TRUCK SERVICE MANUAL
FUEL SYSTEM
Gear Pump And Pulsation Damper
Governors
The gear pump is driven by the pump main shaft and
Idling And High-Speed Mechanical Governor
contains a single set of gears to pick-up and deliver fuel
The mechanical governor, sometimes called "automotive
throughout the fuel system. A pulsation damper
governor," is actuated by a system of springs and
mounted to the gear pump contains a steel diaphragm
weights, and has two functions. First, the governor
which absorbs pulsations and smooths fuel flow through
maintains sufficient fuel for idling with the throttle control
the fuel system.
in idle position; second, it cuts off fuel to the injectors
From the gear pump, fuel flows through the filter screen
above maximum rated rpm. The idle springs in the
and.
governor spring pack positions the governor plunger so
In the PT (type G) fuel pump, to the governor
the idle fuel port is opened enough to permit passage of
assembly as shown in Fig. 5-3.
fuel to maintain engine idle speed.
Currently, PT (type G) gear pumps are equipped with a
During operation between idle and maximum speeds,
bleed line (to the engine injector return line or to tank)
fuel flows through the governor to the injectors in accord
which prevents excessive fuel temperatures within the
with the engine requirements as controlled by the throttle
fuel pump. The bleed line functions primarily when the
and limited by the size of the idle spring plunger
pump throttle is set at Idle speed but when gear pump
counterbore on PT (type G) fuel pumps. When the
output is high, due to engine rotative speed, as occurs
engine reaches governed speed, the governor weights
during down hill operation. A special check valve and/or
have moved the governor plunger to close the fuel
fitting is used In the gear pump to accomplish the bleed
passages to injectors. At the same time another
action.
passage is opened and fuel is dumped back into the
main pump body. In this manner engine speed is
Pressure Valve
controlled and limited by the governor regardless of
The pressure valve is used only In PT (type G) VS
throttle position. Fuel leaving the pump flows through the
governor fuel pump. The valve is located near gear
shut-down valve, supply lines and cylinder head drillings
pump drive shaft. It restricts return fuel coming from
into the injectors.
governor barrel thereby creating low fuel pressure in
housing.
Removal Of Fuel Pump Units
Clean And Mount
Throttle
1. Clean outside of fuel pump thoroughly with an
In both fuel pumps, the throttle provides a means for the
approved solvent. Remove lockwires and seals if used.
operator to manually control engine speed above Idle as
Note: Many solvent cleaners are injurious to aluminum.
required by varying operating conditions of speed and
Make sure your cleaner is suitable before using it on
load.
aluminum.
In the PT (type G) fuel pump, fuel flows through the
2. Remove pulsation damper from gear pump and
governor to the throttle shaft. At idle speed, fuel flows
discard "0" ring. Older style dampers are fitting mounted
through the idle port in the governor barrel, past the
and are removed by screwing damper from fitting on
throttle shaft. To operate above idle speed, fuel flows
gear pump,
through the main governor barrel port to the throttling
3. To remove gear pump from main housing, tap sides
hole in the throttle shaft.
of gear pump with a plastic hammer to loosen from
dowel. Remove gear pump discard gasket.
Shut-Down Valve
Either a manual or an electric shut-down valve is used on
PT (type G) Fuel Pump Service Cooling Kit
Cummins fuel pumps.
1. Remove check valve and orifice elbow assembly
from bottom or back side of fuel pump.
With a manual valve, the control lever must be fully
Note: Current pumps have the check valve located at
clockwise or open to permit fuel flow through the valve.
the gear pump and older pumps should be changed over
at any time a new gear pump is used.
With the electric valve, the manual control knob must be
2. Clean parts in clean fuel oil and dry with compressed
fully counterclockwise to permit the solenoid to open the
air blown through both ends.
valve when the "switch key" is turned on. For emergency
operation in case of electrical failure, turn manual knob
Shut-Down Valve, Pulsation Damper And Gear Pump
clockwise to permit fuel to flow through the valve.
1. Remove shut-down valve from top of main housing
and discard "O" ring.
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