position, the sleeve levers move the sleeves down,
cutting off fuel to the cylinders.
Any and all adjustments to the governor
and governor controls should be made
according to the manufacturers manual and
AUTOMATIC TIMING ADVANCE UNIT.
All current Caterpillar engines use some form of
automatic timing for the fuel injection pump. On sleeve
metering injection systems, this advance is mounted on
the front end of the camshaft of the engine. The gear of
the automatic advance unit meshes with and drives the
fuel injection pump camshaft. The principal parts of the
advance unit are the slides, the springs, and the weights.
Operation of the automatic advance-timing unit is as
The slides are located and driven by two dowels,
attached to the engine camshaft gear. The slides,
in turn, fit into notches within the weights,
thereby transferring their drive from the engine
camshaft gear to the weights.
With the engine running, centrifugal force
exerted by the rotating weight assemblies cause
them to act against the force of the springs.
Since the weights are designed with notches in
them, as they move outward under centrifugal
force, they cause the slides to effect a change in
the angle between the timing advance gear and
the two drive dowels of the engine camshaft.
This relative movement of the timing advance
unit gear will, therefore, automatically advance
or retard the timing of the fuel injection pump in
relation to the engine speed and load.
However, built into the advance unit is a maximum
timing variation of 5 degrees with the timing change
starting at approximately low idle rpm and continuing
on up to the rated speed of the engine; therefore, you
cannot adjust the automatic timing advance unit. The
timing unit is lubricated by engine oil under pressure
from drilled holes at the engine camshaft front bearing.
Scroll Metering Fuel System
The scroll metering fuel system is similar to the
sleeve metering fuel system in that it uses a plunger and
barrel to create high pressure for injection. This system
was designed to create higher injection pressure on
direct-injection engines, offering an approximate 10
percent fuel economy improvement over
precombustion-type engines, along with the ability to
meet long-term EPA exhaust emissions regulations and
better overall engine performance, as well as the ability
to provide greater part commonality between different
In a scroll system two helix cut ports are usedthe
bypass closed port and the spill port Fuel is supplied
from the transfer pump to an internal fuel manifold in
the injection pump housing at approximately 35 psi.
When the pump plunger is at the bottom of its stroke,
fuel at transfer pump pressure flows around the pump
barrel and to both the bypass closed port and spill port,
which are both open at this time to allow fuel to flow
into the barrel area above the plunger. The pump
plunger is moved up and down by the action of a roller
lifter, riding on the injection pump camshaft, which
rotates at one-half of engine speed. As the injection
pump camshaft rotates and the plungers rises, some fuel
will be pushed back out of the bypass closed port until
the top of the plunger eventually closes both the bypass
closed port and the spill port. Further plunger
movement will cause an increase in the trapped fuel
pressure, and at approximately 100 psi, a check valve
will open and fuel will flow into the injection line to the
The fuel pressure of 100 psi is not enough to open
the injection nozzle, which has an opening pressure of
between 1,200 and 2,350 psi for a 3300 series engine
and between 2,400 and 3,100 psi on 3406 engines.
However, as the plunger continues to move up in its
barrel, this fuel pressure is reached very quickly.
A high-pressure bleed-back passage and groove
machined around the barrel are in alignment during the
effective stroke to bleed off any fuel that leaks between
the plunger and the barrel for lubrication purposes.
When the upward moving plunger uncovers the
spill port, injection ceases, and although the plunger can
still travel up some more, this is simply to allow most of
the warm fuel (due to being pressurized) to spill back
into the manifold. As the plunger moves downward in
the barrel, it will once again uncover the bypass closed
port and cool fuel will fill the area above the plunger for
the next injection.
When the spill port is opened,