems is to protect everything from the spray.

You can replace your discrete wiring with a sealed harness. This is the easier but more costly way to protect your lighting. Sealed harnesses can be custom-made if you have custom show-truck lighting or accessories like power lift gates. For “normal” trailers, they come in standard sizes and configurations or in modular sections that can be plugged together. Sealed harnesses are available at all trailer dealers.

The less expensive, but far more time-consuming, alternative is to inspect, repair and (when needed) replace wires and fixtures. One problem with discrete wires is the ability of corrosive materials to enter the tiniest openings in insulation. Once an entry is found, the material wicks through the wire, reducing its conductive properties. This happens for as much as 15 to 20 feet beyond the point where the chemicals entered. It is a leading cause of the flickering you described.

When you have a piece of stripped (insulation removed) wire crimped to a connector and left exposed, you can see obvious signs of the wicking that takes place. But you don’t need that large a section of exposed wire. Even a pinprick will do.

Sometimes technicians use electrical continuity testers to determine current flow. The sharp point is supposed to be placed on bare metal, but many still use them to penetrate insulation when bare metal connectors are too hard to reach. Even the smallest hole opens a pathway for corrosion. Never damage insulation.

Whenever working with trailer wiring, start by making sure the wires are the right gauge or diameter. The longer the wire, the greater the overall resistance to current flow and the greater the voltage drop. Larger wire (lower gauge number) allows current to flow more easily, lessening voltage drop.

If you need to splice wires together, the best practice is to use connectors and tools designed especially for the job. Specialized connectors with heat-activated sealant are available through truck and trailer dealers. They require crimping around both the wire and the insulation. Once the connection is made, a heat gun can be applied to melt the sealant and cause it to flow around the wire strands.

To ensure the seal is waterproof, slide a length of shrink tubing over one end of wire. After the terminal ends are spliced and sealed, connect them and slide the shrink tubing over the connection. There should be about an inch of tubing beyond each end of the connection. Use a heat gun to shrink the tube around the wire and the entire connection. The result should be completely sealed against any corrosion for the life of the vehicle.

You can crimp manually, but the best tools assure a virtually perfect crimp every time. The tools have pairs of jaws, one for the exposed wire and one for wire covered by insulation. Each pair of jaws is color coded by gauge size. Ratcheting action allows steady application of force until the crimp is correct. Then the tool releases. This type of tool is essential for trailer wiring work. It is available from any electrical tool source.

As for the lamps themselves, you can get LEDs that will mount in the same holes as the incandescent lamps you are replacing. They are more costly up front, but you should never have to replace them. For best results, ground the lamps back to the heavy gauge wire leading back to the 7-pin connector. Twisting exposed wire strands around a screw or bolt head just doesn’t cut it anymore.

This should keep your lamps from flickering. LL


DO YOU HAVE A maintenance question?
Send your question to Paul Abelson, senior technical editor, in care of Land Line Magazine, PO Box 1000, Grain Valley, MO 64029; email them to or fax questions to 630-983-7678. Please mark your message Attention: Maintenance Q&A.

Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.