ABS System

1. ABS System Hydraulic Unit and Wheel speed Sensor

2. ABS Solenoids • There are three types of solenoids in the hydraulic unit: • Isolation • Decay • Build Fluid return Isolation Pump Build Accumulator Decay Note: The illustration here shoes the solenoids for only one channel. The hydraulic unit has a set of solenoids for each channel

3. Fail safe mode • When there is no electrical current supplied to the ABS system the isolation solenoid is open – allowing brake fluid to pass through hydraulic unit to the caliper unchanged. • Without electrical power the decay and build solenoids are closed • The brakes work normally but ABS is not available. 500 psi Isolation Build Decay 500 psi

4. ABS isolation mode • The ABS control module closes the isolation solenoid as soon as an impending wheel lockup is detected • This blocks additional pressure from the master cylinder entering the hydraulic circuit for the locked wheel. • Decay and build solenoids remain closed • If the deceleration rate of the wheel remains within limits no further action is taken. 500 psi Isolation Build Decay 500 psi

5. ABS Decay mode • If the deceleration rate of the wheel exceeds the limits the ABS control module will hold the isolation solenoid closed. • The decay solenoid is pulsed open for a few milliseconds to decrease pressure to the caliper. • The build solenoid is kept closed. • The ABS control unit will continue to pulse the decay solenoid until the rate the wheel is turning begins to match the speed of the other wheels. Fluid from decay solenoid returns back to reservoir 500 psi Isolation Build Decay 200 psi

6. ABS Build mode • If the wheel is now turning faster than the other wheels the ABS control module will keep the isolation solenoid closed. • The control module will pulse the build solenoid for a few milliseconds to increase pressure to the caliper. • Decay solenoid is closed. • The ABS control unit will continue to pulse the build solenoid until the rate the wheel is turning catches up to the speed of the other wheels. • In most systems the pressure reapplied to the locking wheel will not exceed the original pressure at the time the control module detected the lockup. 500 psi Isolation Build Decay 500 psi

7. Internal accumulators • Modern ABS systems locate the accumulator inside the hydraulic unit. • External accumulators were used on first generation integrated ABS/booster systems. • The volume of the internal accumulator doesn’t need to be very large since it is not being used to operated a power booster piston. Photo courtesy of Robert Bosch Gmbh Accumulator cover

8. Combined decay and build solenoid • Since the decay and build solenoids are never open at the same time a single, two position solenoid can be used to control decay and build modes. • When one side of the solenoid is energized the decay port is open and the build port is closed. • When the other side is energized the build port is open and the decay side is closed. • This simplifies the design of the hydraulic allowing 8 solenoids to control 4 hydraulic channels.

9. Accumulator pressure sensor Pressure sensor • A solid state pressure sensor is connected to accumulator pressure. • The ABS control unit monitors this pressure. • When the pressure drops below a preset limit the ABS control module turns on the pump. • Once the pressure reaches the upper limit the module turns off the pump. ABS Control Module

10. Other pressure sensors • The two inlet pressure sensors are used to provide data to the ABS as to how hard the driver is pressing the brake pedal. • If there is a significant difference between the front and rear master cylinder chamber pressures a fault code for a master cylinder problem will be set. • There may also be outlet pressure sensor – on sensor for each channel. • The outlet pressure sensor tells the ABS control module the actual pressure in each brake line. • Data from the outlet sensors helps the system modulate pressure to the calipers and wheel cylinders and provides data for self diagnostics.

11. Active and passive accumulators • Most modern cars use an active accumulator that has an electrically driven pump to supply hydraulic pressure for build pressure and for ATC and ASC operation. • First generation ABS systems used passive accumulators which were small cylinders with a spring loaded piston in the hydraulic unit that held the decay fluid under pressure. • When build pressure was needed the accumulator solenoid opened up and allow the pressure in the brake line to return to its original pressure.

12. Active Accumulators • Modern ABS systems use active accumulators that store high pressure brake fluid to restore line pressure as wheel rotation accelerates. • Active accumulators allow the brake pressure to be applied to a caliper when the brake pedal is not depressed – for use by the Automatic Traction Control system or Automatic Stability Control. • The active accumulator uses an electric pump, pressure sensor and relay. • The pump is mounted directly on the hydraulic unit. • The pressure sensor and accumulator are located inside the hydraulic unit.

13. 4 channel hydraulic circuit Pump 2 input pressure sensors Accumulator 4 Isolation solenoids 4 Build solenoids Accumulator pressure sensor 4 Decay solenoids 4 output pressure sensors

14. Normal braking 100 psi Isolation solenoids open Decay and Build solenoids closed 100 psi 100 psi

15. ABS - LF Isolation mode 100 psi LF Isolation solenoid is closed All other isolation solenoids remain open Pressure at LF caliper remains at 100 psi Decay and Build solenoids closed 100 psi 100 psi

16. ABS - LF Decay mode 100 psi LF Isolation solenoid is closed LF Decay solenoid momentarily opens All other isolation solenoids remain open Pressure at LF caliper Drops to 50 psi All other Decay and Build solenoids remain closed 100 psi 100 psi

17. ABS - LF Build mode 100 psi LF Build solenoid momentarily opens LF Isolation solenoid is closed All other isolation solenoids remain open LF Decay solenoid closes Pressure at LF caliper rises back to 100 psi All other Decay and Build solenoids remain closed 100 psi 100 psi

18. ABS wheel speed sensor • The wheel speed sensor consists of two parts. • Sensor coil and magnet • Tone ring • The sensor generates an AC electrical current when the tone ring is rotating. • The faster the tone ring spins the higher the frequency of the signal. Coil Magnet Tone Ring

19. Tone ring • Tone ring also known as: trigger wheel, or exciter wheel. • A toothed iron ring mounted on the constant velocity joint, brake rotor, differential carrier or wheel bearing • Rotates at the same rate as the wheel • If tone wheel is mounted on the differential carrier it will rotate at the average speed of the 2 rear wheels • After an axle or brake rotor replacement the tone ring may have to the removed from the old axle/rotor and pressed onto the new axle/rotor. • When replacing axles or rotors it is important to check the both axles have the same number of teeth • When ordering half shaft assemblies, it’s important to specify that the vehicle has ABS.

20. Wheel speed sensor • Mounted on the steering knuckle, caliper anchor bracket or differential housing • Produces an AC waveform that varies in both frequency and amplitude as the wheel speed changes • Consists of a permanent magnet with a coil of copper wire wrapped several hundred times around the magnet • A two wire electrical connector and cable transmits speed signal to ABS control module Photo courtesy of Robert Bosch Gmbh

21. Wheel speed sensor • Many wheel speed sensor have very long terminal leads that pass through a grommet in the vehicle body so that all terminal connections reside inside the passenger compartment. • The sensor circuit handles very small voltage levels – any unwanted resistance will have a negative effect on the wheel speed sensor signal – wheel speed sensor harnesses are usually replaced rather than repaired to prevent problems of this nature from occurring after a faulty repair.

22. Wheel speed sensor • Also, whatever type of repair made to the harness will cause the harness to be stiff and brittle at that point – this harness must follow suspension and steering movement and will become unreliable in the future. • Some sensor coils are integrated within the wheel bearing/hub assembly – the entire hub assembly must be replaced if the sensor is faulty

23. ABS Electrical circuit Fuse Box CAN Hi CAN Lo OBDII Diagnostic Connector Electronic Brake Control Module Body Control Module Instrument Control Module Powertrain Control Module Yaw and Lateral Acceleration Sensor Instrument Cluster Wheel Speed Sensors Foot Brake Switch Parking Brake Switch Low Fluid Sensor Brake Lights LF RF LR RR

24. Dynamic Rear Proportioning • Dynamic Rear Proportioning does electronically what a proportioning valve does mechanically. • The ABS control module looks at the data from the front and rear pressure sensors at the output of the hydraulic unit. • When the output pressure to the rear brakes reaches a preset level the ABS control module closes the isolation solenoids to the rear brakes – preventing any further rise in pressure. • The pressure to the front brakes may continue to rise but the pressure at the rear is limited. • This process occurs when all ABS wheel speed sensors are at the same RPM.

25. Electronic pressure differential monitoring • The ABS control module does electronically what the pressure differential valve does mechanically. • The control module monitors the difference in inlet pressure between the two lines coming from the master cylinder. • If there is a significant difference in pressure between the two the control module will disable the ABS system, set a failure code and illuminate the ABS warning light.

26. Automatic stability control • The ASC system used two sensors. • Yaw and lateral acceleration • Steering angle • Yaw is the rotation of the vehicle left to right. • The ASC software compares the steering angle to the yaw rate • When the is a significant difference between the two the ABS sends hydraulic pressure to one or both brakes on inside of the turn. Actual Vehicle Path Steering Input Yaw

27. Yaw/acceleration and steering angle sensor • The steering angle sensor connects to the steering shaft and is normally mounted below the steering wheel. • The yaw/lateral acceleration sensor is located near the center of the vehicle usually on the floor below the center console. Photo courtesy of Robert Bosch Gmbh

28. Fun Fact • The sensor chip that detects changes in position in your smart phone and i-pad was originally invented in the 1980s by Bosch for use in automotive automatic stability control systems Photo courtesy of Robert Bosch Gmbh

29. Passive tire pressure monitoring • Prior to model year 2002 some manufactures used the ABS system to monitor changes in tire pressure. • All cars built after 2002 have active tire pressure monitoring that uses a sensor inside each wheel and communicates to the vehicle using radio waves. • Many cars built in the 1990s and early 2000s used the ABS speed sensors to detect a changes in tire pressure. • When pressure in a tire drops the tire gets a little bit smaller and therefore rolls at a slightly higher speed than it would at normal pressure.

30. Passive tire pressure monitoring • The system monitors the differences in wheel speed between all four tires. • As the tires wear the system updates the table of speed differences between all of the tires. • A sudden change in the speed the speed differential for one tire triggers the ‘Tire Pressure’ warning light. • When replacing or rotating tires the system needs to be manually reset. • The reset button is usually in the fuse box – resetting instructions are printed on the fuse box cover. • Failure to reset the system will usually result in the light coming on just as the customer drives out of your parking lot.

31. ABS diagnosis • Because of the number of different systems and brake manufacturers, there will be many variations in the details involved in diagnosing ABS complaints. • A systematic approach needs to be used to help solve ABS problems. • This involves recognizing the symptom(s). • Typically the problem is that the ABS warning indicator is on all the time. • Other symptoms could be excessive jitter or shaking of the brakes when the ABS is activated.

32. Systematic diagnosis • The use of a diagnostic table or flow chart will direct a technician to the component which is at fault • Using a diagnostic flow chart is the quickest way to find the source of the problem and ensure that the problem has been corrected. • Finally, the repair must be verified.

33. Diagnostic Process • Operate the system to verify owners concern. • Check the operation of the system warning lights. • ‘Brake’ light should turn on with the parking brake set and turn off when it is released • ‘ABS’ light should turn on during the ignition on self check and turn off a few seconds afterward • If the vehicle has a ‘Brake systems’ light it should turn on during the ‘power on’ bulb check • The brake system warning light is normally triggered by wear sensors in the brake pads but may be triggered by low vacuum in the booster or low accumulator pressure in an electro-hydraulic booster system.

34. Visual inspection • Brake fluid level • Fluid leaks • Master cylinder • Hydraulic unit • Brake lines • Calipers and wheel cylinders • Electrical wiring • Sensor harness and connectors • Harness connector to hydraulic unit and ABS Control Module • Fuses and relays • Check all fuses • Pull the relay out of its socket in the fuse box and inspect the terminals • You can also listen to hear if the relay clicks during the power up self test

35. Testing fuses • Fuses are tested with a test light without removing them from their socket. • There is a contact on the edge of the fuse. • If the test light illuminates when the probe touches both contacts the fuse is good. • If the test light illuminates when touched to on contact but not the other, the fuse is blown. • If the test light does not illuminate when you touch either contact the circuit the fuse protects is not turned on or the test light is faulty or not properly grounded. Test Contacts

36. Visual Inspection • Check the condition of the tone rings • Do they have any chips or missing teeth • Are they aligned with the wheel speed sensor tip • Does the gap between the tone ring and sensor tip appear normal • Check for recent repairs • New rotors, CV joints and wheel bearings • If the new tone rings that came with a different new of teeth the ABS system will not work.

37. Metal particles on sensor magnet • The wheel speed sensor is a permanent magnet and will attract any loose iron particles that come near it. • If the pads wear down to the steel backing plates the metal particles will attach themselves to the magnet. Also driving through constriction sites where welding is done may steel welding particles. • When iron particles surround the sensor tip the magnetic field is disrupted and the signal produced will be erratic and may cause a loss of wheel speed data. • Cleaning the metal debris is all that is required to fix this problem.

38. Scan tool diagnosis • Scan tool diagnosis is the primary method of diagnosis a ABS system fault where the ABS is illuminated. • In general a professional level scan tool [Snap-On, OTC, Bosch] will be needed to read chassis system DTCs. • Inexpensive amateur grade scan tools use software that follows OBDII guidelines and is relies on the federally mandated hardware interface that was designed to monitor emissions related components.

39. Scan tool interface • The scan tool is plugged into the OBDII diagnostic connector and uses the vehicles CAN [Controller Area Network] to communicate with the ABS control module. • The 16 pin OBDII connector is located inside the car, usually below the dash and within a foot or so of the steering column. • If you are using the Snap-On scan tool you will need to insert a card into the connector cable plug that matches the type of vehicle.

40. Scan tool diagnosis • After connecting the scan too you will need to select the manufacture and identify the model year and chassis type [normally the 4th digit of the VIN]. • ABS code are in the Chassis system. • Instead of ‘P’ codes the chassis codes start with the letter ‘C’. • Just like power train codes if the second digit of the code is a ‘0” the code is generic – if it is a ‘1’ it is specific to the manufacturer.

41. Scan tool diagnosis • Most professional level scan tools will have diagnostic charts to help you through the diagnostic process. • Pin out illustrations are normally available through the scan tool as well. • As with any diagnostic chart you cannot skip over any steps if you expect to reach a correct diagnosis.

42. Testing wheel speed sensors • A DTC for a wheel speed sensor doesn’t necessarily mean the sensor is bad – it means that the ABS control module is not seeing a signal from the sensor or that the signal is erratic. • Before replacing the sensor you need to confirm that the sensor is at fault and not the wiring. • A DVOM [Digital Volt and Ohm meter]is used to test the operation of the sensor.

43. Testing the sensor using a DVOM • Set the function Knob to measure Volts [V]. • Toggle the function button to read AC. • Connect the black lead to the COM socket. • Connect the red Lead to the Volts/Ohms/RPM socket. Function Button Function Knob VOLT/OHM/RPM Socket Com Socket

44. Testing the sensor voltage output • You will need two small jumper wires with very small terminal pins to connect to the sensor electrical terminals. • Spin the wheel by hand and observe the voltage. • Since you can’t keep the wheel spinning at a constant speed the voltage level will go up and down but it wont be zero.

45. Testing the sensor frequency • Testing frequency is a better way to check that the ABS control module is receiving a good signal. • Spin the wheel by hand and observe the frequency. • The frequency will fluctuate when the wheel is turned by hand but it should not be zero • Turn the function knob to the 320 HZ position.