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TM 9-8000
Figure 4-8. Fuel Filter Element Configurations.
(1) The rocker arm is moved up and down by the
nonpositive-type fuel pump regulates fuel line pressure.
engine camshaft. The rocker arm spring causes the
Normal pressures range from 1.5 to 6 psi (10.3 to 41.4
rocker arm to follow the cam lobe.
kPa). The operating range of the pump depends on the
tension exerted by the diaphragm spring.
(2) The rocker arm hooks into an elongated slot in
(8) A venthole is provided under the diaphragm to
pull rod. The other end of the pull rod is attached to
allow the pressure to change in the lower chamber as
the diaphragm flexes.
(3) As the camshaft operates the rocker arm, it
operate the diaphragm against the force of
(9) The pulsation chamber, located above the pump
diaphragm spring.
chamber, uses a soft diaphragm and a sealed chamber
to  cushion  the  pulsating  action  inherent  to  the
(4) As the rocker arm pulls the diaphragm down, the
diaphragm-type pump.
inlet check valve is unseated and fuel is drawn into the
pump chamber. The outlet check valve seals the outlet
(10) An oil seal is provided to keep crank-case oil from
entering the lower chamber and leaking from the
(5) As the diaphragm spring pushes the diaphragm
back up, the inlet check valve seals the inlet and the fuel
d. Mechanical, Positive Type (Fig. 4-10).
in the pump chamber is pushed through the unseated
The positive-type mechanical pump operates in the
outlet check valve and through the pump outlet.
same manner as the nonpositive type. The difference is
that the diaphragm pull rod is solidly linked to the rocker
(6) The action is repeated each time the rocker arm
arm. The pump, therefore, will not regulate fuel line
operates the diaphragm.
pressure. When this type of pump is used, a separate
fuel pressure regulation device must be used that will
(7) Pressure will build in the fuel line and the pump
bypass excess fuel back to the fuel tank.
chamber as the fuel pump fills the carburetor bowl. As
the pressure rises to the desired level in the pump
e. Double Action Fuel Pump (Fig. 4-11).
chamber, it will hold the diaphragm down against the
Vehicles that use vacuum-operated windshield wipers
pressure of the diaphragm spring. The rocker arm will
often will utilize a supply pump that is built Into the fuel
move up and down in the slotted pull rod. There will be
pump. The pump serves to operate the windshield
no pumping action until the fuel line pressure again
wipers during periods of high engine load when manifold
drops below the desired level. In this way, the
vacuum is low (para 17-14). The pump operates from the


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