Troubleshooting guide

6
For more information, visit www.bendix.com or www.foundationbrakes.com • 1-800-AIR-BRAKE (1-800-247-2725)
Air Supply
The vehicle’s compressor takes in fi ltered air — either at
atmospheric pressure from the outside or already at an
increased pressure from an engine turbocharger — and
compresses it. The compressed air is delivered to the air
dryer where water and a small amount of oil is removed.
The air then travels into the air reservoirs (“air tanks”). It
is typically delivered to a rear brake system reservoir and
a front brake system reservoir, as well as any attached
trailer reservoirs. For each system, the air pressurizes
the reservoir and the air hoses all the way to the next
control valve, where the air pressure remains, ready to
use.
A vehicle may use compressed air for many tasks. Some
examples are: to provide force for braking, to deliver air
to a particular component, to off-load bulk goods, etc.
Normal Braking
When the driver applies the foot brake, a plunger within
the foot brake valve moves, opening channels within the
valve that allow the air pressure waiting there to pass
through and be delivered to the rear and front brake
systems. The pressure quickly increases in the brake
chambers and applies force to the push rod, transferring
the force to the air disc brake (or via a slack adjuster, to
a foundation drum brake). (See page 21 for more about
foundation brakes, and government-mandated stopping
distances.) Frictional forces slow the wheels and the
vehicle comes to a stop. When the brakes are released,
the air in the brake chambers is quickly released, enabling
the vehicle to drive away.
Vehicle Parking
Vehicles are parked using powerful springs, which are
part of the spring brake assembly, to engage the brakes
and hold the vehicle in position. When the driver prepares
to move away and releases the parking brake, the spring
force is countered by the introduction of air pressure.
An anti-compounding valve in the system design helps
prevent the application of both the spring and service
brakes together.
Emergency Braking
In emergency situations where system air pressure is
reduced or lost, government regulations require vehicles
to meet specifi ed stopping distances. As an example,
some straight truck system designs use modulated
parking-brake applications to bring the vehicle to a stop.
Electronically-Controlled Braking
The following systems enhance the air brake system by
adding electronic sensors and controllers to modify the
vehicle’s braking characteristics.
• Antilock Braking Systems (ABS)
Most commercial vehicles use electronic Antilock
Braking Systems (ABS) to help improve braking
when excessive wheel slip, or wheel lock-up, is
detected. Bendix
®
Electronic Control Units (ECUs)
use patented technology to monitor wheel speeds
(on all wheels equipped with speed sensors) and use
ABS modulator valves to adjust, or pulse, the braking
force being applied. These valves operate multiple
times per second during an ABS event. ABS typically
improves stability and steerability, and also helps
reduce stopping distances on most surfaces.
In addition to the ABS features above, some advanced
ECUs have a drag torque control feature which helps
reduce drive-axle wheel slip (due to driveline inertia)
by communicating with the engine’s controller and
increasing the engine torque.
Automatic Traction Control
In addition to the ABS function, many Bendix ECU
models provide an Automatic Traction Control (ATC)
feature which can help improve vehicle stability and
traction during vehicle acceleration at low speeds.
• Hill Start Feature
For vehicles with the optional Bendix hill start feature
(often referred to as Hill Start Assist/Assistance or
HSA), the system interfaces between the transmission
and the braking system to help the driver prevent the
vehicle from rolling downhill when moving up a steep
incline from a stationary position.
Section 1: An Introduction
Introduction