Shots in the Dark

Anybody who’s ever pulled a trailer is only too well aware that its lights are a weak point. The best lamps and wiring systems minimize the hassle, but sooner or later even they will develop problems. The environment is just that harsh.

If you feel the urge to have a go at fixing things, remember the first rule in trailer-light diagnostics: start with the easiest and cheapest solution.

First, pin down exactly what the lights are doing. Are they affected by bumps in the road? Do they work sometimes? When was the problem first noticed? Does rain have any effect? Those answers should tell you which lights and circuits are suspect.

Often, shorts and failures can be traced to the light cord between the tractor and trailer. It’s vulnerable to both loose connections and corrosion. The most common problems are loose screws, dirt, and broken wires, but diagnosis is fairly simple.

1. Turn off the lights and remove the light cord from the tractor. First, clean the holes in the plugs. If a plug is full of grease from the fifth wheel, you can clean it with a shot of starting fluid (turn off all the lights and don’t smoke when you do this).

2. Then, take a close look at the plugs and the entire cable. Check for damage caused by the cord rubbing against a hot exhaust pipe. Has the cord sat against the edge of the frame rail or deck plate and chafed? Does the plug show signs of severe wear? Are all the parts there, including spring and clamp?

3. Take the plug apart and look inside. The little screws commonly work loose, and wires can also break just behind them. The screws can fall out and short an entire circuit or two.
The retaining clamp on the plug can also cause grief. When it’s loose or missing, the little wires get jerked loose. When it’s too tight, it breaks the wires. Torque it enough to hold the cable securely, then quit. Be sure it clamps against the heavy outer insulation and not directly against the internal wires.

A cord can also have internal breaks in the wires. If you don’t spot this visually, you’ll have to confirm it with a volt-meter after becoming certain the plugs are OK. This kind of damage often happens when the cord becomes entangled in the air lines, then gets snatched when jackknifing to dock the trailer.

Light cords can often be repaired by cutting off a few inches and installing a new plug, but don’t allow any stray wire strands to frazzle and create a new short. If possible, solder the wire tips before installing the new plug.

When you clip the cord and trim back its heavy insulation, be cautious not to cut into the insulation around the smaller wires which invites corrosion and shorts. Then seal the screws with a dab of fingernail polish to keep them from working loose.