Troubleshooting Trailer Lights

By Tim Murphy and Mark Corke

If your trailer lights were erratic last season, now's a good time to fix them before you hit the road again. Here's how to find the problem.

Dunking the boat trailer lightsDunking boat trailer lights is hard on bulbs and wiring. (Photo: Bruce W. Smith)

When was the last time you thought about your trailer lights? You probably can't remember, right? These essential but often-neglected workhorses have a hard life: They get dunked in water and bounced around on the highway, yet they are expected to work trip after trip. Therefore, they need regular attention.

Regulations may differ from state to state but all trailers require a minimum of taillights, brake lights, turn signals, and a license plate lamp. While these same components may fare well in the RV market, many trailer-light components are not designed for the harsh marine environment, and they're not going to last on a boat trailer. All is not lost, however, and there are ways to keep your trailer lights working properly.

Keeping The Lights On

Many standard trailer lights use traditional incandescent bulbs. While these may be perfectly functional when the trailer is new, over time the seals around the removable lens covers fail, allowing water into the light cases. When backing the trailer down the boat ramp, the lights go underwater. If the seals are gone, as soon as water touches the hot bulb, the thermal shock will often cause bulbs to fail. Water also causes corrosion, and while a little dielectric grease or a shot of WD-40 might help, it's only a matter of time before rusting components, such as bulb holders and connections, stop the lights from working.

How do you keep everything working properly then? First, prevent your lights from getting wet in the first place. One option is to attach the lights to a removable light bar that can be quickly detached from the trailer before backing down the ramp rather than having lights permanently attached to the trailer. You may also want to unplug the trailer wiring harness from the tow vehicle just before your trailer is submerged to help prevent short circuits, unless there's a special-purpose wire for brakes or other safety functions.

A second option is to replace your standard lights with LEDs. These lights are available where the whole unit is epoxy-encapsulated, effectively sealing them for life. Because the price of LED technology is increasingly more competitive against the cost of traditional bulbs, many trailer manufacturers are installing LEDs from the outset.

Use A Multimeter To Check Trailer Wiring

A digital multimeter should be in every boat owner's toolkit. This tool is very useful for checking trailer wiring. In addition to the meter you will also need a length of wire with crocodile clips on each end. The wire should be long enough to reach from the trailer plug to the rear of the trailer where the lights are. Set the meter to ohms, which is the scale used to measure resistance — most meters have the Greek omega symbol Ω for ohms.

Connect one end of the long cable to the pin on the plug that corresponds to the wire you are trying to check, then run it back to the light fixture. Connect the other end of the long wire to one of the meter's probes, either the red or the black — it makes no difference for this check. Then touch the other probe to the same color cable where it connects inside the light. If the meter reads zero or nearly zero, this means there is little to no resistance and the trailer wire is good. If the meter displays "OPEN" or "OPn" in the display, the cable is either broken or there is a missing connection somewhere. Repeat this procedure for each wire in turn, replacing or repairing any that are suspect.

Wiring Woes

Subpar wiring is often a failure waiting to happen. But before you start blaming your trailer wiring, check your tow-vehicle wiring. The trailer connector socket on the back of the tow vehicle often hangs down below the bumper and is exposed to the elements, road dirt, and general neglect. Even if your tow vehicle is in good overall condition, start your troubleshooting by plugging in a tester before you tear apart the trailer wiring. A trailer-light tester is a good investment for a few dollars. This handy device easily confirms whether the tow vehicle wiring is installed correctly and is operating as it should.

Trailer Wiring Color Code chartClick on image to enlarge.

Once you have confirmed that the tow vehicle wiring is functioning, turn your attention to the trailer. Cables get chafed, connections corrode, and the plug may be a less-than-stellar fit. When troubleshooting trailer-light problems, disconnect the trailer tongue and safety chains from the tow vehicle to ensure that the grounding is through the connected wires only.

First start with the simple things: For trailers fitted with conventional incandescent light fittings, remove the lens covers and check the bulbs to make sure none are burned out. While the covers and bulbs are removed, check the electrical connections. If they are corroded, clean them with some fine sandpaper or a pencil eraser, then smear on a little dielectric grease before replacing the bulbs. Checking and replacing bulbs may be all that is required, but if not, you'll need to look at the wiring.

Many trailer problems are due to a poor grounding connection, which is usually the white wire coming out of the trailer plug. If the ground is poor, lights may work intermittently or not at all. Even if the wiring to the plug is adequate, make sure that the ground connections to the trailer frame are good. They should be bright and clean, free from paint and rust, and secure. If you operate one of the turn signals and the brake lights glow, or the lights are not as bright as they should be, suspect that the ground is the cause.

Trailering Safety Tips

Towing a boat

We've collected thousands of boat-trailer claims over the years, analyzed all of them, and found that by following a few tips, you're less likely to endure the problems some of our insureds have had during less-prepared tows.

Weight Distribution. Five to 10 percent of the total weight of the boat, motor, trailer, and gear should be on the trailer ball when the coupler is parallel to the ground. Too much weight on the ball and the tow vehicle will be difficult to steer; too little and it could fishtail (excessive sway). Techniques for redistributing weight at the coupler include shifting gear inside the boat, emptying water and fuel tanks, and adjusting the boat's position on the trailer.

Positioning. To reduce the chances of sagging or oil-canning (flexing) that could permanently disfigure or even weaken the boat's hull, the boat should be evenly supported with rollers or padded bunks concentrated in critical areas, such as the engine and chine. The transom on boats with outboards or I/Os must be well supported.

Securing the boat. When traveling at highway speeds, the boat will be buffeted by near-hurricane winds. Anything loose on the deck or in the cockpit may be blown away. Stow them below or make sure they're secured. Better yet, wrap the boat in a snug-fitting cover, which also protects upholstery.

Use heavy straps to anchor the boat's stern to the trailer. If not, the boat will bounce against (or off!) the trailer. Don't rely solely on the winch cable to tie down the bow. Use a separate line or chain from the bow eye to the trailer. When you're traveling, check the straps and the bow eye itself whenever you stop.

Safety chains. Use crossed safety chains between the car and the trailer coupling. Should the hitch fail, the chains will keep the trailer from flying off the road. Leave enough slack in the chains to allow for proper turning, but not so much that they drag. A shackle/pin is far more secure than relying on the standard S-hooks, which can jiggle loose.

Getting there (in one piece). Slow down. Reducing speed gives you more time to react and reduces the strain on the tow vehicle and trailer. Swing wider at corners so your trailer doesn't hit the curb, and allow extra space when you pass other cars. The additional weight of a trailer dramatically affects braking, so leave considerably more distance than you normally would between your vehicle and the one in front of you. Rely on lower gears rather than brakes to reduce speed when driving downhill. Some states have separate speed limits for cars pulling trailers.

— Charles Fort

The wiring and plug will often have four, five, six, or seven wires, depending on the trailer (see table above). A four-way connector, the most common on smaller boat trailers, provides a ground circuit plus three functions (typically left turn/brake, right turn/brake, and taillights). Larger boat trailers may have additional wires and different trailer-to-tow vehicle plugs. A five-way connector includes all that the four-way does but adds a blue wire for a brake controller or auxiliary function. A six-way plug is often rectangular and builds on the five-way by adding another wire that may be connected to a breakaway control or provide power to an auxiliary battery. A seven-way connector includes all the above plus an override for electronic brakes so you can reverse without the wheels locking up. But verify your wiring by checking the schematics that came with your trailer or asking the manufacturer.

Trailer wire is no place to skimp on quality. As the trailer wiring is likely to get wet from launching and the elements, use tinned cable to impede corrosion. Also avoid simply twisting wires together as a temporary fix — and don't use wire nuts. Instead, use properly sized crimp connectors. The same level of care needs to go into the trailer lighting as on the boat. That means connections should be properly crimped and covered with heat shrink to keep out moisture. 

— Published: February/March 2019


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