W ith the inlet valves held off their seats by the unloader
pistons and plungers, air is merely pumped back and forth
between the two cylinders. When air is used from the air
The function of the air compressor is to build-up and main-
tank and the pressure drops to the low pressure setting of
tain the air pressure required to operate air powered devices
the governor, the governor closes and in doing so exhausts
in the air brake, carrier remote controls, and throttle sys-
the air from beneath the unloader pistons. The unloader
spring forces the pistons and plungers down and the inlet
valves return to their seats. Compression is then resumed.
The compressor runs continuously while the engine is
running, but actual compression of air is controlled by the
governor which stops or starts the compression of air by
INSPECTION. The following service checks should be per-
loading or unloading the compressor in conjunction with its
formed periodically to ensure proper operation of the air
unloading mechanism. This is done when the air pressure in
the system reaches the desired maximum or minimum pres-
During the down stroke of the piston, a slight vacuum
created above the piston causes the inlet valve to move off
Do not use the brake system to hold the machine
its seat. Atmospheric air is drawn in through the com-
while testing or working on the air system. Hold the
pressor intake, by the open inlet valve, and onto the top of
machine by blocking the wheels with wheel chocks.
the piston. As the piston starts its upward stroke, the air
that was drawn in on the down stroke is being compressed.
1. It is of utmost importance that the compressor is taking
Now, air pressure on the inlet valve plus the force of its
in clean air. Be sure the compressor intake is air tight and
spring, returns the inlet valve to its seat. The piston con-
that the connections are tight.
tinues the upward stroke and compresses the air sufficiently
to overcome the discharge valve spring and unseats the dis-
2. Check compressor mounting to be sure it is secure.
charge valve. The compressed air then flows by the open dis-
charge valve, into the discharge line and on to the air tanks.
3. Inspect the oil supply and return lines. Be sure the com-
pressor is getting the proper supply of oil, and just as im-
As the piston reaches the top of its stroke and starts down,
portant, that the oil is returning to the engine properly.
the discharge valve spring returns the discharge valve to its
seat. This prevents the compressed air in the discharge line
4. Check the water lines to and from the compressor.
from returning to the cylinder bore as the intake and com-
pression cycle is repeated.
AIR LEAKAGE TEST. Leakage past the discharge valves
can be detected by removing the discharge line, applying
When the pressure in the air tanks reach the high pressure
shop air back through the discharge port, and listening for
setting of the governor, the governor opens, allowing air to
escaping air. Also the discharge valves and unloader pistons
pass from the air tank through the governor and into the
can be checked for leakage by building up the air pressure
cavity beneath the unloader pistons. This lifts the unloader
until the governor cuts out, then stop the engine. With the
pistons and plungers. The plungers move up and hold the
engine stopped, carefully listen for escaping air at the in.
inlet valves off their seats.
take. To pinpoint Ieakage, if noted, squirt soapy water