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Why lighting systems fail

A scalpel in the hands of a surgeon can be a vital tool, yet the same device can be used by a madman to inflict harm on others. The same can be said of test probes, when used to troubleshoot lighting systems.


A scalpel in the hands of a surgeon can be a vital tool, yet the same device can be used by a madman to inflict harm on others. The same can be said of test probes, when used to troubleshoot lighting systems.

“A test probe is a murder weapon (for lighting systems),” Bill Sumner, Canadian OEM manager for Grote said during a seminar at this year’s Transportation Maintenance and Technology Conference. “It’s the worst tool ever invented. If use properly, it’s a good tool to have in your toolbox, but when you start poking through wire to see if there’s power there, you’re asking for all sorts of trouble.”

The seemingly harmless tiny holes left by a test probe are enough to allow corrosion to set in, agreed Errico Paolucci, Ontario regional manager with Truck-Lite.

“A mechanic should never use test probes to puncture insulation while troubleshooting lighting complaints,” he said. “Salt molecules will penetrate the smallest hole and start causing corrosion.”

If that advice comes too late, existing holes should be immediately sealed using heat shrink tubing, Paolucci added.

“If not, wicking action can cause moisture to travel considerable distances inside the wire, resulting in corrosion at critical locations, destroying the circuit,” he said.

Corrosion is one of the most common causes of lighting system failure. Water, dirt, salt and other contaminants can penetrate the tiniest holes and wreak havoc on a lighting system. Lighting experts suggest using a non-conductive grease to encapsulate connectors and keep corrosion at bay. Another cause of light failure is the constant shock and vibration of travelling over the road. There are lights on the market that have been designed with shock-mounted mechanisms, which cradle the bulb and absorb the effects of road inputs. LEDs are less susceptible to harm from vibration, according to Paolucci.

Another issue that crops up is improper voltage. Paolucci estimates as much as 60% of bulb failures are not as they first appear. Often, they result from voltage spikes as a result of things like starting the engine with the lights turned on. Paolucci suggested avoiding starting the vehicle while the lights or other heavy draw accessories are turned on. He also suggested regularly checking the voltage, noting that just one volt beyond the designed voltage will reduce the life expectancy of a bulb by more than 50%.

Incorrect voltage should be investigated, not ignored.

“To correct voltage problems, discover the real cause,” Paolucci said. “Undervoltage is often caused by poor electrical connections.”

Don’t just ramp up the voltage to compensate for the problem, he added, noting: “Overvoltage is a killer of lamps and batteries.”

Each failed lamp has a story to tell, and Paolucci advised against throwing bulbs away without first investigating the cause of the failure. A bulb with stretched or broken filaments most likely failed due to vibration. A yellowish, whitish or bluish glaze on the bulb indicates a rupture in the bulb’s glass envelope. A dark, metallic finish indicates old age. And a black, sooty bulb indicates a poor seal in the bulb, Paolucci explained. Truck-Lite has found that 20% of the lamps that are discarded are still in working condition. The lamps should be removed and bench tested to see if they can be resurrected.

Another common cause of lighting system failures is a poor ground connection, especially when the trailer is used for a ground. When lamps are grounded through the lamp housing, Paolucci said to ensure there’s a clean metal-to-metal connection. A fifth wheel ground strap can be used for added protection on the chassis ground system.

Incandescent bulbs can have their lives cut short by excessive heat. This is less of a concern with LEDs, which draw less power. Dirt on the lens of an incandescent bulb can cause heat to build up, eventually leading to failure, Paolucci said.

“All lamps will live longer if they run cool and can dissipate heat,” he said.

When cleaning lights, be sure to use a compatible cleaning product, Paolucci suggested. Many commonly used cleaning chemicals can penetrate the plastic and cause cracking.

 Troubleshooting lighting failures

One way to prevent lighting issues is to incorporate a complete inspection of the wiring harness into your preventive maintenance program, Grote’s Sumner suggested.

“Harness systems are installed on a piece of equipment and then forgotten about until there’s a problem, and by that time it’s too late,” he said. “If adding the complete harness system to your PM schedule is not practical, then consider, at least, instructing technicians to inspect the harness system on a regular basis as much as possible.”

While a preventive maintenance program can reduce the likelihood of problems, any shop will encounter some lighting-related issues over time. The first step in troubleshooting a problem, according to Paolucci, is to use a multi-meter tool to measure the voltage at the J560 connector, which is the entry point for the system’s power. If appropriate power is reaching the lamp, the next step is to check the lamp itself for failure. The lamp should be removed from its mounting and unplugged from the harness, Paolucci explained. Also check the lamp’s plug connection.

If the lamp itself appears to be okay, inspect the harness pigtail to ensure it’s working. Look for evidence of corrosion and clean all contacts. Check to ensure the lamp is properly grounded and test the exposed terminals on the harness pigtail using a multi-meter, Paolucci continued.

If the lamp and pigtail are not to blame, move to the nearest wire connection and inspect for problems.

“Follow the wiring harness to the next plugged connection or junction box,” Paolucci explained. “Begin by unplugging the harness, and test for proper voltage on exposed terminals.”

If the problem is still elusive, he said to inspect the J-box, but avoid using test probe wires.

If everything inspected so far has checked out, move on to the J-560 nose box connection and examine the main cable.

“The best way to identify the problem within the harness system is to evaluate the system’s continuity and resistance,” Paolucci said. “To do so, a break in the system must be found, by looking for highs and lows in the system. Highs and lows in the current could be caused by inconsistencies in wire, and can be found in broken wires and even instances of cross-talk within the wire.”

He suggested using a multi-meter to conduct these tests – but only when the system is powered off. The test lead should be connected to either end of the wire and then attached to a multi-meter. A beep should indicate there is no continuity problem.

If the harness is found to be the source of the problem, it’s then time to track down where in the harness the wire is broken. Paolucci suggested starting at the connector, looking for abnormal signs that could indicate a failure, such as water wicking – the travel of water along the inside of a wire. Water wicking is bad news. The water can travel alongside the copper wire, eventually causing corrosion within an entire wiring harness.

If the failure is caused by a damaged connector, Paolucci said to replace the connector, after checking it with a continuity meter to ensure that all wires are making contact with their connector pins. When cutting off the connector, be sure to leave enough wire for working on both sides of the cut, he added.

Harness damage can often be traced back to the inappropriate use of a piercing probe, which leaves a hole that can then be penetrated by water. But road debris can also provide contaminants with a convenient entranceway. Paolucci said to look for swelling in the wire, which could indicate the presence of corrosion.

Occasionally, wires inside the liner can be damaged or broken without any damage appearing on the insulator that contains the damaged wire. This too, can be bad news, as it’s difficult to locate the failure and in some instances, the entire length of wire must be replaced.

Wires that are connected too tightly could experience problems when temperature fluctuations cause them to expand and contract.

“Repeated flexing will stress the wire and can sometimes cause it to break,” Paolucci said. “To avoid this, connectors and splices should be restrained to prevent damage from excessive motion during vehicle movement…Look for sharp bends in the wires of the appearance of a wire kink. Look for these at the secured ends of a tight wire or at the secured ends of an excessively loose wire, that can move a lot during vehicle motion.”

 Repair methods

When a damaged section of wire is located and removed, check it for signs of corrosion, Paolucci advised. Any corroded sections of wire should be removed. Dull or dirty wires can be cleaned using steel wool or fine sandpaper, he added.

For best results, solder the wires together and use heat shrink tubing to seal the connection. Crimp the connection to ensure it stays in tact.

Lighting system problems can be among the most challenging to diagnose and repair, but avoiding the use of test probes and monitoring the condition of the wiring harness on a regular basis can go a long way towards eliminating these types of problems in the first place, Paolucci reiterated.


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