abs know how
TRANSCRIPT
Antilock Brakes, Traction,
and Stability Control
Chapter 59
Objectives
• Describe the reason for an antilock brake
system (ABS)
• Explain the theory of operation of ABS
• Describe the parts of two-, three-, and four-
wheel ABS
• Explain the differences between integral and
nonintegral ABS
Objectives (continued)
• 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
Introduction
• Effectiveness of braking 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 10-20% slip
Antilock Brakes
• Wheel speed sensors measure rotational speed
of the wheel
• If the wheel locks, antilock brake controller
pulsates the pressure to that wheel– Pulsates 10 – 20 times per second
• ABS disabled below a certain speed
• When ABS senses failure, system reverts to
conventional-only braking
• Bump followed by rapid pulsing felt in pedal
Antilock Brake System Components
• ABS computer known as:– Electronic brake control module (EBCM)– Controller antilock brake (CAB)– Electronic brake and traction control module
(EBTCM)
• Wheel speed sensors usually permanent magnet generators– Positioned near tone ring (exciter ring)– Frequency changes with wheel rotation speed
• Hydraulic control valve assembly causes pressure to pulsate
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 usually much larger– Some systems have pressure sensitive switch
• Nonintegral ABS– ABS unit separate from master cylinder, in series
with brake lines– Hydraulic pump circulates fluid– Two wheel or four wheel– One-, three-, or four-channel
Two-Wheel ABS
• Only works on rear wheels– Found on SUVs, light trucks– Designed to stop fully loaded truck
• Rear brakes modulated simultaneously– System needs only one sensor
• Four wheel ABS either three or four channel– Front wheels controlled separately
• Three channel – two sensors in front, one in back
• Four channel – sensor on each wheel
Antilock Brake System Operation
• During two-wheel ABS stop, isolation valve closes
• When action by isolation solenoid not sufficient, dump valve cycles open and closed rapidly
• Dump valve closes when pressure to rear brakes relieved, and wheels turning
• In some three and four channel systems, combination valve used
• Nonintegral systems may use motor pack instead of electric pressure pump
Antilock Brake System Operation
(continued)
• If malfunction occurs in ABS system, computer shuts system off
• When testing ABS you should feel the pedal pulsing
• ABS may limit wheel spin during acceleration– Computer matches traction with engine power– controller disables traction control if brakes
overheat
• Computer stabilizes vehicle in sudden evasive maneuver– Compensates for understeering and oversteering
Antilock Brake (ABS) Service
• Fewer than 1% of problems in brake system
come from the ABS
• Two warning lights:– Amber for ABS problems only
– Red for hydraulic system
• False modulation – system operates when it
shouldn’t– Brake drum linings incorrect coefficient of friction
– Incorrect brake drum return springs
– Two primary linings on one side, secondary on other side
ABS Brake Fluid Service
• Follow correct procedure for inspecting brake fluid level
• Some integral ABS systems must be bled of pressure before service
• Fluid replaced every 2 years, every time pads are replaced
• Majority of ABS problems result from wheel sensor failure– Harsh operating conditions– Demagnetized or polarized by physical impact
ABS Brake Fluid Service (continued)
• Testing a sensor– Spin a wheel while reading AC voltage output
– Follow diagnostic flow chart
• Wheel sensors are magnetic and attract metal
shavings– Result is erratic sensor reading
• Damage to the harness is most common wheel
sensor service problem– Changing resistance or configuration changes the
signal to computer
ABS Brake Fluid Service (continued)
• Gap affects voltage
produced by sensor– Check using brass
feeler gauge
– Looseness in wheel bearing affects air gap
• Look for damaged tone
ring teeth– Use a press when
replacing tone ring
ABS Brake Fluid Service (continued)
• 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
ABS Brake Fluid Service (continued)
• Integral ABS uses same inputs as nonintegral
• RWAL records one soft code at a time– If code not a hard fault, controller shuts off MIL
and codes remain in memory
• Delco VI ABS failure may result in low pedal– Follow diagnostic chart to repair problem
• Speed sensor buffers fail– Check inputs and outputs – Check resistance and AC output voltage of VSS– Failed ABS/TCS relay with bad electrical contacts– Incorrect brake lamp can cause ABS to shut off