The air compressor is the source of energy for the air
The air compressor is located on the engine and operates only when the engine is running. Clean, filtered, turbocharged pressurized air from the intake manifold is force-fed into the air compressor. The compressor maintains the air system whenever the system's pressure falls below preset parameters. When the maximum pressure is reached, the compressor output is diverted. An air governor is used to maintain the air pressure from 85 to 125psi.
Air pressure build up test.
Continue to let the air pressure build; it should not take more than four minutes for the air pressure to go from discharged (5-20 psi) to between 120 and 130 psi.
Governor cut-in/cut-out test.
When air pressure gets between 120-130 psi, the governor should cut out. The dash gauge needle stops moving. When the needle stops, pump the brake pedal to reduce the air pressure to 80 psi, release the brake pedal, the compressor starts pumping air (cutting in)! watch for needle movement. The air governor causes the air compressor to cut in between (85-90 psi).
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
The vehicle’s compressor draws in filtered air, either
at atmospheric pressure from the outside (or already
at an increased pressure, from the engine turbocharger
where permitted), and compresses it.
The brake system needs a supply of compressed air
between a preset maximum and minimum. The
governor (along with a synchro valve for the Bendix®
DuraFlo™ 596 air compressor) monitors the air
pressure in the supply reservoir and controls when
the compressor needs to pump air into the air system
(also known as the “air build cycle” - the compressor
is “running loaded”) and when the compressor should
simply turn over without building pressure (“running
unloaded”). When the air pressure becomes greater
than that of the preset “cut-out”, the governor controls
the unloader mechanism of the compressor to stop
the compressor from building air and also 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 the air dryer 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 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 affect air brake charging system
performance. These conditions may require additional
maintenance and lead 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 leakage from
fittings, connections, lines, chambers or valves, etc. See
page 9 for compressor maintenance and usage
guidelines. Use the 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 oilvapor
mixture to cool between the compressor and
air dryer. The typical size of a vehicle's discharge line,
(see table on page 9) assumes a compressor with a
normal (less than 25%) duty cycle, operating in a
temperate climate. See Bendix and/or vehicle or air
dryer manufacturer guidelines as needed.
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. Air dryer inlet
temperatures play a key role in air system cleanliness
and air dryer performance. 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 fitting to
avoid low points where ice may form and block the
flow. If, instead, ice blockages occur at the air dryer
inlet, insulation may be added here, or if the inlet fitting
is a typical 90 degree fitting, it may be changed to a
straight or 45 degree fitting. For more information on
how to help prevent discharge line freeze-ups, see
Bendix Bulletins TCH-08-21 and TCH-08-22. Shorter
discharge line lengths or insulation may be required in
The air dryer contains a filter that collects oil droplets,
and a desiccant bed that removes almost all of the
remaining water vapor. The compressed air is then
passed to the air brake service (supply) reservoir. The
oil droplets and the water collected are automatically
purged at the dryer when the governor reaches its “cutout”
For vehicles with accessories that are sensitive to small
amounts of oil, we recommend installation, downstream
of the air dryer, of a Bendix® PuraGuard® QC™ oil
coalescing filter to minimize the amount of oil present.