Foot Throttle Valve (Accelerator Control Valve)
The foot throttle valve is a treadle operated air valve mounted on the cab floor. The valve controls the air pressure to the
throttle cylinder mounted on the engine which controls engine speed. Air pressure is supplied from the air swivel to the
foot throttle valve.
The throttle cylinder is bracket mounted on the engine and connected to the engine throttle linkage. It moves the throttle
lever in response to an air pressure from the foot throttle valve in the cab. The push tube adapter is threaded to permit
connection to the throttle lever.
When the reservoir air pressure is below the pressure protection valve setting the push tube extends to the fast idle
position. When the pressure protection valve opens, the push tube returns to the slow idle position in which the adapter
shoulder is in contact with the cylinder bushing.
At normal system pressures, control of the throttle cylinder is through the throttle valve and connecting line. When the foot
throttle is depressed, air pressure is supplied through the throttle valve port and acts on the effective area of the throttle
piston. When the force developed by the air pressure is greater than the force developed by the graduating spring on the
opposite side of the throttle position, the push tube is extended toward the full throttle position to the extent of the air
pressure delivered by the throttle valve.
As pressure is reduced or completely exhausted from the throttle cylinder through the throttle valve, the push tube is
moved toward the idle position by the graduating spring.
The secondary reservoir provides the supply for the R12 relay valve for the service brakes application and the supply for
the dual brake valve (pedal) in the cab.
R-12 Relay Valve
The relay valve functions as a relay station to speed up the application, modulation, and release of the service brakes. It
can be considered to be a remote mounted, air controlled brake valve that releases or delivers air to the actuators in
response to the signals received from the parking brake valve or the dual brake valve.
The rapid reaction of the relay valve to changes in control pressures is in part due to the relatively small volume of air
required between the valve cover and the relay piston. The area of the relay piston and the pressure of the quick release
also contribute greatly to the fast application and release of the actuators.