Brakes (ABS modulator): Description and operation (page 2 of 2)

The ABS modulator is a 4 channel unit that controls the supply of hydraulic pressure to the brakes in response to inputs from the SLABS ECU. The modulator is attached by three mounting bushes to a bracket on the LH inner front 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.

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

The ABS modulator has three operating modes: Normal braking, ABS braking and active braking.

Normal braking mode When the brake pedal is pressed, pressurised fluid from the master cylinder assembly moves the shuttle valves to open lines 'A' and close the shuttle valve switches. Pressurised fluid then flows through the open inlet solenoid valves to operate the brakes. The closed shuttle valve switches supply a brakes on signal to the SLABS ECU. If the SLABS ECU determines that EBD is necessary, it energises the inlet solenoid valves for the brakes of one axle. The inlet solenoid valves close to isolate the brakes from any further increase in hydraulic pressure.

ABS braking mode When in the normal braking mode, if the SLABS ECU determines that ABS braking is necessary, it energises 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 pressurised 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 SLABS ECU then operates the inlet and outlet solenoid 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 SLABS ECU determines that active braking is necessary, it starts the return pump. Hydraulic fluid, drawn from the reservoirs through the master cylinder, shuttle valves and lines 'B', is pressurised by the return pump and supplied to lines 'A'. The SLABS 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).