Lesson 2/Learning Event 1
An unloading mechanism on the cylinder head unloads compressor
compression whenever the air pressure reaches a predetermined
maximum. The unloader mechanism generally has a diaphragm connected
to linkage, so that when air pressure is applied at one side of the
diaphragm, the diaphragm is moved to hold either the unloader valves
or the intake valves open. At this time, the compressor will
continue to run with the engine but will not compress air. When the
air pressure drops to a certain predetermined amount, the pressure is
released from the diaphragm, permitting the intake or unloader valves
to close and the compressor pumps up the pressure again.
The operation of the unloader mechanism is controlled by the air
governor. Many different designs of governors are used, but they all
serve the same purpose and operate on the same basic principles.
Primarily, any air governor is a valve held closed by spring tension.
Air pressure from the compressor and air reservoir is applied to a
diaphragm, piston, or a similar device that opposes the spring
tension in an attempt to open the valve. When the air pressure
reaches a desired maximum of about 110 to 120 PSI, air pressure
overcomes the spring tension and the governor valve opens. Air
pressure flows through the open valve to the unloader mechanism
opening the unloader valves. When the air pressure drops to a
desired minimum, spring tension on the governor valve overcomes the
air pressure closing the valve. This releases the air pressure to
the unloader allowing the unloader valves to close.
The air governor is mounted on the engine side of the cowl
(firewall). At least two air lines must be connected to a governor:
one pressure line from the air supply and one line to the unloader on
the compressor. A third line, for the pressure exhaust may be
connected between the governor and the vehicle vent system. The
governor contains a filter to strain the air that passes through it.
Most governors have an external adjustment that allows the mechanic
to change the tension on the spring holding the air valve closed,
which will in turn change the amount of air pressure required to open