Operation

OPERATION

The ABS2E anti-lock braking system is controlled by an electronic control unit (ECU) mounted on the modulator unit in the engine compartment. The ECU derives its input from four speed sensors mounted at the wheels. The input from these sensors allows the ECU to control the fluid pressure delivered to the wheel cylinders during braking via the hydraulic modulator, preventing the wheels from locking. A 12 volt supply is supplied from fuse 4 on a G/K wire to the ECU when the starter switch is turned on.

The operation of the ABS system is completely dependent upon electronic signals. To prevent the system responding to any inaccurate signals, a built in safety circuit monitors all electric and electronic signals, including battery voltage. If an inaccurate signal, low battery or high battery voltage is detected, the ECU will shut down the ABS system immediately and illuminate the warning light in the instrument pack.

The warning light in the instrument pack is fed from fuse 8 on a Y wire. When the starter switch is initially switched on, the ECU supplies an earth path to the warning light whilst the ECU performs a system check.

The system check is carried out in two stages, the first a static check and the second a dynamic check at speeds over approx 4 mph (6 km/h). In the event of a fault being detected the warning light will be illuminated and the system shut down.

When the vehicle is stationary the wheel sensors supply no signal to the ECU. When the vehicle is moving the sensors provide a sine wave signal to the ECU the frequency of which is proportional to the wheel speed.

The hydraulic modulator contains 3 solenoid valves. The front wheel cylinders are controlled by separate solenoid valves and the rear wheel cylinders are controlled by a single solenoid valve and a copy valve. The solenoid valve relay is supplied from fuse 4 on a G/K wire. An earth path for the relay is supplied by the ECU, energising the relay. With the relay energised, a feed from a 40A fusible link supplied on a N/K wire is passed through the relay to the 3 solenoid valves.

When the brake pedal is depressed a signal is supplied to the ECU from the brake pedal switch on a G/W wire. The ECU acting upon the signals from the wheel cylinders uses the solenoid valves and the return pump to control the fluid pressure being exerted on each wheel cylinder.

If the wheels start to accelerate or decelerate to a speed beyond that required for optimum braking, the ECU will switch an earth path for the return pump relay. With the relay energised, a feed from the N/K wire is passed to the return pump. With the pump operating, fluid pressure is quickly restored to the correct level.