Interpretation ID: nht74-3.18

DATE: 11/12/74

FROM: AUTHOR UNAVAILABLE; R. B. Dyson; NHTSA

TO: Dana Corporation

TITLE: FMVSS INTERPRETATION

TEXT: This responds to Dano Corporation's October 22, 1978, request for a statement by the National Highway Traffic Safety Administration that Standard No. 121, Air brake systems, does not require antilock systems on the axles of air-braked trailers subject to the standard, and, if antilock systems are provided, that the standard does not specify the number or location of the speed sensing or logic components which constitute the system.

Standard No. 121 requires that wheels not lock-up under certain conditions (S5.3.1, S5.3.2) but it does not require the use of an antilock system to prevent wheel lockup. If a manufacturer chooses to install an antilock system on his vehicle, the standard requires that an anti-lock warning signal be installed (S5.1.6), that electrical failure of the antilock system not increase the actuation and release times of the service brakes (S5.5.1), and that an antilock system on a trailer be powered through the stop lamp circuit (S5.5.2).

There are no other requirements for antilock systems used on air-brake equipped vehicles subject to Standard No. 121. This means that the manufacturer may choose the number of wheel speed sensors and logic modules that he includes in his antilock system. It should be noted, however, that the "controlled lock-up" exception of S5.3.1 (a) and S5.3.2(a) would not apply to a wheel which is not equipped with a wheel sensor that contributes to the control of the reduction of air pressure.

If the number and location of these components becomes a safety problem in the future, the NHTSA would consider appropriate specifications for them.