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Common Questions About Truck Lighting

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Basic Truck Lighting Questions


What are combination marker/clearance lamps, and how do I mount them?
It is one light doing the work of two. It must be at a 45 degree angle (on corners) to be legal. It needs

How hard should I tighten down (torque) plastic lights or devices?
I know over-torqueing will generally cause cracking or distortion. Use the torque spec provided by the manufacturer. A marker lamp will generally be in the neighborhood of 8 to 20 inch lbs.. The "rule of thumb" is to use the minimum torque necessary to hold the device.

Are heavy-duty harnesses worth the extra cost?
Heavy duty harnesses provide an independent (secure) ground, less voltage drop and are completely sealed to avoid corrosion.

What is the "12 Square Inch rule?
On December 1, 1991, the National Highway Traffic Safety Administration (NHTSA) made effective the final ruling regarding the "12 Square Inch Rule." This ruling requires that the total luminous lens area for a stop and turn lamp must not be less than 75 centimeters squared (11.625 square inches) when used on vehicles over 80" wide. It must be noted that the reflex reflector ring cannot be counted as square inch lens area. When the reflex ring is subtracted from the total 12 inch lens, it leaves less than the 12 square inches required.


Troubleshooting Truck Lighting Issues


My light doesn't work. Is it the light or the wiring?
Use a tester or multi-meter to check for open or broken filaments in the lights, --or for voltage from the vehicle. Another method is to substitute a known good light. Lack of vehicle voltage means there is a problem with the vehicle wiring or ground.

Why are my lights cracking?
Most lights are made from polycarbonate which is the toughest, highest melting plastic generally available. However, it can be attacked (cracked) by some chemicals or cleaners? The answer is to avoid the incompatible material and/or use Diamond Shell protected products.

What are the best ways to make lights last longer?
Use sealed chock-resistant lights employing a long-life LED or filament. Do not forget to grease the plug connection, if recommended.

Can you tell me approximately how long a bulb burned and what caused it to go out?
Yes, by looking at the filament under a microscope and by examining other telltale signs about the bulb.

Some of the electrical pin connectors in my plugs have completely corroded away. What can I do about this?
We use the most corrosion resistant metal available in the plugs, but if water or "conductive" dirt gets in the plug, it can provide a "short cut" for the voltage which "corrodes" away the metal in the plug. The answer is to use new plugs and plenty of grease. If possible, move the lights to a more protected area and/or add a module box for protection.

Why are the lenses on the back of my truck melting?
It is probably due to extended contact of the lights with the insulating material provided by loading dock air seals. Turn the lights off when at loading docks or move them to a more protected location or use a bracket protector or switch to LED's which generate less heat.

Most of your lights seem to last a long time, but I have this one that's always failing. Why is that?
There is something different in the way it is being used; otherwise, there would be a problem with all your lights. Look for a loose mounting bracket or excessive vibration in the vehicle body at that location. If you find it, try to find a way to stop it. Look for impacts from loading docks, tailgates, doors, or cargo and try to reduce them if they exist or find an improved shock-mounted light, or an LED.



Legal Questions About Truck Lighting


Would it be alright to mount a light at an angle that isn't "square" on the vehicle?
"Maybe." You need to contact the Lab for a test in that position; otherwise, it can be questioned by NHTSA or enforcement people.

Can I ever use the Model 40 with Relfex reflector ring legally?
Yes, as follows: On any vehicle under 80" wide. As an "extra" light on over 80" (reflex is legal). On vehicles made in 1991 or earlier, when installed by the vehicles owner, although most states enforce the Federal requirement.

Is "SAE" or "DOT" lettering required on lights?
No lettering is required by NHTSA (108) except for headlights and conspicuity tape. The Federal Motor Carrier Safety manual mentions that some lights should be marked "SAE" but this is not enforced. Truck-Lite marks all their lights with "SAE" and/or "DOT" markings.

Are there any special lighting requirements for vehicles hauling flammable or explosive materials?
There are no "special" Federal requirements as "108" covers all over-the-road vehicles. Some states or municipalities have regulations, but these cannot "supersede" 108, they can only add to it. Truck-Lite’s sealed lights and wiring systems have been tested to all known standards and certification. Reports are available from the Truck-Lite lab.

"108" says that some lights must be mounted as high as "practical." What is "practical?"
NHTSA has stated that the vehicle manufacturer has the responsibility to choose the best mounting location "in light of the particular design/configuration of the vehicle involved" and they will not contest it unless it is "clearly erroneous." Recently, Canada has started to strictly enforce the word practical as meaning "capable of being done." In the case of sidemarkers, not more than six inches from the edge of the vehicle.

My customer wants to use a light for a "side turn" that isn't listed for that function. Is this OK, seeing as how side turns aren't required anyway?
Be careful that it doesn’t violate some state laws regarding color or location. If it is also used as a side marker, that function must be listed on the lens code.

My customer wants to add some auxiliary lights (ie. illuminated signs, deceleration lights, decorative lamps). Are these legal?
Yes, provided they do not: Render inoperative any device in compliance with "108". Negatively affect the performance of required equipment.Create confusion or misunderstanding of lighting signals. Always contact the lab if you have any questions.


Information Provided by Truck-Lite.


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