Technical information
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Technician Guidelines for Antilock Braking Systems
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Brake Proportioning The limiting of brake air pressure to a specific axle or tandem
to compensate for varying vehicle loading. Brake proportioning
is most beneficial during bobtail tractor operation.
Braked Wheel Behavior The study of wheel reactions during braking, particularly be-
tween the road surface and the tire.
Category (I, II, & III) A means of categorizing ABS performance used in Europe.
Chamber Pressure The air pressure in the brake chambers during a brake
application.
Channel The electrical connection between the ECU and the modulator.
The term is also used to describe the number of individual
modulators in a particular antilock system.
Chuff Test Also called ignition blowdown test. A test—designed to simplify
diagnostics—used to exercise the ABS modulator(s) upon initial
power-up. The “chuff” sound is made by air escaping from
rapid exercising of the exhaust solenoid (and supply solenoid)
on each modulator.
Coefficient of Friction A measure of the friction (such as between a tire and the road
surface) available to use as surface retardation. The ratio is
defined as “Force Required to Overcome Friction/Weight” and
is denoted by the Greek letter µ. See also “Mu.”
Control Algorithm The specific configuration of logical decisions implemented to
determine the characteristics of an ABS cycle. Apply, release,
hold, etc., determinations are made in the control algorithm,
which is implemented in the ABS software contained in the
electronic control unit (ECU).
Control Pressure The air pressure applied from the foot/hand valve which con-
trols the brake application pressure either directly or through a
relay valve. The ABS interrupts this pressure by adding a modu-
lator in series such that the air pressure at the individual brake
chambers may vary from the control pressure. During ABS
operation, therefore, chamber pressure may be equal to or less
than the control pressure.
Controller Another name for the electronic control unit (ECU). See
Electronic Control Unit.