1988 JEEP CHEROKEE 6 cylinder

3800 engines were suffering from exactly this. The point is that a
lack of detail could cause misdiagnosis.
As you might have guessed, a lab scope would not miss this.
RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE (1)









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Dwell Meter (2) Duty Cycle Meter
1
.................................................... 1%
15 .................................................. 25%
30 .................................................. 50%
45 .................................................. 75%
60 ................................................. 100%
( 1) - These are just some examples for your understanding.
It is okay to fill in the gaps.
( 2) - Dwell meter on the six-cylinder scale.









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THE TWO TYPES OF INJECTOR DRIVERS
OVERVIEW
There are two types of transistor driver circuits used to
operate electric fuel injectors: voltage controlled and current
controlled. The voltage controlled type is sometimes called a
"saturated switch" driver, while the current controlled type is
sometimes known as a "peak and hold" driver.
The basic difference between the two is the total resistance
of the injector circuit. Roughly speaking, if a particular leg in an
injector circuit has total resistance of 12 or more ohms, a voltage
control driver is used. If less than 12 ohms, a current control driver
is used.
It is a question of what is going to do the job of limiting
the current flow in the injector circuit; the inherent "high"
resistance in the injector circuit, or the transistor driver. Without
some form of control, the current flow through the injector would
cause the solenoid coil to overheat and result in a damaged injector.
VOLTAGE CONTROLLED CIRCUIT ("SATURATED SWITCH")
The voltage controlled driver inside the computer operates
much like a simple switch because it does not need to worry about
limiting current flow. Recall, this driver typically requires injector
circuits with a total leg resistance of 12 or more ohms.
The driver is either ON, closing/completing the circuit
(eliminating the voltage-drop), or OFF, opening the circuit (causing \
a
total voltage drop).
Some manufacturers call it a "saturated switch" driver. This
is because when switched ON, the driver allows the magnetic field in
the injector to build to saturation. This is the same "saturation"
property that you are familiar with for an ignition coil.
There are two ways "high" resistance can be built into an
injector circuit to limit current flow. One method uses an external
solenoid resistor and a low resistance injector, while the other uses
a high resistance injector without the solenoid resistor. See the left
side of Fig. 1.
In terms of injection opening time, the external resistor
voltage controlled circuit is somewhat faster than the voltage
controlled high resistance injector circuit. The trend, however, seems
to be moving toward use of this latter type of circuit due to its
lower cost and reliability. The ECU can compensate for slower opening

CURRENT WAVEFORM SAMPLES
EXAMPLE #1 - VOLTAGE CONTROLLED DRIVER
The waveform pattern shown in Fig. 4 indicate a normal
current waveform from a Ford 3.0L V6 VIN [U] engine. This voltage
controlled type circuit pulses the injectors in groups of three
injectors. Injectors No. 1, 3, and 5 are pulsed together and cylinders
2, 4, and 6 are pulsed together. The specification for an acceptable
bank resistance is 4.4 ohms. Using Ohm's Law and assuming a hot run
voltage of 14 volts, we determine that the bank would draw a current
of 3.2 amps.
However this is not the case because as the injector windings
become saturated, counter voltage is created which impedes the current
flow. This, coupled with the inherent resistance of the driver's
transistor, impedes the current flow even more. So, what is a known
good value for a dynamic current draw on a voltage controlled bank of
injectors? The waveform pattern shown below indicates a good parallel
injector current flow of 2 amps. See Fig. 4.
Note that if just one injector has a resistance problem and
partially shorts, the entire parallel bank that it belongs to will
draw more current. This can damage the injector driver.
The waveform pattern in Fig. 5 indicates this type of problem
with too much current flow. This is on other bank of injectors of the
same vehicle; the even side. Notice the Lab Scope is set on a one amp
per division scale. As you can see, the current is at an unacceptable
2.5 amps.
It is easy to find out which individual injector is at fault.
All you need to do is inductively clamp onto each individual injector
and compare them. To obtain a known-good value to compare against, we
used the good bank to capture the waveform in Fig. 6. Notice that it
limits current flow to 750 milliamps.
The waveform shown in Fig. 7 illustrates the problem injector
we found. This waveform indicates an unacceptable current draw of just
over one amp as compared to the 750 milliamp draw of the known-good
injector. A subsequent check with a DVOM found 8.2 ohms, which is
under the 12 ohm specification.
Fig. 4: Injector Bank w/Normal Current Flow - Current Pattern