Technician Guidelines for Antilock Braking Systems
Air-Braked Trucks, Tractors and Trailers

This is the ABS Spec'ing Considerations section of the reprint in HTML format of the government document FHWA-MC-98-008. The original document can be found at the following listed website: http://mchs.fhwa.dot.gov/safetyprogs/brakesaft.htm

ABS SPEC’ING CONSIDERATIONS

The Federal Government’s requirement for full-time electrical power to ABSs has prompted both equipment users and manufacturers to reconsider the way trailers are supplied with such power. Since a particular powering configuration is not required in the ABS rule, manufacturers and equipment users can decide for themselves how to achieve the full-time power requirement. There are several different methods of supplying full time power to the trailer ABS:

• If the auxiliary circuit of the seven-pin connector is not in use, it can be used to supply full-time power as long as the circuit is always “on” or “hot” when the key switch is “on.” NOTE: Unless otherwise specified, many manufacturers will supply a “hot” auxiliary circuit as standard equipment. It is very important that vehicles use this option if they are commonly coupled to vehicles in other fleets.
• A second connector can be used specifically to power the trailer ABS. (for example, the ISO 3731 connector)
• A special connector which is compatible with the existing seven-pin connector can be used if it can accommodate additional circuits (for example, a 13-pin connector).

Each of these methods has certain advantages and drawbacks. However, it is the consensus of the members of The Maintenance Council that the existing seven-pin connector design should be preserved if possible for important reasons of compatibility, safety, and maintainability.

Another important consideration is ensuring that adequate power is available for proper ABS function. Voltage drops between the battery and the last unit of a combination vehicle can impact the amount of power available for the ABS, especially in doubles and triples combinations.

For these reasons, TMC developed two recommended practices to promote power supply and connector standardization—TMC RP 137, “Antilock Electrical Supply From Tractors Through the SAE J560 Seven-pin Connector,” and TMC RP 141, “Trailer ABS Power Supply Requirements.”

To ensure adequate power is provided to the trailer from the tractor, TMC RP 137 recommends that at least 12.5 volts be available at the J560 connector with a 10-amp load on both the stop lamp and auxiliary circuit. Industry consensus is that meeting this minimum recommendation will ensure adequate power for trailer ABSs.

Additionally, TMC RP 141 recommends that:

• Pin 7 of the J560 connector be reassigned as a continu-ous power circuit, activated when the ignition is on. NOTE: For the purposes of RP 141, Pin 7 of the SAE J560 seven-pin connector (so designated by SAE as the auxiliary circuit) is referred to as the continuous power circuit.
• There be a minimum of 9.5 volts (which includes a 1- volt safety margin) available at the trailer ABS ECU— when coupled to a tractor complying with RP 137— to ensure adequate power for proper ABS operation.
• Trailer manufacturers provide equipment purchasers with written information regarding the voltage and current characteristics of the stop lamp and auxiliary circuits at the SAE J560 seven-pin receptacle. TMC recommends this information be included in the owner’s manual.

TMC does not recommend specific wiring gauge sizes, lighting technology, ABS power consumption, or grounding methods. It is the consensus of TMC’s membership to leave those decisions to the manufacturer/equipment user. All vehicle/component manufacturers have agreed to incorporate TMC’s recommendations into their vehicle design as either standard or at the request of the equipment user.

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