Section III. TROUBLESHOOTING AND TESTING THE ANTI-LOCK
BRAKE SYSTEM WITH THE WABCO TEST UNIT
This section contains information on the use of the test unit for the Anti-Lock Brake System
(ABS). The ABS is an electronic system that monitors and controls wheel speed at all times, and
controls braking only during emergency situations. The ABS controls the braking of each wheel
separately, which prevents wheel locking, maintains steerablility. and reduces stopping distance.
The ABS has two diagonal circuits. Each circuit connects the front wheel of one side of the
tractor to the rear wheels of the opposite side. In case of a system fault, only half of the ABS
stops working. Control of that half is returned to the standard brake system. The ABS uses a
tooth wheel and sensor on the wheel end of axles. The sensor sends wheel speed information to
the Electronic Control Unit (ECU). The ECU signals the modulator valve for that wheel to
increase, reduce, or keep the same air pressure in the brake chamber.
Prior to performing the tractor test, ensure that the daily preventive maintenance inspections
and procedures have been performed on the tractor.
Disconnect the cab harness from the ECU under the passenger seat, and connect the 35-pin
connector of the cab harness to the test unit.
Location of ABS Components in Tractor