Service manual

Group 59 Anti-Lock Brake System (Bendix) Design and Function
ABS Electronic Control Unit (ECU)
The ECU is the brain of the ABS system. It receives
wheel speed information from the sensors and sends
signals to the ABS modulator valves.
The ECU is powered through a 30–amp fuse on the igni-
tion circuit. The ECU receives power only when the
ignition is ON.
The ECU compares the wheel speed of each wheel to
the speed of the other wheels to determine if any one
wheel is locked up (i.e. not spinning or spinning very
slowly in relation to the other wheels). This comparison
is performed many times every second. If any wheel is
determined to be locked up while the other wheels are
spinning, the ABS sends signals to the modulator valve
for the locked wheel to reduce its braking force until it
starts to spin again. This series of events may occur as
many as 15 times each second, giving the modulator
valve the appearance of having a pulsating action.
If the Traction Control System (TCS) feature is installed,
the ECU provides signals to the ATR valve to control
driving wheel spin. The ATR valve is energized to apply
air to the brake chambers on the set of drive wheels that
are spinning. This action will slow the drive wheels to a
speed consistent with vehicle speed to increase the trac-
tion of those wheels.
If the engine is equipped with electronic controls, the
ABS ECU will also send a signal through a J1939 or
J1922 data link to the engine ECU to reduce engine
torque during wheel spin conditions. Above 25 mph, the
ECU will only send the engine signal; no braking will
occur.
W5000951
Bendix ABS ECU, current model
The Bendix ABS ECU has built-in diagnostic capabilities
and is equipped with a series of light emitting diodes
(LEDs) that are visible from the outside. These LEDs are
used in troubleshooting the faults in the anti-lock brake
system. The LEDs are marked so that the technician can
determine the location of a problem detected by the
ECU. The designations are grouped to aid in determin-
ing the problem location and component.
The first three LEDs identify which axle has the faulty
component and are labeled FRT for front, MID for mid-
dle, and RER for rear. The next two LEDs identify the
side of the vehicle with the fault; RHT for right, LFT for
left. The next four LEDs identify the faulty component;
TRC for traction control components, MOD for modula-
tor, SEN for sensor, and ECU. The final LED, which is
the only green LED, is marked VLT. This LED indicates
whether the ECU is receiving 12 volts. The VLT LED is
ON during normal conditions.
W5000953
Left side of ECU showing LEDs
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