GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

General Description

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See Figures 1, 2, 3 and 4



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Fig. Fig. 1: 1994-95 4WAL EBCM system schematic



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Fig. Fig. 2: 1994-95 4WAL VCM system schematic



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Fig. Fig. 3: 1996-99 4WAL system schematic

There are some slightly different four wheel anti-lock brake systems used on the pick up models. One system utilizes a separate EBCM, like the RWAL and some 4WAL systems, while the other system utilizes a Vehicle Control Module (VCM). On the VCM equipped model, the computer control module (which is mounted in the engine compartment) controls both the ABS system and the engine emission control/fuel injection systems.

All versions of the four 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 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: The Electro-Hydraulic Control Unit (EHCU) is located near the master cylinder and controls the hydraulic pressure within the brake lines

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 ANTI-LOCK 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

See Figures 5 and 6



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Fig. Fig. 5: View of the ECHU and related components

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.



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Fig. Fig. 6: The 4WAL Brake Pressure Modulator Valve (BPMV)

Front Wheel Speed Sensors

See Figure 7

On most 2 and 4WD 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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Fig. Fig. 7: Common front wheel speed sensor mounting

 
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