Brakes (ABS): Description and operation

ABS

The ABS is a full time, four channel system that gives individual speed control of all four wheels to provide the vehicle with anti-lock braking (ABS), Electronic Traction Control (ETC) and Hill Descent Control (HDC) functions. In addition to the components of the non ABS brake system, vehicles with ABS have an ABS modulator, four ABS sensors and an ABS Electronic Control Unit (ECU).

ABS modulator

The ABS modulator controls the supply of hydraulic pressure to the brakes in response to inputs from the ABS ECU. The modulator is attached by three mounting bushes to a bracket on the RH inner wing, and connected to the primary and secondary hydraulic circuits downstream of the master cylinder assembly. Three electrical connectors link the ABS modulator to the vehicle wiring.

Internal passages in the ABS modulator, separated into primary and secondary circuits, connect to the various components that control the supply of hydraulic pressure to the brakes. Shuttle valves and check valves control the flow through the internal circuits. Shuttle valve switches, connected in series to the ABS ECU, provide a brakes on/off signal. A damper chamber and restrictor are included in each circuit to refine system operation. Inlet and outlet solenoid valves control the flow to the individual brakes. An expansion chamber is connected to each circuit to absorb pressure. A return pump is connected to both circuits to provide a pressure source.

The ABS modulator has three operating modes:

• Normal braking mode: When the brake pedal is pressed, pressurized fluid from the master cylinder assembly moves the shuttle valves to open lines A’ and close the shuttle valve switches. Pressurized fluid then flows through the open inlet solenoid valves to operate the brakes. The shuttle valve switches supply a brakes on signal to the ABS ECU.
• ABS braking mode: When in the normal braking mode, if the ABS ECU detects that a wheel is about to lock, it energizes the inlet and outlet solenoid valves of the related brake and starts the return pump. The inlet solenoid valve closes to isolate the brake from pressurized fluid; the outlet solenoid valve opens to release pressure from the brake into the expansion chamber and the return pump circuit. The brake releases and the wheel begins to accelerate. The ABS ECU then operates the inlet and outlet valves to control the supply of hydraulic pressure to the brake and apply the maximum braking effort (for the available traction) without locking the wheel.
• Active braking mode: When ETC or HDC are enabled, and the ABS ECU determines that active braking is required, it starts the return pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, shuttle valves and lines B’, is pressurized by the return pump and supplied to lines A’. The ABS ECU then operates the inlet and outlet solenoid valves to control the supply of hydraulic pressure to the individual brakes and slow the wheel(s).

ABS sensors

The ABS sensors supply the ABS ECU with a speed signal from each wheel. An inductive sensor is installed in a bush in each wheel hub, sensing off a 60 tooth exciter ring. A fly-lead connects each sensor to the vehicle wiring. The exciter rings are fitted to the outer diameter of the constant velocity joint on each drive shaft and shielded by the hub centre boss.