See Figures 1 and 2
The ABS system employs a speed sensor at each wheel sending signals to the ECU. The control unit can trigger any or all of 4 solenoids within the hydraulic actuator. This system allows optimal control of the braking effort at each wheel. This model use a Dual Proportioning Valve (DPV) to further control distribution to the rear wheels. On this model the primary circuit is the right front and left rear wheels.
The ABS system uses a 3 solenoid system within the actuator. Brake fluid line pressure is controlled separately to each of the wheels, while the signal line to both rear wheels is controlled by the remaining solenoid. This model uses only 3 sensors, one at each front wheel and one mounted in the side of the differential housing.