Technical information
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Technician Guidelines for Antilock Braking Systems
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pressures to at least one front axle and one rear axle. The ABSs
on semi-trailers and dollies must control at least one axle of the
vehicle. Additionally, the ABSs on tractors must control one of
the rear axles with two modulator valves so that the brake
pressure on one end of the axle is independent of the brake
pressure on the other end. The performance requirements of
FMVSS 121 can require an ABS on additional axles.
NHTSA defines an ABS as a portion of a service brake
system that automatically controls the degree of rotational
wheel slip during braking by:
• Sensing the rate of angular wheel rotation.
• Transmitting signals regarding the rate of wheel
rotation to one or more devices, which interpret these
signals and generate responsive controlling output
signals.
• Transmitting those signals to one or more devices
which adjust braking forces in response to the signals.
Other aspects of NHTSA’s rule stipulate that:
• ABSs on trailers be capable of being powered by the
trailer’s stop lamp circuit.
• New tractors—built on or after March 1, 1997—
provide constant electrical power to a tractor-to-trailer
electrical connector for powering trailer ABSs.
• Vehicles required to have an ABS also have a yellow
ABS malfunction indicator lamp which lights up to
indicate most malfunctions.
• The power unit’s ABS malfunction lamp be “in front of
and in clear view” of the driver. It lights when the
ignition key is first switched “on” for a bulb check.
• The ABS malfunction lamp on trailers be mounted on
the left side of the trailer, near the rear side marker
lamp. On dollies, the lamp is located on the left side
where it can be seen by someone standing about 10
feet from the lamp. The lamp lights for a short bulb
check when the vehicle is stopped and the ABS starts
receiving electrical power. This lamp will no longer be
required after February 2009.
• Air-braked tractors and trucks which tow other air-
braked vehicles—built on or after March 1, 2001—
have an in-cab warning lamp which indicates