GM Celebrity/Century/Ciera/6000 1982-1996 Repair Guide

General Description

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The Teves Anti-Lock Brake System used on the General Motors A-Body cars is manufactured by Alfred Teves Technologies of West Germany. The 4-wheel system uses a combination of wheel speed sensors and a microprocessor to determine impending wheel lock-up and adjust the brake pressure to maintain the best braking. This system helps the driver maintain the control of the vehicle under heavy braking conditions.


CAUTION
Some procedures in this section require that hydraulic lines, hoses and fitting be disconnected for inspection or testing purposes. Before disconnecting any hydraulic lines, hoses or fittings, be sure that the accumulator is fully depressurized. Failure to depressurize the hydraulic accumulator may result in personal injury.

The use of rubber hoses or parts other than those specified for the ABS system may lead to functional problems and/or impaired braking or ABS function. Install all components included in repair kits for this system. Lubricate rubber pats with clean fresh brake fluid to ease assembly.

SYSTEM OPERATION



Under normal driving conditions the Anti-lock system functions the same as a standard brake system. The primary difference is that the power assist for normal braking is provided by the booster portion of the hydraulic unit through the use of pressurized brake fluid.

If a wheel locking tendency is noted during a brake application, the ABS system will modulate hydraulic pressure in the individual wheel circuits to prevent any wheel from locking. A separate hydraulic line and 2 specific solenoid valves are provided for each front wheel; both rear wheels share a set of solenoid valves and a single pipe from the master cylinder to the proportioner valve or tee. The proportioner valve splits the right and left rear brake circuits to the wheels.

The Pontiac 6000 AWD incorporates a differential lock mounted on the transaxle transfer case. The lock is engaged when the vehicle is in all wheel drive and the transfer case differential is locked. Due to the rigid coupling of the front and rear axles through the drive train, the wheel speed sensors cannot relay accurate data to the controller. The differential lock disables the ABS system when the vehicle is in all wheel drive.

The ABS system can increase, decrease or hold pressure in each hydraulic circuit depending on signals from the wheel speed sensors and the electronic brake control module.

During an ABS stop, a slight bump or a kick-back will be felt in the brake pedal. This bump will be followed by a series of short pulsations which occur in rapid succession. The brake pedal pulsations will continue until there is no longer a need for the anti-lock function or until the vehicle is stopped. A slight ticking or popping noise may be heard during brake applications with anti-lock. This noise is normal and indicates that the anti-lock system is being used.

During anti-lock stops on dry pavement, the tires may make intermittent chirping noises as they approach lock-up. These noises are considered normal as long as the wheel does not truly lock or skid. When the anti-lock system is being used, the brake pedal may rise even as the brakes are being applied. This is normal. Maintaining a constant force on the pedal will provide the shortest stopping distance.

Anti-Lock Warning Light

Vehicles equipped with the ABS have an amber warning light in the instrument panel marked ANTILOCK. Additionally, some models using this system will flash other ABS related messages on the Graphic Control Center or other message panels. The warning light will illuminate if a malfunction in the anti-lock brake system is detected by the electronic controller. In case of an electronic malfunction, the controller will turn on the ANTILOCK warning light and disable some or all of the anti-lock system. If only the ANTILOCK light is on, normal braking with full assist is operational but there may be reduced or no anti-lock function. If the ANTILOCK warning light and the red BRAKE warning light come on at the same time, there may be a fault in the hydraulic brake system.

The ANTILOCK light will turn on during the starting of the engine and will usually stay on for approximately 3 seconds after the ignition switch is returned to the RUN position.

Due to system de-pressurization over time, a vehicle not started in several hours may have the BRAKE and ANTILOCK warning lights stay on up to 30 seconds when started. This is normal and occurs because the ABS pump must restore the correct pressure within the hydraulic accumulator. Both lamps will remain on while this recharging is completed.

Brake System Warning Light

The Anti-lock Brake System uses a 2 circuit design so that some braking capacity is still available if hydraulic pressure is lost in 1 circuit. A BRAKE warning light is located on the instrument cluster and is designed to alert the driver of conditions that could result in reduced braking ability. Certain models may display brake related messages on screens or other panels, these messages supplement the brake warning light.

The BRAKE warning light should turn on briefly during engine starting and should remain on whenever the parking brake is not fully released. Additionally, the BRAKE warning lamp will illuminate if a sensor detects low brake fluid, if the pressure switch detects low accumulator pressure or if certain on-board computers run a self-check of the dashboard and instruments.

If the BRAKE warning light stays on longer than 30 seconds after starting the engine, or comes on and stays on while driving, there may be a malfunction in the brake hydraulic system.

SYSTEM COMPONENTS



See Figure 1



Click image to see an enlarged view

Fig. Fig. 1: Teves anti-lock brake system main component locations

Electronic Brake Control Module (EBCM)

The Electronic Brake Control Module (EBCM) monitors the speed of each wheel and the electrical status of the hydraulic unit. The EBCM's primary functions are to detect wheel lockup, control the brake system while in anti-lock mode and monitor the system for proper electrical operation. When 1 or more wheels approach lockup during a stop, the EBCM will command appropriate valve positions to modulate brake fluid pressure and provide optimum braking. It will continue to command pressure changes in the system until a locking tendency is no longer noted.

The EBCM is a separate computer used exclusively for control of the anti-lock brake system. The unit also controls the retention and display of the ABS trouble codes when in the diagnostic mode. As the EBCM monitors the system or performs a self-check, it can react to a fault by disabling the ABS system and illuminating the amber ANTILOCK warning light. The EBCM is located on the right side of the dashboard, generally behind the glove box.

Wheel Speed Sensors

A wheel speed sensor at each wheel transmits speed information to the EBCM by generating a small AC voltage relative to the wheel speed. The voltage is generated by magnetic induction caused by passing a toothed sensor ring past a stationary sensor. The signals are transmitted through a pair of wires which are shielded against interference. The EBCM then calculates wheel speed for each wheel based on the frequency of the AC voltage received from the sensor.

Hydraulic Components

The ABS uses an integrated hydraulic unit mounted on the firewall or cowl. This unit functions as a brake master cylinder and brake booster. Additionally, the hydraulic unit provides brake fluid pressure modulation for each of the individual wheel circuits as required during braking. The hydraulic unit consists of several individual components:

MASTER CYLINDER/BOOSTER ASSEMBLY

This portion of the hydraulic unit contains the valves and pistons necessary to develop hydraulic pressure within the brake lines. Pressure in the booster servo circuit is controlled by a spool valve which opens in response to the amount of force applied to the brake pedal. The rate at which the vehicle decelerates depends on the type of road surface and the pressure applied to the brake pedal.

The master cylinder portion uses a 3-circuit configuration during normal braking; individual circuits are provided for each front wheel while a shared circuit is used for the rear wheels. The 3 circuits are isolated so that a leak or malfunction in one will allow continued braking on the others.

The master cylinder/booster is a non-serviceable component and should never be disassembled.

VALVE BLOCK

The valve block is attached to the right side of the hydraulic unit and includes the 6 solenoid valves used to modulate pressures in the 3 circuits during anti-lock braking. Each circuit is equipped with an inlet and outlet valve.

During normal braking, the inlet valves are open and the outlet valves are closed. When anti-lock control begins, the EBCM switches 12 volts to the appropriate valve circuit. This allows the fluid pressure in each circuit to be increased, decreased or held constant as the situation dictates. The position of the valves can be changed as quickly as 15 times per second when ABS is engaged.

The valve block may be serviced separately from the master cylinder/booster assembly but should never be disassembled.

MAIN VALVE

The main valve is a 2-position valve controlled by the Electronic Brake Control Module (EBCM). Except for testing, the valve is open only during ABS stops. When open, the valve allows pressurized brake fluid from the booster servo into the master cylinder front brake circuits to prevent excessive pedal travel.

The main valve is not serviceable as a component; the master cylinder/booster assembly must be replaced.

ACCUMULATOR

The hydraulic accumulator is used to store brake fluid at high pressure so that a supply of pressurized fluid is available for ABS operation and to provide power assist. The accumulator uses a rubber diaphragm to separate high-pressure nitrogen gas from the brake fluid.

Nitrogen in the accumulator is pre-charged to approximately 870 psi (6000 kPa). During normal operation, the pump and motor assembly charges the accumulator with brake fluid to an operation range of 2000-2600 psi (13,800-18,000 kPa).

Because of the high pressures in the system, it is extremely important to observe all safety and pressure reduction precautions before performing repairs or diagnosis.

PUMP/MOTOR ASSEMBLY

The ABS system uses a pump and motor assembly located on the left side of the hydraulic unit to pressurize fluid from the reservoir and store it in the accumulator. When pressure within the system drops, the pressure switch on the hydraulic unit grounds the pump motor relay which energizes the pump motor and pump.

The pump/motor assembly is serviceable only as an assembly; the pump must never be disconnected from the motor.

FLUID LEVEL SENSOR

Found in the fluid reservoir, this sensor is a float which operates 2 reed switches when low fluid level is detected. One switch will cause the red BRAKE warning light to illuminate; the other signals the EBCM and possibly other computers of the low fluid situation. Depending on model and equipment, other messages may be displayed to the driver. The Electronic Brake Control Module (EBCM) will engage the amber ANTILOCK warning light and disable the ABS function.

PRESSURE SWITCH

The pressure switch is mounted on the pump/motor assembly and serves 2 major functions, controlling the pump/motor and providing low pressure warning to the Electronic Brake Control Module (EBCM).

The switch will allow the pump/motor to run when system pressure drops below approximately 2030 psi (14,000 kPa) and will shut the pump/motor off when pressure in the accumulator is approximately 2610 psi (18,000 kPa). Should pressure within the accumulator drop below approximately 1500 psi (10,300 kPa), internal switches will both signal the EBCM and turn on the red BRAKE warning lamp. If the system re-pressurizes and reaches at least 1900 psi (13,100 kPa), the switches will reset.

PROPORTIONER VALVE

Included in the rear brake circuit is a proportioner valve or tee assembly which limits brake pressure build-up at the rear brake calipers. Since the front brakes do the majority of the braking, less pressure is required for the rear brakes under certain conditions. The proportioner valve improves front-to-rear brake balance during normal braking.

 
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