GxT Ferret 92 Color Labscope - Manual (Page 21)

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Port Fuel Injector:
Fig. 16 is a capture of a typical saturated switch fuel injector. It is a
high speed digital signal and is not viewable in real time. As you can
see the relatively short Time Base and ground at the bottom gives
maximum resolution. Time cursors were used to measure the pulse
width at 2.99mS.
Fig. 17 is a capture of the same injector as Fig. 16, but with a much
longer Time Base allowing plenty of room for change in pulse width if
we decide to snap the throttle to watch the system respond. As you
can see, a Voltage Scale of 50 volts was used here, allowing us to see
the voltage spike at 44.3 volts amplitude.
This injector has power to one side of the winding when the ignition is
turned on and the driver completes the ground side of the circuit to
pulse it. The proper way to connect your scope leads here is negative
to a good ground and positive to the control, or negative, side of the
injector. We used a negative slope trigger because the driver pulls the
control side of the circuit to ground to pulse the injector. This means
that voltage will decrease when the injector is pulsed. Trigger level
is set fairly close to the middle of this voltage transition. If excessive
resistance is present in the control side of the circuit, voltage may not
be pulled to ground. If trigger level was set too low the voltage level
might not fall past the set level, in which case, the scope would not
trigger.
Peak & Hold Injector:
Fig. 18 is a peak and hold injector. We have our scope leads hooked
up the same as we did in Fig. 17 and used similar settings. You can
see that the waveform for this injector is different than the saturated
switch injector we just looked at. it has two voltage spikes. This type
of injector is used on CFI and TBI systems where it must open much
more often that port fuel injectors. Because of this it must open faster,
so it has a very low resistance winding. If it were controlled the same
as a port injector the high current draw would produce a lot of heat
which would cause the driver to fail. To overcome this obstacle two
drivers are used, One driver switches the circuit to ground to snap the
injector open and shuts off after a very short time, a little over 1 mS.
This gives the driver plenty of time to cool down, The second driver
completes the circuit through a resistor, reducing the current to only
what is needed to hold the injector open for the remainder of its on time.
The fi rst voltage spike occurs when the peak driver circuit is opened
and the second when the hold driver is opened.