Anti Lock Brakes

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Anti Lock Brakes
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Purpose

• To maintain steering control in panic stop
• To decrease stop distance on most surfaces

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Wheel slip

• Negative slip vs. positive slip
• Brake systems operate normally unless negative
  wheel slip is indicated
• A skidding vehicle has 100 slip
• Slip causes reduced traction and increased stop
  distances
• Engineers found 10-20 slip is optimum under
  most conditions

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ABS pressure control

• ABS will pump the brakes to maintain 10-20
  slip
• Capable of changing pressure to wheels more than
  15 times per second

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Three pressure control modes

• Isolate or hold mode
• ABS controller senses wheel speed dropping
  excessively or faster than other wheels
• ABS isolates wheel from master cylinder
  maintaining pressure at the wheel

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• Release or dump mode
• If wheel is still approaching lockup some
  pressure is released or dumped off
• Apply or increase mode
• If wheel speed continues to rise to vehicle speed
  pressure is reapplied

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System types or classifications

• 2 or 4 wheel
• 1,2,3 or 4 channel systems
• Integral or non-integral
• Most manufactures have made both types
• Current manufactures
• Bendix, Bosch, Delco Moraine, Teves,
  Kelsey-Hayes, Lucas-Girling

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Warning lamps

• Typically two lamps
• Amber indicates problem in ABS
• Red indicates problem in hydraulic system
• Most modern systems store/flash trouble codes
• Many systems now have data stream

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Wheel speed sensors

• Typically AC generator
• Frequency and amplitude increases with speed
• Often damaged by debris on road

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Two wheel systems

• Includes RWAL or RABS systems
• Usually a 1 channel system
• Affects only rear brakes, usually trucks
• Usually uses one speed sensor
• Uses one isolation and one dump valve
• May have one or two accumulators
• May have a pump for fluid return