Table of Contents Fundamentals of ABS Subject Page Purpose of System.3 Anti-Lock Brake System Theory.3 Brake Regulation.4 Bosch ABS Overview.6 Braking Forces.7 Road Surface.9 ABS Operation.10 Pressure Build.10 Pressure Hold.11 Pressure Drop.12 Overview of ABS Systems.14 Bosch ABS System l-P-0 .15 Teves ABS System l-P-0 .16 Initial Print Date: 1/03 Revision Date Model: Fundamentals of Anti-Lock Brakes Production: All with ABS Objectives: After completion of this module you will be able to: Understand Basic ABS operation 2 Fundamentals of ABS Purpose of System Anti-Lock Brake System Theory The ability to slow or stop a vehicle depends upon the braking forces applied to the wheels and the frictional contact that exists between the tires and the road surface. Very low fric¬ tional forces exist when the tire is locked (or skidding). A locked tire also causes a loss or the lateral locating forces that effect directional control of the vehicle. The result of a locked tire (ortires) is the loss of steering control and stability. The majorforces that affect how easy a tire will lock include: The braking force applied from the vehicles braking system. Environmental factors - rain - ice - snow - etc. Type and condition of the road surface. Condition of the tires (tread and design). The anti-lock braking system is designed to allow the maximum amount of braking force to be applied to the wheels without allowing the wheels to lock or skid. The advantages that AB5 provides includes: Driving stability - by maintaining the lateral locating forces between the tires and the road surface Steerability- allowing the driver to continue to steerthe vehicle while stopping (even dur¬ ing panic stops) or accident avoidance maneuvers. Provides optimum braking distances - the rolling wheels transfer higher frictional forces to slow the vehicle. 3 Fundamentals of ABS Brake Regulation In order to prevent the wheels from locking during braking, yet provide the optimum brak¬ ing force for maximum braking efficiency, the AB5 braking system must: Have the ability to monitor the wheel rotation rates Be able to regulate the braking forces applied to the wheels. The ABS system carries out these functions with an electronic control system. The com¬ ponents of the ABS system include: The electronic control module The wheel speed sensors The brake hydraulic unit The brake master cylinder The four wheel speed sensors are used as inputs to the control module. The module uses these signals to determine wheel speed, wheel acceleration and deceleration. ABS con¬ trolled braking starts when the module detects that one or more wheels are about to lock. Once activated, the ABS control module pulses the brakes on the affected wheel rapidly (2- 15 times a second). This allows the vehicle to be slowed down while still maintaining steer¬ ability and directional stability. The ABS pulses the brakes through solenoids mounted in the hydraulic unit. The solenoids regulate the pressure to the affected wheel through three phases of control: Pressure Hold Pressure Drop Pressure Build Fundamentals of ABS 5 Fundamentals of ABS Bosch ABS Overview WHEEL SPEED SENSOR INPUTS . -® ABS INDICATOR naimur i 6 Fundamentals of ABS Braking Forces When the brakes are applied, brake force counters the inertia of the moving vehicle. This force is created by the brake pads acting on the rotors and through the wheel and tire to the roadway. Even in the best of conditions, some wheel slip occurs. Up to a point this wheel slip is acceptable and in most cases it can even be helpful. When braking, the transmitted brake force concentrates at the tire "foot print", where the rubber meets the road. This causes a distortion which, when excessive, promotes wheel slip. When controlled, the distortion can actually enhance the transmission of brake force. Therefore, the ABS logic allows wheel slip up to 20-25%. Beyond that the AB5 system lim¬ its the application of additional brake force. This allows the transmission of maximum brake force while reducing the stopping distance. 0% SLIP KAAAA/i KVVX4VW 25% SLIP 7 Fundamentals of ABS Braking Forces (continued) The transmission of braking forces and the retention of Lateral Locating Forces are inverse. That is to say as braking forces increase the locating forces decrease. As indicated in the chart, the rolling wheel has a wheel slip value of 0% which provides the best Lateral Locating Forces. As the applied brake force increases the locating force decreases. Depending on the pre¬ vailing road surface friction, the optimum transmission of brake force is at the end of the "stable range” with a wheel slip value of 20-25%. Additional brake force at this point is clearly counter-productive as the additional brake force only increases wheel slip and reduces Lateral Locating Forces. Therefore the AB5 system limits wheel slip by regulating the application of brake force while providing the shortest possible stopping distance. STABLE UNSTABLE BEST WORST 0 -► 20 % 25 %- 50 %- 00 % WHEEL ROLLING WHEEL SLIP EXAMPLE: WET BLACK TOP WHEEL LOCKED 8 Fundamentals of ABS Road Surface Clearly the condition of the roadway and weather conditions are significant influences regarding wheel slip and the retention of Lateral Locating Forces. As road surfaces vary and weather conditions impact the tire's ability to maintain good rolling contact, the function of the ABS remains unchanged. Only the stopping distances increase due to the regulated transmission of braking force. Whatever the road surface or weather, the wheel slip will still be limited to 20-25%. Regardless of the ABS system, good judgement and common sense are still required. 1.0 0.8 - 0.6 0.4 0.2 0 0 20 % 25 % 50 % - 100 % WHEEL ROLLING WHEEL SLIP WHEEL LOCKED 9 Fundamentals of ABS ABS Operation Normal Braking - "Pressure Build" The ABS control module constantly monitors and compares the wheel speed sensor sig¬ nals. When all four signals are at the same frequency within a small window of tolerance, the ABS system is not active and normal braking takes place. RETURN LINE PUMP MOTOR ] The inlet solenoid valves of the hydraulic unit are de¬ energized. This maintains an open passage from the master cylinder to the brake calipers. The outlet solenoids are also de-energized. This main¬ tains a closed outlet of the brake circuit back to the master cylinder reservoir. The inactive scenario is equal to a conventional braking system where the driverapplies hydraulic pres¬ sure from the brake pedal and the brake calipers react by compressing the brake pads on the rotor. Fundamentals of ABS Pressure Hold If the control module detects a decrease in the frequency (rate of deceleration) of one or more of the individual signals it perceives this as possible wheel lock. The control module energizes the inlet valve for that specific brake circuit. This closes the inlet port and prevents any additional hydraulic pressure from being exerted on the brake caliper by the driver. RETURN LINE PUMP MOTOR 1 APPLIED HIGH PRESSURE HELD PRESSURE NO PRESSURE Fundamentals of ABS Pressure Drop The control module de-energizes the inlet and outlet valves. This returns the brake circuit back to normal braking and the hydraulic pressure is once again determined by the driver's pedal force. This sequence continues rapidly until the wheel speed signals are once again acceptable and the contact of the road and the tire surfaces are restored. PRESSURE BUIILD UP NO PRESSURE ] Return Pump Activation When the system is in reg¬ ulation, the brake pedal could progressively sink to the floor. This would be the result of bleeding hydraulic pressure during AB5 regu¬ lation. The return pump maintains the pressure between the master cylinder and the inlet valve to prevent the pedal from sinking com¬ pletely to the floor. This entire sequence of events happens repeatedly in a split second. The func¬ tion of the pump is felt in the brake pedal during AB5 regulation. Fundamentals of ABS Pressure Drop With the inlet valve closed the pressure on the caliper is stabilized and isolated. The con¬ trol module energizes the outlet valve which opens the outlet port and drops the pressure in the isolated portion of the circuit. The brake fluid flows back to the master cylinder reservo RETURN LINE PUMP MOTOR ] APPLIED HIGH PRESSURE RELEASED PRESSURE NO PRESSURE Fundamentals of ABS Overview of ABS Systems BMW uses two basic ABS systems, one manufactured by Bosch and the other by Teves. As of 1991 Model Year - the ABS systems are connected to the diagnostic link* for trou¬ bleshooting purposes. Bosch E32 E34 1988- 1994* 1989- 1994* Bosch ABS 5 E32 1994 E32 1995 Teves Mark IV - Open E36 1992-1994 Teves Mark IV G (Closed) E36 Z3 1995-1996 Teves Mark 20 1 E36 318 Z3 1997 (limited production) As of 1997 Model Year- Traction Control became standard equipment on all models, with the noted exception of the Mark 20 I ABS system. This system was equipped on a limited number of early production 1997 model year E36 - 318i and Z3 1.9 vehicles produced at Plant 10 in Spartanburg, SC. Fundamentals of ABS Bosch ABS (Typical) LF RF LR RR ABS or MAIN RELAY KL 15 ALTERNATOR d + -H T} / \ / ' l_* i i \ / T} / \ / ' L_# \ i \ / ■0 'tt / \ i ' > i \ / ■0 5 / ' i ' > i \ / BRAKE LIGHT SWITCH - ABS PUMP RELAY ABS LAMP HYDRAULIC UNIT r \AA/~ \AA^ ABS PUMP RELAY GROUND CONTROL KL 30 VALVE RELAY Fundamentals of ABS Teves ABS TXD KL 15 ABS RELAY CONTROL PUMP RELAY CONTROL HYDRAULIC UNIT SOLENOIDS