the end of first and the end of second
counter is the length of the Signal Pulse
Width (SPW) (or the period during which
the pump solenoid is energized).
To change pump timing the ECM either
lengthens to retard or shortens to ad-
vance, the first counter’s count. If fuel de-
livery is not to be changed, then the sec-
ond counter count will have to be adjusted
the same amount. If fuel quantity changes
are needed without changes in pump tim-
ing, the first counter count will remain the
same and the second counter count will
be decrease to cutback on fuel or in-
creased to increase fuel. A combination
of changes in both timing and fuel deliv-
ery quantity are possible by changes in
both first and second counter counts.
ship never changes. Due to this fixed pump
cam to engine relationship a means of
communicating cam position to the ECM
is not needed.
To control the pump a high current signal
generated by the ECM, is sent to the
pump’s fuel control solenoid, this in turn
controls the positioning of the poppet valve
which creates the spilling and pumping
events.
Figure 1.14 shows a cam profile, pump-
ing events and ECM signals. On the cam
profile there are two ranges, filling and
pumping. The filling range, or the retraction
ramp side of the cam profile, is where the
plungers bores are completely filled with fuel
at transfer pump pressure. The pumping
range, or the pumping ramp side of the cam
profile, is where the plungers are driven in-
wards; displacing the fuel in the plunger
bores.
When the ECM receives a signal from the
engine drive shaft Top Dead Center (TDC)
sensor, it starts two internal clocks
(counters). These clocks count off engine
degrees of rotation. The first clock controls
the timing of the pumping event by count-
ing off an ECM determined amount of de-
grees of engine rotation before turning
power on to the pump solenoid which forces
the poppet valve onto its seat initiating the
start of the pumping event.
To determine the length of the pumping pe-
riod (fuel quantity) the ECM monitors vari-
ous engine inputs and load demand infor-
mation. Once the fuel quantity need is de-
termined the ECM sets the number of
counts (degrees) for the second counter.
The second clock controls when the pump-
ing event ends by counting off an ECM de-
termined amount of engine degrees before
turning off the power to the pump solenoid,
ending the pumping event by starting the
second spill event. The difference between
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