GM Blazer/Jimmy/Typhoon/Bravada 1983-1993 Repair Guide

General Description

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See Figures 1, 2, 3 and 4

The four wheel anti-lock brake system was first used on these vehicles on the 1991 4-door models. The system became standard equipment on 1992 and later models including the Typhoon. The 1991-92 systems are unique in that they used 4 wheel speed sensors (one located at each wheel) while the other systems use the Vehicle Speed Sensor (VSS) signal for rear wheel speed.



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Fig. Fig. 1: The 4WAL Brake Pressure Modulator Valve (BPMV)



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Fig. Fig. 2: Common front wheel speed sensor mounting



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Fig. Fig. 3: 4WAL system schematic - 1991-92 vehicles

The 4 wheel anti-lock system are 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 repair guide, 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 systems utilize a Brake Pressure Modulator Valve (BPMV), formerly known as the Electro-Hydraulic Control Unit (EHCU). The EHCU/BPMV is located near the master cylinder and controls the hydraulic pressure within the brake lines.



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Fig. Fig. 4: 1993 4WAL EBCM/BPMV system schematic

SYSTEM OPERATION



In a severe brake application, the EHCU/BPMV 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/BPMV 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/BPMV 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.

SYSTEM COMPONENTS



EHCU/BPMV Valve

The BPMV valve (formerly referred to as the EHCU) is mounted near or under the master cylinder and combination valve assemblies. The valve is not serviceable and must be replaced if malfunctioning. The valve is also the sight for the only additional attention required to properly bleed the 4WAL hydraulic system.

Front Wheel Speed Sensors

On most 4WAL vehicles covered by this information, 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 2wd vehicles, the rotor must be removed for access. 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 rotor on 2wd vehicles and the front hub and bearing assembly on 4wd vehicles.

A properly operating speed sensor should have a resistance value of 900-2000 ohms.

 
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