Mr. Larry W. Strawhorn
Vice President of Engineering
American Trucking Associations
2200 Mill Road
Alexandria, VA 22314-4677

Dear Mr. Strawhorn:

This letter responds to your request for an interpretation of the antilock power circuit requirements set forth at S5.1.6.3 of Standard No. 121, Air Brake Systems. This provision states that

S5.1.6.3 Antilock Power Circuit for Towed Vehicles. 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. (Emphasis added.)

You believe that the phrase "separate electrical circuit" allows for the continued use of the single SAE J560 connector if one of the seven pins provides full-time power for the ABS. You further believe that the ABS malfunction signal can be multiplexed on any circuit of the connector and that the other trailer devices can be powered off the circuit as long as the circuit is adequate to enable the antilock system on each towed vehicle to be fully operable.

In the March 10, 1995 final rule, NHTSA decided to adopt the proposed full-time power requirement for trailer ABSs. (60 FR 13216) The agency explained that it amended the standard's wording to clarify that towing vehicles must have a corresponding separate circuit specifically provided to power the antilock system on the towed vehicle or vehicles. The agency stated that requiring a separate circuit "will ensure the strongest possible source of electrical power from the tractor to ensure the functioning of all the ECUs and modulators that are employed in the antilock brake system, or systems, on single trailers, or multiple trailers and converter dollies in multi-trailer combinations.@ It also stated that this requirement will ensure a continuous malfunction indication whenever a malfunction exists. The agency further stated that it has left the decision about which type of connector should be used to the industry.

In response to your question about the use of one of the pins in the seven-pin connector to provide full-time power for the ABS, the use of such a pin would be permissible provided that the

pin services a "separate" electrical circuit to "specifically provide" full time power for the trailers in combination vehicles. This means that the circuit's sole function must be to provide ABS powering, i.e., other trailer devices may not be powered off this separate electrical circuit. This would preclude the use of the pin to power the ABS malfunction signal. Since the requirement for the ABS malfunction circuit did not specify that the circuit used for transmitting the malfunction signal be a "separate" one, ABS malfunction signals can be multiplexed on other circuits with pins in the electrical connector, but not on the circuit and pins used to power the ABS system.

It is important to note that the ABS semitrailer fleet study report (DOT HS 808 059) concluded that the voltages delivered by powering system approaches that employed dedicated separate circuits (i.e., the Cole Hersee, ISO, and 6-pin auxiliary systems) were well within the required limits for ECU powering; whereas, the voltages delivered through the stoplamp circuit did not perform as well. The agency concluded that these data indicate the superiority of a separate circuit powering of the trailer ABS and therefore, justify the separate circuit requirement.

As you are aware, NHTSA received several petitions for reconsideration about the separate electrical circuit. The agency anticipates that the final rule in response to these petitions for reconsideration will have a detailed discussion of these requirements. In addition, the agency may decide to modify these requirements.

I hope this information has been helpful. If you have any other questions, please contact Marvin Shaw of my staff at this address or by phone at (202) 366-2992.


Samuel J. Dubbin Chief Counsel

ref:121 d:11/17/95