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:
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
- 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
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
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
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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