Antilock Brakes / Traction Control Systems: Description and Operation
BASE BRAKING MODE
Under normal operating conditions, the brake system will operate using conventional braking by means of brake pedal force, the vacuum booster, and the master cylinder. Each front channel consists of a motor, a solenoid, an expansion spring brake (ESB), a ball screw, a piston, and a check valve. As illustrated, under normal operating conditions (base braking), the piston is held in the upmost or "home" position. This is accomplished by the screw via the motor turning the ball screw and driving the nut upwards.
Once the piston is at the upmost position, it is held by an ESB. An ESB is a spring that is retained in a housing at a close tolerance. One end of the spring is in contact with the motor drive dog, and the other end is in contact with the pinion drive dog. In normal braking, brake pressure is present on the top of the piston, applying a downward force. This force applies a counterclockwise torque to the motor pinion, which tries to rotate the spring counterclockwise. This torque expands the spring outward with-in the housing and prevents the gear from rotating.
While the piston is at the upmost position, it unseats the check valve and opens a path for the brake fluid.
Two paths are available to transfer brake fluid to the calipers: (1) through the modulator, around the open check valve, and out to the caliper; and (2) through the modulator, through the normally open solenoid and out to the caliper. The solenoid in the front circuits provides an alternate brake fluid pressure path to the caliper. With this arrangement, if the ABS were to lose power or malfunction with the piston out of the "home" position, an equivalent brake fluid path would always be available.
The rear channels operate in a similar manner except: (1) both rear channels are controlled together and (2) no solenoid exists. Both rear channels are controlled by one motor to simplify the design. Since ABS operates on a select low principle (if either rear wheel begins to lock, brake pressure to both rear wheels is reduced to maximize vehicle stability), both rear brakes are controlled together.
There is no rear solenoid due to the nature of the braking system. The vast majority of the braking is accomplished with the front brakes. An ABS failure that affected the operation of the rear base brakes would cause a DTC to be stored and the electronic brake control module (EBCM) would illuminate both the amber ABS indicator and the red BRAKE indicator.
ANTILOCK BRAKING MODE
ABS VI has been designed to improve the controllability and steerability of a vehicle during braking. ABS VI accomplishes this by controlling the hydraulic brake pressure applied to each front caliper and the rear wheel cylinders. Antilock braking occurs only when the stoplamp switch is closed and a microprocessor, located in the electronic brake control module (EBCM), determines that at least one wheel is about to lose traction during braking. The EBCM will then allow the hydraulic modulator to change the brake pressures several times each second to keep the wheel(s) from locking and provide the driver with maximum vehicle controllability. ABS VI cannot increase the brake pressure above the master cylinder pressure applied by the driver and cannot apply the brakes by itself.
The result is greatly improved braking, enabling the driver to maintain steerability more easily and to bring the vehicle to a controlled stop. ABS VI provides effective braking and directional control over a wide range of read surfaces and driving conditions.
If any wheel begins to approach lockup, the EBCM will control the three motors and two solenoids, appropriately, to control brake pressure to the affected wheel(s). During front wheel ABS operation, the solenoids are turned on to isolate that brake pressure path to the caliper.
The EBCM then provides controlled current to the motors to regulate the speed and amount of movement. When the motor is activated and tries to drive the ball screw nut, the end of the expansion spring brake (ESB) in contact with the motor drive dog rotates inward, causing the spring to contract in its housing and allowing the motor to rotate the modulator gear.
The most common application of this principle is in window crank mechanisms, where the weight of the window or the force of the window will not allow the window to move downward, but a small amount of force on the regulator handle allows the window to be lowered or raised. For the ESB, brake pressure on the top of the pistons corresponds to the weight of the window and the motor corresponds to the window regulator handle. As the motor moves backwards, the piston follows the nut downward, allowing the check valve to seat. The brake pressure to the caliper is now a function of the controlled volume within the piston chamber. To reduce brake pressure, the motor drives the nut further downward. To reapply or increase brake pressure, the motor drives the nut and piston upward. If ABS were entered during low brake pressure, such as in ice, and dry pavement is then encountered during reapply, the piston is driven all the way to the top. This results in the unseating of the check valve and a return to base braking until sufficient brake pressure exists to cause the wheel to approach lockup again. At this point, the ABS cycle would start again. Total brake pressure during ABS is limited to the brake pressure present when ABS was entered. Also, anytime wheel brake pressure exceeds the brake pressure at the master cylinder (caused by reduced force on the brake pedal), the check valve unseats and a small volume of brake fluid is returned to the master cylinder to equalize pressure. In this manner, ABS VI cannot increase the brake pressure above the master cylinder pressure applied by the driver and cannot apply the brakes by itself. When ABS is no longer required, the pistons are returned to their upmost position, and the solenoids on the front channels are opened to provide an optional base braking path again.
The rear channel operates in a similar manner except that no solenoid is used.
Both rear brakes' pressures are controlled by the same motor, and both rear brakes' pressures are maintained at nearly equal levels.
Initialization
The ABS indicator will be illuminated for about 3 seconds when the ignition switch is first turned ON. System initialization will occur when the vehicle speed reaches approximately 8 km/h (5 mph). A slight mechanical noise may be heard during system initialization. This is a normal occurrence, the result of the hydraulic modulator pistons returning to their upmost or "home" position. A driver who is applying the brake during system initialization may feel a slight pedal bump. When this occurs, system initialization is interrupted.
Indicator Operation
The standard brake system uses a single red BRAKE indicator located in the instrument cluster. The antilock brake system uses two lamps, the red BRAKE indicator, and an amber ABS indicator.
1. When the ignition switch is turned ON, before starting the engine, the amber ABS indicator should illuminate for approximately 3 seconds. The red BRAKE indicator will quickly flash on also.
2. As the engine is cranked, the red BRAKE indicator and the amber ABS indicator should illuminate steadily.
3. In 3 to 4 seconds after the engine is started, the ABS indicator should be off. The red BRAKE indicator will go off immediately.
TIRES AND ABS
Replacement Tires
Tire size is important for proper performance of the ABS. Replacement tires should be the same size, load range, and construction as the original tires. Replace tires in axle sets and only with tires of the same tire performance criteria (TPC) specification number. Use of any other size or type may seriously affect the ABS operation.
ABS SYSTEM COMPONENTS
The ABS VI Antilock Braking System (ABS) consists of a conventional hydraulic brake system plus antilock components. The conventional brake system includes a vacuum booster, master cylinder, front disc brakes, rear leading/trailing drum brakes, interconnecting hydraulic brake pipes and hoses, a brake fluid level sensor and the BRAKE indicator.
The ABS components include a hydraulic modulator/ motor pack assembly, an electronic brake control module (EBCM), a system enable relay, two system fuses, four wheel speed sensors (one at each wheel), interconnecting wiring, a lamp driver module, and the ABS indicator. See "Antilock Brake System; Locations; Components" for the general layout of this system.
The hydraulic modulator/motor pack assembly is located at the firewall on the right side of the vehicle.
The basic hydraulic modulator configuration consists of drive gear subassemblies, ball screws, nuts, pistons, and hydraulic check valves. The hydraulic modulator/ motor pack assembly controls hydraulic pressure to the front calipers and the rear wheel cylinders by modulating hydraulic pressure to prevent wheel lockup.