Saturn Coupes/Sedans/Wagons 1991-1998 Repair Guide

General Information


See Figure 1

The Saturn Anti-Lock Brake System (ABS) is lightweight, compact and highly efficient at controlling wheel locking. The basic premise of ABS is that in order to slow a vehicle, the brake system needs tire traction (friction). When wheel lock-up occurs during braking (a tire skids rather than rolls), the friction between the tire and road surface decreases enormously. This is extremely undesirable since friction is necessary for both steering and slowing of the vehicle. The Saturn ABS prevents wheel lock-up, increasing braking and steering ability (under certain, otherwise adverse, conditions such as wet pavement) by monitoring wheel speed and activating to relieve brake pressure to the portion of the system which is approaching lock-up. In other words, the system monitors how fast each wheel is turning and, if it detects there is an odd man out (a tire is moving much slower than the rest) it will regulate the hydraulic brake pressure to that tire' caliper or piston and prevent wheel lock-up. Without ABS activation under these conditions, the system acts as a normal hydraulic brake system and does not regulate hydraulic pressure.

Many other ABS systems use only electric solenoids to control fluid flow. The solenoid is commanded to one of three positions, allowing line pressure to build, release or hold. On the Saturn system, quick-response electric motors are used to alter the position of displacement pistons. The pistons serve as movable valves, altering the line pressure in small increments. Expressed another way, the solenoid systems can be compared to digital circuits, being either on or off while the motor-operated valves in the Saturn system compare to an analog circuit with continuous modulation. This modulation of brake line pressure yields much smoother operation with less pedal vibration during operation.

The system uses input signals from each of 4 wheel sensors. Four output channels (2 front, 2 rear) are controlled by the control module, though only the 2 front channels can be considered independent. Each front channel is provided with a separate piston, solenoid valve and motor, so they can be controlled separately, while the rear wheels share a single motor driving 2 pistons and are, therefore, activated in unison. When locking is detected at one wheel, a solenoid closes the hydraulic path from the master cylinder. The electric motor for that brake circuit cycles the piston up and down to modulate braking force. The system cannot increase the brake pressure above that developed in the master cylinder; the system cannot apply the brakes by itself.

During normal braking, the ABS system is transparent to the operator. Internally, each control piston is in the uppermost or home position allowing brake fluid pressure to pass to the wheels unrestricted. A small internal Expansion Spring Brake (ESB) device is applied to each piston, preventing it from being forced downward by the pressurized fluid passing above it.

When impending wheel lock is noted at one or more wheels, the Electronic Brake Control Module (EBCM) commands the system into ABS mode. Solenoids in each front wheel circuit close. The brakes on the pistons are released and the pistons are driven downward by the electric motors through a system of driven gears and against the spring pressure. The amount of current applied to the motors controls the speed and distance travelled. As the motors move backwards, the piston moves downward, allowing a check valve to seat. The brake pressure to the wheel is now a function of the controlled volume within the piston chamber.

To reduce pressure, the motor continues to drive the piston downward. If an increase in pressure is necessary, the piston is driven upward. Total pressure available is limited to the amount present when ABS was entered.

The rear brakes are controlled in similar fashion. Wheel speed signals are received from each rear wheel. The EBCM uses a Select Low strategy, controlling a single motor to control output to the rear brakes based on the wheel with the greatest tendency to lock.

Many of the service procedures, including troubleshooting and control assembly overhaul require the use the Saturn Portable Diagnostic Tool (PDT) or an equivalent scan tool. If a tool is not available for these procedures, the vehicle should be taken to a qualified repair shop.

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Fig. Fig. 1: The Saturn Anti-Lock Brake System