Troubleshooting guide

9
For more information, visit www.bendix.com or www.foundationbrakes.com • 1-800-AIR-BRAKE (1-800-247-2725)
The Charging, Air Treatment and Storage System: Compressors
leakage from fi ttings, connections, lines, chambers or
valves, etc. See page 11 for compressor maintenance
and usage guidelines. Use the Bendix
®
BASIC
™
test (p/n
5013711) where the amount of oil present in the air brake
system is suspected to be above normal.
The discharge line allows the air, water vapor and oil vapor
mixture to cool between the compressor and air dryer.
The typical size of a vehicle’s discharge line, (see table
on page 10) assumes a compressor with a normal (less
than 25%) duty cycle, operating in a temperate climate.
See Bendix and/or the vehicle or air dryer manufacturer
guidelines as needed.
Air dryer inlet temperatures play a key role in air system
cleanliness and air dryer performance. When the
temperature of the compressed air that enters the air
dryer is within the normal range, the air dryer can remove
most of the charging system oil. If the temperature of the
compressed air is above the normal range, oil (as oil-
vapor) is able to pass through the air dryer and into the
air system. Larger diameter discharge lines and/or longer
discharge line lengths can help reduce the temperature.
The discharge line must maintain a constant slope
down from the compressor to the air dryer inlet fi tting
to avoid low points where ice may form and block the
fl ow. If, instead, ice blockages occur at the air dryer
inlet, insulation may be added here — or if the inlet fi tting
is a typical 90-degree fi tting — it may be changed to a
straight, or 45-degree fi tting, to prevent moisture build-
up and freezing. For more information on how to help
prevent discharge line freeze-ups, see Bendix Bulletins
TCH-008-021 and TCH-008-022. Conversely, shorter
discharge line lengths (or insulation) may be required in
extreme cold climates to maintain proper dryer operation.
Bendix Air Compressors
The air compressor is the source of energy for the air
brake system.
Usually driven by the vehicle engine, the air compressor
builds the air pressure for the air brake system. The air
compressor is typically cooled by the engine coolant
system and lubricated by the engine oil supply. (Certain
models have self-lubricated and/or air-cooled versions
available.) Note: Air compressor shafts can rotate in
either direction.
The vehicle’s compressor draws in fi ltered air — either
naturally aspirated (at atmospheric pressure) from
the outside (or already at an increased pressure from
the engine turbocharger where permitted) — and
compresses it until system pressure is achieved.
To function, the brake system needs a supply of
compressed air within a preset maximum and minimum.
The governor (along with a synchro valve for certain
Bendix
®
air compressors) monitors the air pressure in
the supply reservoir and monitors when the compressor
needs to pump air into the air system (also known
as the “air build cycle” — where the compressor is
“running loaded”). When the system has suffi cient air,
the compressor can simply turn over without building
pressure (“running unloaded”). It also monitors when
the air pressure becomes greater than that of the preset
“cut-out”, the governor controls the unloader mechanism
of the compressor, stops the compressor from building
air and causes the air dryer to purge. As the service
reservoir air pressure drops to the “cut-in” setting of the
governor, the governor returns the compressor back to
building air and cycles the air dryer back to air drying
mode.
As the atmospheric air is compressed, all the water vapor
originally in the air is carried along into the air system (as
well as a small amount of the compressor lubricating oil)
as vapor.
The duty cycle is the ratio of time the compressor spends
building air, relative to the total engine running time. Air
compressors are designed to build air (run “loaded”) up
to 25% of the time. Higher duty cycles cause conditions
(such as higher compressor head temperatures) that may
adversely affect air brake charging system performance.
These conditions may require additional maintenance
due to a higher amount of oil vapor droplets being passed
along into the air brake system. Factors that add to the duty
cycle are: air suspension, additional air accessories, use
of an undersized compressor, frequent stops, excessive
Air Dryer — Typically with
a Bendix
®
PuraGuard
®
Oil
Coalescing Filter
Governor
Safety
Valve
Air
Compressor
Supply Reservoir
Safety
Valve
Discharge Line
Purge
Valve
Drain Valve
Air Treatment System