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 (ABS) system is supplied by three fuses. There is a 10 Amp fuse located in the Power Distribution Center (PDC) cavity 10. This fuse supplies power to the (ABS) controller, Powertrain Control Module (PCM) and the fuel pump relay on the F12 circuit. Power for the fuse is supplied on the A21 circuit from the ignition switch. This fuse is HOT when the ignition switch is in the START/RUN position.

The second fuse is located in the PDC cavity 19. It is a 40 Amp fuse and used to supply power to the ABS Pump Motor relay on circuit A10. This relay is located in the ABS relay box. The fuse is HOT at all times and powers the contact side of the relay.

The third fuse is also located in the PDC cavity 2. It is a 20 Amp fuse and used to power the contact side of the ABS System relay on circuit A20. The fuse is HOT at all times.

There are 3 grounding points used in the ABS system. One is a case ground on the Controller Anti Lock Brake (CAB). The second is on the Z1 circuit from the ABS Hydraulic Control Unit/ABS Relay Box. This ground terminates at the right frame rail.

The third ground is also on the a Z1 circuit from the CAB and terminates at the right frame rail.