How many burned-out diodes will ‘fail’ an LED tail lamp?
When is a red tail light on your truck or trailer officially “failed”?
It’s obvious if it’s an old-style incandescent lamp with a bulb that either works or doesn’t. If the bulb’s burned out, the lamp has legally failed, and you need to replace that bulb or maybe even the entire fixture, if it’s damaged or corroded. If the bulb is dim, it might mean a bad ground or some other problem with the lamp.
It’s not so simple with modern lamps made of LEDs, or light-emitting diodes. A typical tail lamp has five or more diodes – small semiconductor dots that glow when electricity passes through them. They use very little voltage, stay cool and are long-lasting.
But everything has a lifespan, and as they age, diodes grow dim and eventually go dark. Unless power to the entire lamp is interrupted, they burn out individually or in groups. The question is, how many of those diodes must be dark before the entire lamp is declared failed?
You’d think a federal standard would set a number, but none does, according to Kerri Wirachowsky, a former vehicle inspector in Ontario, Canada, and now the director of inspection programs for the Commercial Vehicle Safety Alliance. Federal Motor Vehicle Safety Standard 108 and some state regulations cover brightness levels defined in minimum distances and in photometrics. These gauge intensity of light from the side as well as straight to the rear – but inspectors don’t carry light meters.
“Right now, the inspector does a judgment call based on amount of light he sees in the given distance,” Wirachowsky said.
This can be iffy. A better idea is to establish a percentage that an officer can objectively use for guidance. ATA’s Technology & Maintenance Council has just come up with that, and CVSA might adopt it in a few months. CVSA is not a government agency but an entity comprised of state and provincial motor vehicle safety officials in the United States, Canada and Mexico; it sets up standards for colleagues to follow.
“This has been going on since 2004,” Wirachowsky said of the diodes issue. Peterson Manufacturing, a major maker of vehicle lighting products, brought it to a head by asking CVSA to issue a rule on the matter. In the meantime, the company tested LED lamps to determine when they stopped emitting sufficient light under FMVSS 108.
That happens when 44% of a lamp’s diodes go dark, the company concluded and informed CVSA, which had requested some guidance. Robert Koelzer, a Peterson engineer involved in the lamp testing, also related that information to TMC members at a meeting in September. In a resulting recommended practice, that number became a more workable 50%. That will be taken up by CVSA officials at their meeting this spring.
The TMC group also discussed a maintenance-oriented figure, one that truck and trailer operators could use to plan for the replacement of lamps with diodes beginning to burn out. Most members concluded the dark-diode threshold should be 20%, but not all agreed, and the recommended practice does not mention it.
“Twenty percent makes sense for a maintenance standard,” Wirachowsky said. “You’ve got a window of opportunity to plan to get the lamp replaced before it reaches the failure percentage,” whether it’s 50% or something else. She added that the failure of one tail lamp is a violation, but it won’t put the vehicle out of service. If the other tail lamp still works, you’re legally good to go, but you need to get the failed one fixed. Likewise for a single stop lamp.
“It’s another matter with turn signals,” she pointed out. “If one is out, that’s out of service” for the vehicle. That’s true whether it’s a single-function turn lamp or a multi-function tail/stop/turn signal lamp.
You, the conscientious owner-operator, can use all this thinking to evaluate your tail lamps’ condition. For instance, if a lamp has six diodes and one has gone out, it’s still OK, because that’s under the suggested 20%. If two have gone dark, that’s 33%, and you ought to plan to replace that lamp soon.
If three of six diodes are out, that’s 50%. The lamp probably has become too dim for continued service and is likely to catch an inspector’s eye. You should install a new one ASAP. Be sure to use one with “DOT” molded into the lens so you know it’s legal performance-wise. Name-brand products will have this marking, and some also list an SAE number. But so might counterfeit parts, so beware.
Here’s another view: A recent thread on an online discussion site carried debate on LED lights and when to replace them. “They’re not that expensive,” one trucker-participant declared. “If one (diode) is out, replace the light. It just looks better.” LL