Transcript Antilock Brakes, Traction, and Stability Control

Antilock Brakes, Traction,
and Stability Control
Chapter 59
© 2012 Delmar, Cengage Learning
Objectives
• Describe the reason for an antilock brake
system (ABS)
• Explain the theory of operation of ABS
• Describe the parts of two-, three-, and fourwheel ABS
• Explain the differences between integral and
nonintegral ABS
© 2012 Delmar, Cengage Learning
Objectives (cont'd.)
• Explain how ABS provides traction control and
stability enhancement
• Explain ABS and normal brake warning light
operation
• Describe how to bleed ABS brakes
• Describe service procedures for ABS brakes
© 2012 Delmar, Cengage Learning
Introduction
• Ability of brakes to do their job
– Limited by tire grip to road surface
– Skids could be avoided if driver could release
brake pressure just before wheel locks
– When wheel stops turning, friction generates
heat, causing tire to lose traction
– Slip rate of 50% means the wheel is rolling 50%
slower than freely rolling tire
– Maximum traction occurs at ten to twenty percent
slip
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Antilock Brakes
• Wheel speed sensors and computer
– Monitor wheel speed
– Wheel speed sensors measure rotational speed
of the wheel
• Wheel locks: antilock brake controller pulsates the
pressure to that wheel
– ABS is disabled below a certain speed
– ABS senses failure: system reverts to
conventional-only braking
– Pedal feel: bump followed by rapid pulsing
© 2012 Delmar, Cengage Learning
Antilock Brake System
Components
• Include:
– ABS computer
• Known as: electronic brake control module,
controller antilock brake, or electronic brake and
traction control module
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–
–
–
Sensor inputs
Pressure modulator valves
Self-test
Wheel speed sensors
• Variations and wiring
– Hydraulic control valve assembly
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Types of Antilock Brake
Systems
• Integral ABS
– Combine master cylinder, power brake booster,
ABS hydraulic circuitry in single assembly
• Early systems used pump for pressure
• Reservoir is usually much larger
• Some systems have pressure sensitive switch
• Nonintegral ABS
– ABS unit is separate from master cylinder and is
in series with brake lines
• Two or four wheel
• One, three, or four-channel
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Two-Wheel ABS
• Only works on rear wheels
– Found on SUVs and light trucks
– Designed to stop a fully loaded truck
• Rear brakes: modulated simultaneously
– Centrally located, single sensor
• Four wheel ABS
– Either three or four channel
• Four channel: sensor on each wheel
– Front wheels controlled separately
© 2012 Delmar, Cengage Learning
Antilock Brake System
Operation
• During two-wheel ABS stop: isolation valve
closes
• Action by isolation solenoid not sufficient
– Dump valve cycles open and closed rapidly
• Pressure to rear brakes is relieved and wheels
are turning
– Dump valve closes
• Three- and four-channel systems
– Some use single combination valve
© 2012 Delmar, Cengage Learning
Antilock Brake System
Operation (cont'd.)
• Nonintegral systems
– May use motor pack
• Malfunction occurs in ABS system
– Computer shuts system off
• When testing ABS
– Pedal pulsing should be felt
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Traction Control System
• Traction control system (TCS) or acceleration
slip regulation (ASR)
– ABS limits wheel spin during acceleration
• Computer matches traction with engine power
• Controller disables traction control if brakes
overheat
• Electronic stability control
– Computer stabilizes vehicle in sudden evasive
maneuver
• Compensates for understeering and oversteering
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
Antilock Brake (ABS) Service
• Fewer than one percent of problems in brake
system come from the ABS
– Warning lights
• Amber: ABS problems only
• Red: hydraulic system
• False modulation
– System operates when it shouldn’t
• Drum brake shoes have incorrect coefficient of
friction
• Incorrect brake drum return springs
© 2012 Delmar, Cengage Learning
ABS Brake Fluid Service
• Follow correct procedure for inspecting brake
fluid level
– Some integral ABS systems must depressurized
before checking
• Some operate under extremely high pressure
– Fluid should be replaced every two years
• Majority of ABS problems
– Result from wheel sensor failure
• Harsh operating conditions or abuse
• Demagnetized or polarized by physical impact
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
ABS Brake Fluid Service
(cont'd.)
• Testing a sensor
– Follow diagnostic flow chart
• Wheel sensors are magnetic
– Attract metal shavings
• Result is erratic sensor signal
• Damage to the harness
– Most common wheel sensor service problem
• Changing resistance or configuration changes the
signal to computer
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
ABS Brake Fluid Service
(cont'd.)
• Gap affects voltage
produced by sensor
– Check using brass
feeler gauge
– Looseness in wheel
bearing affects air
gap
• Look for damaged
teeth
– Use a press when
replacing tone ring
© 2012 Delmar, Cengage Learning
ABS Brake Fluid Service
(cont'd.)
• Precautions common to all antilock systems
– Do not fast charge battery with computer
connected
– Do not use charger on fast charge setting to
jump start a vehicle with ABS
– Do not arc weld on frame with computer
connected
– Do not install antenna near ABS controller
– Do not change tire size other than width
– Do not disconnect or reconnect electrical ABS
parts while ignition is on
© 2012 Delmar, Cengage Learning
ABS Brake Fluid Service
(cont'd.)
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Integral ABS uses same inputs as nonintegral
RWAL records one soft code at a time
Delco VI ABS failure may result in low pedal
Speed sensor buffers fail quite often
– Check inputs and outputs
– Check resistance and AC output voltage of VSS
– Check for failed ABS/TCS relay with bad
electrical contacts
– Incorrect brake lamp can cause ABS to shut off
© 2012 Delmar, Cengage Learning