ABS

BRAKES

70-10

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.) Hill Descent Control (HDC) and Electronic Brake force 
Distribution (EBD) functions.

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. A multi-pin connector links 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. Separation valves and check valves control 
the flow through the internal circuits. Damper chambers and restrictors are included in each circuit to refine system 
operation. Inlet and outlet solenoid valves control the flow to the individual brakes. An accumulator is connected to 
each circuit to absorb flow surges. A common return pump is connected to both circuits to provide a pressure source 
and return fluid to the reservoir.

The ABS modulator has three operating modes: 

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Normal braking mode: When the brake pedal is pressed, pressurized fluid from the master cylinder assembly 
flows through the open separation valves and inlet valves to operate the brakes.

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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 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.

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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, is 
pressurized by the return pump. 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).