1. Chapter 73
Anti-Lock Brakes,
Traction Control, and
Stability Control

2. Contents Anti-lock brake systems (ABS)
Traction and stability control systems
ABS service
Traction and stability control system service
Final system check

3. Anti-Lock Brake Systems (ABS)
An ABS uses wheel speed sensors, a computer (ECM), and a modulator unit to prevent skidding during hard braking

4. Anti-Lock Brake System

5. ABS Operation
For good drivers to get maximum braking and control, extreme tire slip must be avoided
On slick pavement the tire slips, its friction with the road surface drops, and stopping distance increases
ABS modulates braking effort at the controlled wheels to reduce the stopping distance and increase directional stability when braking

6. With and Without ABS

7. Without ABS
Without ABS cars tend to slide to the right because of higher tire adhesion on the road

8. With ABS cars tend to travel further during hard braking

9. Basic ABS Components

10. ABS Components

11. ABS Control Module
Uses wheel speed sensor inputs to control the electro-hydraulic modulator

12. Wheel Speed Sensor Sensor Rotor
Wheel sensors produce an ac signal that corresponds to wheel and tire speed
Sensor Rotor
moving teeth on the sensor rotor induces current in the wheel sensor coil

13. Wheel Speed Sensor/ Sensor Rotor
Wheel sensors produce an ac signal that corresponds to wheel and tire speed
The signal increases in frequency as the tire rotates faster
As the tire rotates, moving teeth on the sensor rotor induce current in the sensor coil

14. Sensor Signal Frequency

15. Sensor Signal Frequency
A rapid decrease in a sensor’s signal frequency indicates that a tire is starting to lose traction and lock up
The control module then sends an electrical signal to the hydraulic modulator to pulsate hydraulic pressure to the affected wheel cylinder, modulating braking effort

16. Electro-Hydraulic Modulator

17. Electro-Hydraulic Modulator
Regulates the fluid pressure applied to each wheel brake assembly during ABS operation
Controlled by the ABS control module
Pressure modulation refers to the rapid cycling of pressure to the brake assemblies, preventing wheel lockup
occurs 15 to 20 times per second

18. ABS Circuit Warning Light

19. ABS Warning Light Mounted in the dash
Alerts the driver to an ABS malfunction
Turns on during engine cranking and ABS self-check at startup to verify operation

20. ABS Accumulator
The accumulator in the ABS system stores high pressure fluid and Caution should be used when servicing!

21. During Hard Braking
When the ABS system is in operation you will feel a pulsing in the brake pedal; this is caused by the solenoid valves cycling pressure on and off. Some ABS systems can cycle up to 15 times per second.

22. ABS Warning Light Mounted in the dash
Alerts the driver to an ABS malfunction
Turns on during engine cranking and ABS self-check at startup to verify operation

23. A:ABS control module
B: booster pump & motor
C: wheel speed sensor
D: sensor rotor
E: rotor

24. Modulator Components Fluid reservoir Solenoid valve block Accumulator
holds an extra supply of brake fluid
Solenoid valve block
coil-operated valves that control brake fluid flow to the wheel brake cylinders
Accumulator
chamber for storing fluid under high pressure

25. Modulator Components Hydraulic pump and motor Pressure switch
motor-driven pump that provides brake fluid pressure for the system
Pressure switch
monitors system pressure and controls the operation of the motor-driven pump
Master cylinder-booster assembly
conventional master cylinder with power assist for normal braking

26. ABS Hydraulics
Here, the brakes are applied, and pressure is being dumped from one wheel to prevent lockup

27. ABS Modes of Operation Three modes of ABS operation are used:
isolation mode
dump mode
reapply mode

28. Integrated ABS
Combines the power booster, master cylinder, and modulator units into one assembly
More common on early systems
More expensive and difficult to repair

29. Nonintegrated ABS Major units are separated
Uses a conventional master cylinder and booster
The electro-hydraulic modulator is a separate unit
More common on recent ABS units

30. ABS Channels
Separate hydraulic circuits that feed one or more wheel cylinders or calipers
One-channel ABS
operates only the rear wheel brakes together
Two-channel ABS
two separate hydraulic circuits are controlled by the modulator
rear-wheel ABS with separate control for each wheel brake

31. ABS Channel Three-channel ABS Four-channel ABS
three separate hydraulic circuits are controlled by the modulator
one channel for each front wheel brake
one channel for both rear wheel brakes
Four-channel ABS
four separate hydraulic circuits are controlled by the modulator
provides a separate hydraulic circuit for each wheel brake

32. ABS Operation (Normal Braking)
Master cylinder pressure flows normally to the brake assembly
No pump or solenoid operation occurs
The ABS system is not functioning during normal driving
The ABS system only becomes active during tire slippage

33. Above: Using a hand-held scope and measuring the wheel sensor signal
Testing a Wheel Sensor
Above: Using a hand-held scope and measuring the wheel sensor signal

34. Traction and Stability
Control Systems

35. Traction Control Systems
Designed to prevent the vehicle’s wheels from spinning and losing traction under hard acceleration
Most systems work with the anti-lock brake system to cycle hydraulic pressure to the wheel spinning the fastest

36. Traction Control Systems
The control module is capable of applying only one wheel brake at a time
Some systems also reduce the engine’s power output to reduce slipping
The indicator light is illuminated anytime the traction control system is activated
warns the driver that the tires are losing traction

37. Stability Control Systems
Advanced systems that reduce tire spin upon acceleration and prevent tire skid when cornering too quickly
Use more input signals from various sensors to provide greater control under severe cornering, braking, and acceleration conditions

38. Understeer
When a vehicle has a tendency to turn less sharply than is intended
If understeer is detected, the control module will apply braking force to the rear wheel on the opposite side of the vehicle
this brings the front of the vehicle back under control for making the turn

39. Understeer

40. Oversteer
When a vehicle has a tendency to turn more sharply than is intended
If oversteer is detected, the control module will apply braking force to the outside front wheel
this prevents the rear of the vehicle from sliding sideways in a turn

41. Oversteer

42. Stability Control System Inputs
Yaw sensor
measures the direction of the thrust generated by vehicle movement
Throttle position sensor
measures the demand for power
Brake pressure sensor
measures the amount of brake system hydraulic pressure generated by the drive
Wheel speed sensors
detect individual wheel speeds
Steering angle sensor
measures how sharply the steering wheel is rotated
Lateral acceleration sensor
measures how much side force is generated by a turn

43. Stability Control System Inputs

44. ABS Service
To service anti-lock brake systems properly, you should be familiar with how a normally operating system feels during operation
You should also be familiar with the exact type of ABS being repaired
Designs and procedures vary
Follow the service manual procedures

45. ABS Inspection Inspect for the following conditions:
ABS indicator light on
low or contaminated brake fluid
brake fluid leaks
brake pad lining and rotor wear
tires that will not rotate freely
loose or worn wheel bearings
different tire size or poor tire condition
wheel speed sensor damage

46. Scanning ABS
Perform a diagnostic circuit check by starting the car and watching the ABS warning light
the lamp should illuminate and then go out after the system self-check is complete
If the ABS light stays on, use a scan tool to check for diagnostic trouble codes

47. Typical scan tool connections
Scanning ABS
Typical scan tool connections

48. Scanning ABS
Connect a scan tool to the data link connector or separate ABS diagnostic connector
Check for a code history and review all trouble codes
When available, perform manual or automatic tests, which will verify system operation
pump, lamp, and solenoid operation

49. Scanning ABS
This tester can be placed on the seat to check operating values during a road test

50. ABS Pinpoint Tests
Use pinpoint tests to determine the exact source of a problem
Use a multimeter or oscilloscope to check circuit components
Compare the voltage and resistance values to the service manual specifications

51. Pinpoint Test Chart

52. Speed Sensor Service Make sure the sensor is mounted properly
Check that all wire connectors are tight
Check for chipped or broken trigger wheel teeth

53. If applicable, use a non-metal feeler gauge to check the air gap
Speed Sensor Service
If applicable, use a non-metal feeler gauge to check the air gap

54. Speed Sensor Service
Unplug the sensor connector from the wiring harness
Connect a hand-held scope across the sensor leads
Rotate the tire by hand while watching the sensor waveform

55. Oscilloscope Connection

56. Sensor Waveform

57. Replace any clips that retain the wiring
Sensor Replacement
Replace any clips that retain the wiring

58. ABS Modulator Check
Some manufacturers support output tests that can verify solenoid and pump operation
These tests may allow the technician to make scan tool selections or jump specified diagnostic connector terminals to activate ABS outputs

59. ABS Modulator Check Raise the vehicle on a lift or jack stands
Release the parking brake and rotate each wheel by hand

60. ABS Modulator Check
Connect a jumper wire or make scan tool selections to apply or release the wheel brake assemblies as needed

61. ABS Modulator Check
With the brake applied, rotate each wheel, verifying the ability to apply or vent pressure at each brake assembly

62. Modulator Replacement

63. Modulator Replacement
Transfer the necessary parts from the old unit to the new one

64. ABS Bleeding Nonintegral system Integral system
when servicing a master cylinder, bench bleed the unit before installation
install the unit and re-bleed it at the lines
Integral system
the master cylinder requires very specific bleeding procedures
Bleeding may require a scan tool to energize the solenoids and pump motor

65. ABS Bleeding
Special procedures may be required; follow the manufacturer’s instructions

66. ABS Test-Drive Use extreme caution
If scan data must be monitored, have a second technician ride along to monitor the data
Where necessary, wet a small strip of pavement about a foot wide and 20 feet long

67. ABS Test-Drive
Position the car so the right or left tires ride on wet pavement
Use this area to trigger an anti-lock event while braking
This will allow you to verify ABS operation and to make sure the ABS indicator light stays off

68. Traction and Stability Control System Service
The procedures are similar to ABS service
Perform a careful system inspection
Using a scan tool, check for diagnostic trouble codes and check scan data
Use pinpoint tests to isolate any problem
After service, verify the repair

69. Test-Driving the Vehicle
Similar to an ABS test-drive
Use extreme caution
A wet pavement or a gravel surface lowers friction, allowing you to produce wheel slip on acceleration
Have a second technician ride along to monitor the scan data

70. Final System Check
Before releasing the vehicle, perform some final checks:
clear the diagnostic trouble codes
check the brake fluid level
carefully test-drive the vehicle
simulate a panic stop
make sure the ABS indicator light stays off