Federal Motor Vehicle Safety Standards; Stability and Control of Medium and Heavy Vehicles During Braking

This document amends Federal Motor Vehicle Safety Standard (FMVSS) No. 121, Air Brake Systems, to allow the alternate placement of the external antilock braking system (ABS) malfunction indicator lamp on trailers that have limited or non-existent structures to which the lamp and associated wiring can be attached. The purpose of the malfunction indicator lamp is to inform drivers, and maintenance and inspection personnel, of malfunctions in a trailer's ABS. The agency will permit the placement of the lamp on certain trailers (such as liquid tank, dry bulk, container chassis, and lowbed trailers) on the left side of the trailer near the red rear side marker lamp, or the front face of the left rear fender of trailers equipped with fenders. In addition, this document defines the methodology that is used to measure distances between the lamps (closest edge of the effective projected luminous lens area of each lamp). This rulemaking allows designers and manufacturers maximum design flexibility in the location of the malfunction indicator lamp while still ensuring that the lamp will serve its purpose.