Four Wheel Antilock Brake System (4WAL)

FOUR WHEEL ANTILOCK BRAKE SYSTEM (4WAL)

Fig. 1 Four Wheel Anti-lock Brake System:

This braking system Fig. 1 is designed to reduce the occurrence of wheel lock-up during severe braking conditions. To accomplish this function, the system regulates hydraulic brake line pressure by using an Electronic Hydraulic Control Unit (EHCU) module. This control module has two types of valves. Isolation valves which maintain pressure to each front wheel separately and rear wheels combined. Dump valves which dump pressure to each front wheel separately and rear wheels combined. These valves are controlled by a microcomputer which is part of the EHCU module. During a severe braking condition, the EHCU is designed to permit the valves to do one of three functions or a combination of all three. The EHCU will allow the valves to either maintain the same hydraulic pressure, release hydraulic pressure the dump valves into the accumulators, or increase pressure.
The EHCU module controls this system based on the signals received from the wheel speed sensors and the brake pedal switch. The speed sensors are connected directly to the EHCU through the 8-pin connector. If the EHCU module detects wheel slip, the module will adjust the hydraulic pressure to compensate for the wheel slip condition.
The 4WAL system is connected directly to the ``Antilock'' warning lamp on the instrument panel. An indication of the proper 4WAL operation and a bulb check is performed each time the ignition is turned ``On.'' The warning lamp will remain lit for two seconds during this bulb check. A 4WAL system malfunction will be indicated by the ``Antilock'' warning lamp remaining on.