1. Techs should never use test probes to puncture insulation while troubleshooting lighting complaints.
If it ever becomes necessary, then the hole from the probe must be resealed. If not, wicking action can cause moisture to travel considerable distances inside the wire, resulting in corrosion at critical locations and destroying the circuit. When correcting illumination problems, don’t just turn up the voltage output. Find the source that caused the failure. Over-voltage is a killer of lamps and batteries. Always replace trailer light cords with the same, or better, quality and gauge of wiring than that used in the original specifications. Otherwise you lose current carrying capability.
2. To correct voltage problems, discover the real cause. Under-voltage often is caused by poor electrical connections. To correct under-voltage, don’t just turn up the voltage—find out the root of the problem for a more efficient operation.
3. Don’t just throw away lamps if they are not burning. Play detective when solving lamp problems to learn if another source is the cause of your lamp failure. Testing the lamp with a meter, power supply or battery can yield a truthful determination of the lamp’s cause of failure.
• A bulb with stretched or broken filaments most likely failed as a result of 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
• A black, sooty bulb indicates a poor seal in the bulb
The Truck-Lite Laboratory estimates that up to 20% of discarded lamps are still in workable condition. Save the removed lamps and bench-test them. The ones that aren’t working can be turned over to your lamp supplier. They can analyze them to find out why the lamps have failed.
4. Treat the electrical system like you would the chassis. Lubricate sockets, pigtails, battery terminals and connections with NYK-77 “nonconductive” anti-corrosion compound. The purpose of the sealant is to completely encapsulate and protect against corrosion and water. Mechanics should carry their own supply of lubricant as a sealant. Grease or any type of corrosion-preventive compounds may be used, as long as they are not carbon-based greases.
5. Inspect for improper ground connections. This is a major cause of lamp failure, especially when the trailer is used for a ground. When lamps are grounded through the lamp housing, make sure there is a clean connection (i.e., metal-to-metal). Also, a fifth wheel ground strap may be used for added protection on the chassis ground system.
6. Look for loose, bare or unsupported wire and fixtures. Harness and wiring should be on the underside of top frame members rather than on the bottom where dirt and road splash connect.
7. Heat is an enemy of lamps. Lamps must “breathe” or the heat buildup will shorten life. All lamps will live longer if they run cool and can dissipate heat. The use of LED lamps, and their lower power draw, can alleviate some concerns with heat. However, dirt on the lens can increase the heat with a lamp, so keeping your lamps clean can keep them operating in a cooler environment.
8. There are many household cleaning products that are not compatible with polycarbonate lenses and housings.
Is it the lamp or the harness?
In locating the source of your electrical malfunction, the first step is to ensure you have proper voltage at the J560 connector, where the system power comes in. Using a multi-meter, test for adequate power. If a multi-meter is not available, substituting a “test” lamp (a new lamp sure to be problem-free itself) for the malfunctioning unit will show if power is present.
If it is found that appropriate power is being fed to the lamp, the next step is to check the lamp itself for failure. Remove the lamp from its mounting application, unplug it from the harness, and test the lamp using a lamp checker or meter to verify its status. It is recommended to first use a new lamp (or one known to be in good working condition) to ensure the testing device is in proper working order.
According to the lighting experts at Truck-Lite, it also is recommended to closely review the lamp’s plug connection to be sure it is in good, working condition. Lamps can easily be damaged at the connection, and as a result could cause a seemingly working lamp to malfunction.
If it is found that the lamp failure is not the result of a malfunctioning lamp, check to see that the harness pigtail is in proper working condition.
Check for evidence of corrosion and clean all contacts to ensure an even contact with the current. Check to be sure the lamp is properly grounded, and test the exposed terminals on the harness pigtail with a multi-meter. Testing should also be performed with a working test light.
Wiring connections
Once it has been found that the lamp and pigtail are not responsible for the problem, move to the nearest wire connection to locate any problem areas. Follow the wiring harness to the next plugged connection or junction box. Begin by unplugging the harness, and test for proper voltage on exposed terminals. Next, the terminals in the J-box should be checked, as well. In this instance, do not use test probe wires. Probe wires can puncture an otherwise healthy wire, opening the door for future failure.
After successfully testing the wiring connections and lamps, testing should be performed at the J-560 (nose box) connection. At this point, the cause of the fault likely lies in trouble within the main cable.
The best way to identify the problem within the harness system is to evaluate the system’s continuity and resistance. 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.
In order to test the system for continuity and resistance, a multi-meter should be used. However, a multi-meter must not be used while a system is powered up.
To test the harness for continuity issues, begin by connecting one end of a test lead to one end of the wire, and the other end of the test lead to the other end of the same wire. When connected to the multi-meter, listen for the beep that will identify no continuity problems.
Cross talk should also be checked for, ensuring no wires are receiving power from other wires within a circuit. A beep from the multi-meter in this instance means there is a presence of cross talk.
As far as multi-meter setup is concerned, checking for resistance problems can be done with the same procedure as when checking for continuity issues.
First, look for a low reading. If a low number is found, the wire is good, and you should move on. At this point, look for a high reading. A high reading can indicate a problem, and likely represents damage from corrosion, cross talk or a break (overload).