Four Wheel Antilock Brakes (4WAL)

Fig. 1 Four Wheel Anti-lock Brake System:

GENERAL DESCRIPTION
The four wheel anti lock system (4WAL) is designed to reduce the occurrence of wheel lockup during severe brake applications. The system does this by regulating hydraulic brake pressure. The pressure is regulated by an Electro-Hydraulic Control Unit (EHCU) located on the left fender (figure 1). The EHCU has two types of valves. Isolation valves maintain pressure to each front wheel separately and the rear wheels combined. Dump valves relieve the pressure from each front wheel separately and the rear wheels combined. These valves are controlled by a microprocessor in the EHCU. During a severe brake application, the EHCU permits the valves to do one of three functions or a combination of all three. The EHCU allows the valves to either maintain the same hydraulic pressure, relieve hydraulic pressure, or increase hydraulic pressure.

The EHCU receives input signals from the vehicle speed sensor (VSS) buffer for the rear wheels, speed sensors located at each front wheel, and the brake lamp switch. The wheel speed sensors connect to the EHCU through the eight pin connector.

The 4WAL system includes an ANTILOCK warning lamp in the instrument panel. This lamp serves as an indication of 4WAL system operation and malfunctions. A bulb check occurs each time the ignition switch is turned to the "ON" position. The ANTILOCK lamp should light and remain on for about two seconds. The ANTILOCK lamp also indicates 4WAL system malfunctions. To determine the specific cause of the malfunction, refer to "Diagnostic Circuit Check."