Description
Probe will use a redesigned anti-lock brake system (ABS) beginning with models built in March 1991. New components on this ABS system include:
^ ABS control module
^ Hydraulic actuation assembly
^ ABS relay
^ Master cylinder
^ Brake booster
^ Proportioning valve
Components that are carryover from the previ . ous
system are:
^ Speed sensors
^ Sensor rotors
^ Front disc brakes
^ Rear disc brakes
1991
06-09-2 Anti-Lock Brake System 06-09-2
DESCRIPTION (Continued)
The hydraulic actuation assembly is a lightweight, four-solenoid unit. The new design eliminates the reservoir, accumulator, and pressure switch used on the earlier model. The new assembly houses: four flow control valves, four solenoid valves, a motor, two pumps, two buffer chambers, two damper chambers, two inlet ports and four outlet ports, and is serviced as a unit.
1991
06-09-3 Anti-Lock Brake System 06-09-3
DESCRIPTION (Continued)
Operation
The ABS system functions by releasing and applying fluid pressure to the calipers during special braking conditions. The ABS system does not function under normal braking conditions, nor does it affect front to rear brake proportioning. When one or more wheels approach a slip condition, the ABS system automatically senses the slip and activates the pressure control function. The ABS system is controlled by a module located under the driver seat. Through its preprogramming, the ABS control module decides which wheel or wheel's brake pressures need modulation. Once the decision is made, the control module sends appropriate signals to the solenoid valves located in the hydraulic actuation assembly. These solenoid valves allow adjoining flow control valves to modulate fluid pressure, resulting in a pressure reduction at the calipers to prevent further lock up.
1991
06-09-4 Anti-Lock Brake System 06-09-4
DESCRIPTION (Continued)
Anti-Lock Brake System Operation
1991
06-09-5 Anti-Lock Brake System 06-09-5
DESCRIPTION (Continued)
Electrical System
The ABS electrical system consists of a newly designed control module, hydraulic actuation assembly, and relay. These new components work together, along with the wheel speed sensors, sensor rotors, and other brake system components to control braking. This three-channel system has independent front wheel control and select low resistance rear wheel control. The hydraulic actuation assembly consists of four solenoids which control hydraulic fluid flow to each brake caliper.
^ Two solenoids for front wheel control receive independent braking signals, depending on wheel speed.
^ Rear wheel control has two solenoids. The control module selects the wheel with the lowest resistance, and sends both rear solenoids the same braking signal.
The control module continuously calculates wheel speed, and thus vehicle speed. The module activates the anti-lock brake system only when it senses wheel lockup under severe braking conditions. If the module senses wheel lockup, it sends a pressure reduction signal through the relay to the hydraulic actuation assembly. The solenoid controls the flow of hydraulic fluid into a buffer chamber located in the hydraulic actuation assembly. When the wheel is no longer in danger of lockup, the module sends pressure increase signals through the relay to the hydraulic actuation assembly. One signal tells the solenoid to allow hydraulic fluid pressure to increase while the other signal activates the pump motor to further increase pressure by returning fluid from the buffer chamber. This operation continues for all wheels until the wheels are no longer in danger of lockup. A fail-safe function is programmed into the control module that reverts the system to normal braking if a malfunction is detected. A self-diagnosis function checks for problems within the anti-lock brake system. The control module stores malfunction codes in its memory to aid the technician in diagnosing problems. The ABS warning light is illuminated while the engine is on to notify the driver of a present failure. The control module system performs the following functions:
^ Calculates wheel speeds
^ Estimates vehicle speed
^ Senses wheel lockup
^ Activates anti-lock brake system
^ Utilizes self-diagnosis and fail-safe functions
^ Stores malfunction codes in memory