Interpretation ID: nht95-6.38

TYPE: INTERPRETATION-NHTSA

DATE: September 6, 1995

FROM: Earl Eisenhart -- Vice President, National Private Truck Council; Larry W. Strawhorn -- Vice President - Engineering, American Trucking Associations

TO: John G. Womack -- Acting Chief Counsel, NHTSA

TITLE: NONE

ATTACHMT: ATTACHED TO 11/17/95 LETTER FROM Samuel J. Dubbin to Larry W. Strawhorn (A43; Std. 121)

TEXT: Dear Mr. Womack:

This letter is a request for an interpretation of the phrase "separate electrical circuit" in Section 5.1.6.3 Antilock Power Circuit for Towed Vehicles of Federal Motor Vehicle Safety Standard (FMVSS) 121, Air Brake Systems, which states:

"Each truck tractor manufactured on or after March 1, 1997 and each single unit vehicle manufactured on or after March 1, 1998 that is equipped to tow another air-braked vehicle shall be equipped with one or more separate electrical circuits, specifically provided to power the antilock system on the towed vehicle(s). Such a circuit shall be adequate to enable the antilock system on each towed vehicle to be fully operable."

Our interpretation is that the phrase "separate electrical circuit" allows the continued use of the single SAE J560 connector if one of the seven pins provides full-time power for the ABS. It is also our interpretation that the ABS malfunction signal can be multiplexed on the power circuit or any other circuit of the SAE J560 connector and that the other trailer devices can be powered off the circuit as long as the performance required by the last sentence of Section 5.1.6....3 is achieved, i.e., the circuit is adequate to enable the antilock system on each towed vehicle to be fully operable.

The bases for our interpretation are:

We interpret separate to mean full-time power is supplied to the ABS. The preamble appears to use terms continuous, dedicated, full-time and separate interchangeably but fails to define these terms. In contrast, we interpret shared to mean that the power is supplied only when switched ON by some means, e.g., turn signal switch, light switch, brake pedal, etc. The preamble (FR60-47, p13250) states that "The agency further notes that requiring that trailer ABSs receive "full-time power will not prohibit multiplexing." Therefore, we interpret that the ABS malfunction signal could be multiplexed on the power circuit. We further interpret that other trailer devices could also be powered through this circuit if they were automatically switched OFF if the trailer is equipped with an ABS, except when the vehicle is stationary. This would require that the trailer be capable of signalling the tractor that it has an ABS. The tractor would

have to be capable of receiving that signal and automatically deactivating the power to other trailer-mounted devices that now recieve power through the auxiliary pin. This solution would protect these devices from being continually powered if the trailer was equipped with an ABS, yet would still allow for the provision of power when the combination was not moving. The majority of today's applications for powering trailer equipment through the auxiliary pin, only require power when the vehicle is stationary.

Our interpretation of the word circuit as used in FMVSS 121 is that it refers to a single conductor between the towing and towed vehicle(s). This interpretation is consistent with preamble statements and SAE's commone use of the term. For examples, Notice 3 of Docket 88-18 (FR57-134, p30911) states, "The stop lamp circuit is powered through one of the pins on a seven-pin connector" and in SAE Standard J560, Seven Conductor Electrical Connector for Truck-Trailer Jumper Cable, in Sectino 6.4 defines the function and color code each of the seven pins, each of which is defined as a circuit. With the understanding that a circuit refers to one conductor between towing and towed vehicle, one can comply with Section 5.1.6.3 without having to provide a separate ground return fro the circuit to provide full-time power to towed vehicle ABS.

Utilizing a single ground on vehicles is considered to be good engineering practice since multiple grounds can cause EMI problems and ground loops which have in the past resulted on severe damage to vehicle electrical systems and even caused numerous fires. Without the need to provide a separate ground circuit, vehicle/antilock suppliers have the flexibility needed to provide the desired level of trailer antilock power in the most efficient method.

Because of the need to finalize design decisions, especially for truck-tractor manufacturers who must meet the requirements of the final rule on March 1, 1997, we would appreciate an early answer to this request.

Please contact us if there is a need for further information.

Sincerely