On a good day, troubleshooting a busted light on a trailer goes something like this. The offending component is a marker lamp at the bottom edge of the trailer, within easy reach. You pop off the plastic lens, hoping a simple bulb replacement will do the trick, but when you gaze at the filament — hmm, it doesn’t appear broken. With a sinking feeling, you work your way through the parts shelves in the back of your shop until you find a spare, figuring you should at least give it try. You insert the bulb, flip the switch, and presto — it lights.

With a sigh of relief, you button things back up and go get a coffee. Elapsed time: 15 minutes.

Scene two: Squinting through a cold rain, you spy a darkened clearance light above the trailer door, 13 feet off the ground. You go back to your service truck for a ladder and when you’re finally face-to-lens you discover that it’s a sealed unit. You replace it, throw the old lamp in the warranty bin, and turn on the juice. Nothing. With a sigh of defeat, you button up your coat and go get a coffee. You’re going to be a while.

This is the New Century, the Electronic Age, yet the big bugaboo in trailer maintenance continues to be the electrical system, and wiring in particular. It’s no wonder, given the exposure to road grime, extreme temperatures, the cla-thunks caused by corduroy roads, the power draw of casino-style lighting, and the evils of bug light testers and MacGuyver-style crimp repairs.

The fact is, however, you can avoid all kinds of electrical problems if you spec the system with components that can stand up to the myriad abuses touched on above. Typically, that means choosing — and paying for, no doubt — higher quality wiring, harnesses, connectors, and lamps.

The basic wiring harness and its connectors have improved markedly over the years, making it simpler to create a system that will stand up to the ravages of chafing, yanking, moisture, and various contaminants. Better harnesses use modular, plug-in connections, meaning sections of wiring come in set lengths and mate with other sections, and lights, to make up a complete system. This modular approach should placate wiring experts who pound in the theme that anything that breaks the airtight and watertight seal formed by the insulation will invite corrosion and failure.

Connectors have evolved, too. Ten years ago, most lamps had female connections (imagine a connector wiggling loose and hot male pins from the wire striking the trailer frame — sparks will fly). Male-pin lighting connectors quickly developed into a standard because of their reliability, simplicity, and safety. Look for a sealed connection where the harness attaches to the light fixtures. Often this will be some sort of boot-like rubber or plastic; at the very least, it should include heat-shrunk coverings (electrical tape is a bad sign).

Getting Wired
Despite these advances, the physical wiring on your trailer hasn’t changed much in more than a decade, and is seen as a weak link in the system because buyers don’t clearly understand what they need.

Automotive wiring is comprised of multiple strands of copper wire covered with polyvinylchloride (PVC) insulation. The amount of copper in the wire determines how thick it is, and therefore its gauge. Gauge ranging from 8 (the biggest) to 20 (the smallest) may by used, depending on how many volts and amperes must be carried, and how far. The smaller the wire, the greater the resistance to voltage and current (amps).

When you choose wiring size, two factors come into play. The first is voltage drop, with the length of the wire being the primary cause. The second is current-carrying capacity, with ambient temperature being one of the influences (the higher the temp, the lower the wire’s capacity). Wire that’s too small may carry the load, but it may overheat in the process. So measure the distance from the most distant light to the power source (the plug-in at the nose of the trailer, or, with a truck body, the vehicle’s batteries). Take into account the path of the wires as they snake their way from one point to the other. To get to an overhead clearance lamp, for instance, wire will run east-west but also north-south — from the floor to the header above the door.

Next determine the load in amps. For lighting, this is simply a total of all the lamps on a circuit. For example, if you have 11 lamps on a trailer drawing 0.69 amps each, the total load is 7.59 amps. For safety, round up to 8 amps.

Your trailer dealer or manufacturer will have a chart that factors in amp load and distance to the most distant lamp to determine the proper size of wire. For a 12-volt electrical system, a 60-foot distance combined with an 8-amp load would require a 12-gauge wire size. If the distance were 80 or more feet, you’d need 10-gauge wire. It’s not unusual to see full 12-gauge wire on the trailer, and as high as 8-gauge for the ground return.

With respect to the lamps themselves, federal standards (CMVSS 108 in Canada, FMVSS 108 in the United States) dictate what lights you need, where they go, and the minimum illuminating power they must have. The regulation, complete with illustrations, is online at www.tc.gc.ca/roadsafety/reg/108/en/menu.htm. With few exceptions, vehicles built today meet these requirements.

The market for lamps continues to evolve as LED (light-emitting diode) lights plunge in price and the number of individual diodes needed for each assembly drops, too. Early stop/tail/turn lamps required as many as 128 separate diodes. Now, at Grote, Peterson, and Truck-Lite, the three key lamp manufacturers, we’re down to 10, seven, and six, respectively.

Adding to the economics, LEDs have a 100,000-hour life expectancy which should exceed the life of the trailer. They use far less voltage than incandescent lamps-the total current draw for a typical dry van trailer with LEDs is about 2.95, compared with 19.34 for their incandescent cousins. And their solid-state construction means they hold up well against shocks and jolts.

If you spec LEDs, remember that the lamp’s performance is affected by the ambient temperature: the lower the temp, the more efficient they are, for a given amount of current being driven through them. The right choice of wiring size will help keep things cool.

Getting the Signal Across
What’s shaping developments in electrical systems today are demands for powered accessories on the trailer, as well as regulations requiring an in-cab warning light to alert the driver to a malfunction in his trailer’s antilock braking system on new vehicles starting on March 1, 2001.

With the seven-pin SAE J560 tractor-to-trailer connection maxed out, the industry faced having to string a second connector just to carry the fault-light signal. Instead, a consortium of industry suppliers endorsed a multiplexing technology called powerline carrier, or PLC.
Multiplexing is way of sending a coded signal from the trailer to tractor using the present J560 cable. A “chirping chip” in the trailer ABS module piggybacks a signal on the No. 7 blue circuit through the connector to a chip in the tractor’s ABS module. The chip pulls fault-indication signals off the wire and relays a message to the instrument panel malfunction lamp when there’s a problem.

The bonus of multiplexing is that it increases the number of electrical signals the J560 connector can handle. Companies like Air-Weigh and Wheel Monitor are creating tractor-trailer communications controls that let you control all kinds of accessories-on-board scales, electronic brake stroke indicators, lift axles, and, of course, the ABS malfunction signal.

The transition to multiplexing hasn’t been smooth. Initially, not every supplier agreed that PLC was the best multiplexing protocol to use, creating confusion in the marketplace that exists today. Also, the multiplexed signal can generate interference with other electrical equipment in the tractor — blower motors and fans, for example — so you may need a filter to prevent this. Phillips Industries makes one that’s integrated into its receptacle/wiring harness; the PLC signal is segregated from the rest of the tractor power system and only clean DC power reaches the rest of the cab.

The more pressing concern is that not every trailer manufacturer is using multiplexing to power the fault light. Some cut into the No. 7 blue wire, an understandable notion, since it was traditionally the circuit used to power trailer accessories. On tractors and trailers made after February 1998, however, the blue circuit must be “hot” when the tractor ignition is on in order to deliver full-time power to the ABS as required by law. Anyone hacking into the blue wire to power other devices is inviting problems with delivering full-time juice to the ABS.

The same goes for cutting into other circuits. It leads to non-standardization, which could generate malfunctions related to overloads and switching requirements. Worse, you risk confusing the heck out of your technicians, who count on colour-coded cicuits to bring order. to their troubleshooting. Cross those signals and, on a bad day, you just might get your lights punched out.