1991 MITSUBISHI MONTERO engine

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)









\









\









\









\









\









\







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.









\









\









\









\









\









\







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

EXAMPLE #2 - VOLTAGE CONTROLLED DRIVER
This time we will look at a GM 3.1L V6 VIN [T]. Fig. 8 shows
the 1, 3, 5 (odd) injector bank with the current waveform indicating
about a 2.6 amp draw at idle. This pattern, taken from a known good
vehicle, correctly stays at or below the maximum 2.6 amps current
range. Ideally, the current for each bank should be very close in
comparison.
Notice the small dimple on the current flow's rising edge.
This is the actual injector opening or what engineers refer to as the
"set point." For good idle quality, the set point should be uniform
between the banks.
When discussing Ohm's Law as it pertains to this parallel
circuit, consider that each injector has specified resistance of 12.2
ohms. Since all three injectors are in parallel the total resistance
of this parallel circuit drops to 4.1 ohms. Fourteen volts divided by
four ohms would pull a maximum of 3.4 amps on this bank of injectors.
However, as we discussed in EXAMPLE #1 above, other factors knock this
value down to roughly the 2.6 amp neighborhood.
Now we are going to take a look at the even bank of
injectors; injectors 2, 4, and 6. See Fig. 9. Notice this bank peaked
at 1.7 amps at idle as compared to the 2.6 amps peak of the odd bank (
Fig. 8 ). Current flow between even and odd injectors banks is not
uniform, yet it is not causing a driveability problem. That is because
it is still under the maximum amperage we figured out earlier. But be
aware this vehicle could develop a problem if the amperage flow
increases any more.
Checking the resistance of this even injector group with a
DVOM yielded 6.2 ohms, while the odd injector group in the previous
example read 4.1 ohms.
Fig. 8: Injector Odd Bank w/Normal Current Flow - Current Pattern

Fig. 9: Injector Even Bank w/Normal Current Flow - Current Pattern
EXAMPLE #3 - VOLTAGE CONTROLLED DRIVER
Example #3 is of a Ford 5.0L V8 SEFI. Fig. 10 shows a
waveform of an individual injector at idle with the Lab Scope set on
200 milliamps per division. Notice the dimple in the rising edge. This
dimple indicates the actual opening of the injector (set point)
occurred at 400 milliamps and current peaked at 750 milliamps. This is
a good specification for this engine.
The next waveform pattern in Fig. 11 shows an abnormality
with another injector. With the Lab Scope set on 500 milliamps per
division, you can see that the current waveform indicates a 1200
milliamp draw. This is a faulty injector.
Abnormally low resistance injectors create excessive current
draw, causing rough idle, and possible computer driver damage.
Fig. 10: Single Injector w/Normal Current Flow - Current Pattern

PFI VIN [3]. It is a perfect example of the peak and hold theory. The
waveform shows a 1-amp per division current flow, ramping to 4 amps
and then decreasing to 1-amp to hold the injector open.
Fig. 13: Injector Bank w/Normal Current Flow - Current Pattern
EXAMPLE #6 - CURRENT CONTROLLED DRIVER
This next known-good waveform is from a Ford 5.0L V8 CFI VIN
[F]. See Fig. 14. The pattern, which is set on a 250 milliamps scale,
indicates a 1.25 amp peak draw and a hold at 350 milliamps.
Fig. 14: Single Injector w/Normal Current Flow - Current Pattern
EXAMPLE #7 - CURRENT CONTROLLED DRIVER
The known-good current controlled type waveform in Fig. 15 is
from a GM 2.0L TBI VIN [1]. With the lab scope set at 2 amps per
division, notice that this system peaks at 4 amps and holds at 1 amp.
The next waveform is from the same type of engine, except

Fig. 17: Single Injector w/Normal Current Flow - Current Pattern
VOLTAGE WAVEFORM SAMPLES
EXAMPLE #1 - VOLTAGE CONTROLLED DRIVER
These two known-good waveform patterns are from a Ford 4.6L
V8 VIN [W]. Fig. 18 illustrates the 64 volt inductive kick on this
engine, indicating no clamping is occurring. The second pattern,
Fig. 19 , was taken during hot idle, closed loop, and no load.

Fig. 21: Injector Bank - Known Good - Voltage Pattern
EXAMPLE #4 - CURRENT CONTROLLED DRIVER
From 1984 to 1987, Chrysler used this type injector drive on
their TBI-equipped engines. See Fig. 22 for a known-good pattern.
Instead of the ground side controlling the injector, Chrysler
permanently grounds out the injector and switches the power feed side.
Most systems do not work this way.
These injectors peak at 6 amps of current flow and hold at 1
amp.

Fig. 22: Single Injector - Known Good - Voltage Pattern
EXAMPLE #5 - CURRENT CONTROLLED DRIVER
These two known-good waveform patterns are from a Chrysler 3.
0L V6 VIN [3]. The first waveform, Fig. 23, is a dual trace pattern
that illustrates how Chrysler uses the rising edge of the engine speed
signal to trigger the injectors. The second waveform, Fig. 24, was
taken during hot idle, closed loop, and no load.