See Figure 1
The four wheel anti-lock brake system was first used on these trucks on the 1991-92 MFI-Turbo Syclone. The 4 wheel anti-lock system is designed to reduce brake lock-up during severe brake application. The basic function of each system is similar to the RWAL system described earlier in this manual, the major difference simply being that the 4 wheel system monitors and controls wheel spin/lockup on the front wheels as well as the rear.
Instead of the APV (control valve) used on RWAL systems, the 4WAL system utilizes a Electro-Hydraulic Control Unit (EHCU) module. The EHCU module is located on the left fender, near the master cylinder. The module contains two types of valves, and controls the hydraulic pressure within the brake lines. The system also uses 4 wheel speed sensors (one located at each wheel).
In a severe brake application, the EHCU valve will, depending on the circumstance: allow pressure to increase within the system, maintain (isolate) the pressure within the system, or release existing pressure through the dump valves into the accumulators.
The EHCU valve operates by receiving signals from the speed sensors, located at each wheel, and from the brake lamp switch. The speed sensors connect directly to the EHCU valve through a multi-pin connector.
The system is connected to the ANTILOCK warning lamp on the dashboard. The warning lamp will illuminate for about 2 seconds every time the vehicle is started as a lamp check. The warning lamp will illuminate if the computer detects a problem within the anti-lock system during vehicle operation. If the warning light comes on when the vehicle is started and does not go out, or if the light comes on and remains on during vehicle operation, trouble has been detected by the computer module.
See Figure 2
The EHCU module is mounted near the master cylinder and combination valve assemblies on the left fender. The module contains two types of valves. Isolation valves maintain pressure to each front wheel separately and the rear wheels combined. These valves are controlled by a microprocessor in the EHCU module. The module is not serviceable and must be replaced if malfunctioning. The module is also the sight for the only additional attention required to properly bleed the 4WAL hydraulic system.
See Figure 3
On most 2 and 4wd vehicles covered by this manual, the front wheel speed sensors are permanently mounted to the brake rotor splash shield which must be replaced as an assembly should the sensor fail. On 4wd vehicles, the hub and bearing assembly must be removed.
The front wheel speed sensors operate with the help of sensor tone wheels. The tone wheels are metal rings equipped with teeth on their outer diameter. The AC voltage is produced as the teeth come into and leave alignment with the sensor. The tone wheels are attached to the front hub and bearing assembly on 4wd vehicles.
A properly operating speed sensor should have a resistance value of 900-2000 &omega.Rear Wheel Speed Sensors
The rear wheel speed sensors on the 1991-92 MFI-Turbo vehicles are held in position by 2 bolts at each rear wheel. The brake drum and primary brake shoe must be removed for access.