Trailer Talk

Quality Parts Should Go Into Trailer Electrical Systems, Updated TMC Recommendations Say

September 24, 2013

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Updated recommendations from TMC for trailer electrical systems say sealed wiring, water- and corrosion-resistant connectors and low-amperage lamp designs are the things to get for long life – not new advice, but information newly included in the group’s Recommended Practice 704.
The rewritten RP, designated 704C because it’s the third update over the years, is one of 10 recently sent out for balloting by members of the Technology & Maintenance Council of the American Trucking Associations. RP-704C and others were formally approved early this month at TMC’s fall meeting in Pittsburgh.
“The RP-704B, recommended practice for Heavy-Duty Lighting Systems for Trailers, was last updated in January 1992 and was badly in need of an update,” said the leader of the project, Brad Van Riper of Truck-Lite Inc.
“We reviewed the RP704b and found that a complete rewrite was in order.” So they:

  • Made the RP light-source neutral, and eliminated bulb types and ratings to allow for technology improvements;
  • Removed and replaced obsolete references;
  • Updated lighting locations; and
  • Added wiring harness information, “focusing on the system,” he said.

“The purpose of the RP is to help equipment purchasers specify a safe and effective heavy-duty lighting system that is low-maintenance, durable and corrosion-free for a minimum service life of 12 years,” a duty cycle agreed on by task force members as typical for widely used trailer types, especially dry-freight vans,  Van Riper explained.
“Since lighting is moving away from incandescent light sources and more toward LED technology, the committee decided to de-emphasize the bulb and look at the complete system. We believe that the light source is no longer the weak link in the system, and teaching fleets to focus on a quality system will improve the durability and reduce maintenance on their new trailers.”
The rewrite took about two years, with participation from varying numbers of task force members, he said. They included people from fleets, industry associations, parts distributors, lighting and harness manufacturers, and trailer builders. They reviewed the final draft at a session in February, and with a few more changes it then went to ballot.
The laborious process for this and other TMC recommended practices ensures accuracy and acceptability from a large majority of members. Forty-one members voted for the updated RP and only one person voted against it; his objections involved typographical issues which Van Riper is now resolving.
RP-704C provides guidelines for how a trailer’s electrical system should be spec’d, and refers often to standards set by the Society of Automotive Engineers, Truck Trailer Manufacturers Association and others. And it leaves room for various trailer types and applications, he said.
“As you’d probably guess, there are as many wire harness systems as there are applications,” Van Riper said. “A ‘quality system’ is one that meets the needs of the end-use customer. Included are wiring jacketing, wiring gauge, terminals, connectors, processors, lamps and installation.
“The RP also included some installation Best Practices which can provide guidance to help improve the performance of the system.”
TMC’s recommended practices cover many types of truck vehicles and equipment, and maintenance on them. Access to the RPs is a benefit of TMC membership; information on that is available at


  1. 1. Charles [ December 17, 2013 @ 04:54PM ]

    You touched on an issue about LED lights i can't let go. About LED's failing, and being up to the DOT Officers discretion. As most of us know, California is it's own little world of enforcement. By the standards of many DOT Officers ive had the pleasure of meeting, even one (1) diode out in a multi diode light is a ticket. Regaurdles of trailer age. The rest of the country DOT will let it slide if a few are out, but if half of the diodes are out, they will write it up


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Author Bio

Tom Berg

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Senior Contributing Editor

Journalist since 1965, truck writer and editor since 1978. CDL-qualified; conducts road tests on new heavy-, medium- and light-duty tractors and trucks. Specializes in vocational and hybrid vehicles.


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