Troubleshooting guide
73
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Drum brake — A brake system in which two brake shoes with
friction material expand into a rotating drum to stop the vehicle.
Dual air brake system — Two independent air systems
designed to retain braking ability in situations where one
system fails. The two systems are typically called the primary
and secondary.
ECU / ABS — Electronic Control Unit. A microprocessor that
evaluates how fast a wheel is rotating by using the electrical
signals generated by the wheel speed sensors from toothed
rings that spin with the wheel.
Edge codes — Developed by the Friction Materials Standards
Institute (FMSI), the edge code is a double letter code (e.g.,
EE, FF, GG, FG) to designate its range of aggressiveness. The
code is printed on the edge of brake blocks.
Emergency brake system — Not a separate system, but is
defi ned as the braking that remains after any single system
service system failure. Also referred to a secondary braking in
an ECE-13 based system. Emergency braking (in the event of
air loss) can involve various portions of the parking and service
brake systems. See spring brake.
Engine brake — An optional device that uses a diesel engine
as a power-absorbing device to slow a vehicle on downgrades.
A type of retarder.
ESP/ESC — Bendix
®
Electronic Stability Program
®
(also
generically known as Electronic Stability Control). This
technology, commonly referred to as “full stability,” is capable
of sensing/controlling both directional (yaw) and roll (lateral
acceleration) events to help to maintain vehicle stability.
Exhaust brake — Another type of retarder. An optional device
that uses engine exhaust back-pressure to slow a vehicle on
downgrades.
FMVSS 121 — Federal Motor Vehicle Safety Standard number
121 is the minimum standard of performance for air-braked
highway vehicles in the US.
Foot valve — A foot-operated valve used to control the air
pressure delivered to the brake chambers.
Foundation brake balance — The torque balance and
thermal balance of the brakes themselves, and between
different axles. Aspects include brake linings, tire sizes, air
chamber size, slack length, etc.
Foundation brake system — The mechanical components
involved in providing braking force such as: slack adjusters;
brake chambers; rotors; brake drums; and brake linings.
GAWR — Gross Axle Weight Rating. It is the total weight
capacity of the axle (single, tandem, or tridem).
Gladhand — A mechanical pneumatic connector used to
attach a tractor's or converter dolly's service (i.e., control) and
emergency (i.e., supply) air lines to those on a trailer.
Caliper — The clamping device in an air disc brake system
that contains friction material mounted to pads. The caliper
applies braking force to both sides of the rotor, when actuated.
Carrier (air disc brake) — structural element which supports
the caliper and mounts the brake assembly to the torque plate.
Channel / ABS — The number of channels in an ABS system
refers to the number of valves its Electronic Control Unit (ECU)
is capable of independently controlling. Wheel ends may be
controlled individually or as pairs.
Check valve — A one-way check valve allows air fl ow in one
direction only. A two-way check valve activates by allowing air
to fl ow from the highest of two pressure sources.
Clevis pin — The pin that connects the arm of a slack adjuster
to a brake chamber push rod yoke.
CMVSS 121 — Canadian Motor Vehicle Safety Standard
number 121 for air brake vehicles.
Confi guration — The primary objective is to identify a “normal”
set of sensors and modulators for the Electronic Control Unit,
so that it will identify future missing sensors and modulators.
Connectors / ABS — Plugs that link the ABS wiring system
to the Electronic Control Unit (ECU), wheel speed sensors and
modulator or relay valves using sealed, corrosion-resistant
connectors and a shielded wiring harness.
Crack pressure — This is the control pressure required to
achieve the initial delivery of an air valve. [Typically measured
in pounds per square inch (psi)].
Diagnostics/ABS — A self-check of the ABS system performed
each time the truck's ignition is turned on. The system is also
monitored continuously during the vehicle operation.
Diagnostic connector — Diagnostic receptacle in the vehicle
cab to connect to the J1587 hand-held, or PC-based, test
equipment. The tester can initiate test sequences, and can also
read system parameters.
Diagnostic Trouble Code (DTC) — An ECU alert caused by a
condition that interferes with the generation — or transmission
of — response or control signals in the vehicle's ABS system
that could lead to the functionality of the ABS system becoming
inoperable in whole or in part. A series of codes displayed by
the self diagnostic portion of the ABS unit, isolating the section
of the system that is now or has previously malfunctioned.
Differential Braking — Application of brake force to a spinning
wheel so that torque can be applied to wheels which are not
slipping.
Disc brake — See air disc brake.
Drain valve — An air reservoir-installed valve used to remove
contaminants. The valve may be manual or automatic in
operation; automatic versions may be heated electrically to
prevent the valve freezing in the open position.
Glossary, continued