Antilock Brakes / Traction Control Systems: Description and Operation

DESCRIPTION
When conventional brakes are applied in an emergency stop or on ice, one or more wheels may lock. This may result in loss of steering control and vehicle stability. The purpose of the Bendix ABX-4 brake system is to prevent lockup under heavy braking conditions. The system offers the driver increased safety and control during braking. ABS operates only at speeds above three miles per hour.

OPERATION
Under normal braking conditions, the Anti-Lock Brake System (ABS) functions as a standard brake system with a diagonally split master cylinder and conventional brake booster.

If a wheel lock is detected during braking, the system will enter ABS mode. During ABS mode, hydraulic pressure in the four wheel circuits is modulated to prevent any wheel from locking.

Each wheel circuit has a set of electrical valves to provide modulation to each wheel. Front wheels receive separate signals and rear wheels receive the same signal.

CIRCUIT OPERATION
Power for the Anti-Lock brake system is supplied by two fuses. There is a 15 Amp fuse located in the fuse block cavity 10. This fuse supplies power to the Controller Anti Lock Brake (CAB), and the Anti-lock Brakes (ABS) system relay on the F12 circuit. The F12 circuit also supplies power to the Powertrain Control Module (PCM). Power for the fuse is supplied on the A21 circuit from the ignition switch. This fuse is HOT in the START/RUN position.

The second fuse is located in the Power Distribution Center (PDC). It is a 40 Amp fuse. This fuse is HOT at all times and supplies power to the contact side of the ABS pump motor relay.

There are three grounding points used in the ABS system. One is a case ground on the CAB. The second is the Z12 circuit at the CAB connector cavity 5. This ground is spliced, and terminates at the left side of the radiator closure panel.

The third ground is on the Z1 circuit. This ground is used for the ABS hydraulic modulator, ABS system relay and the ABS pump motor. This ground, like the Z12 ground splices and terminates at the left head lamp ground.