1991 FORD FESTIVA Service Manual

number of windings, the more potential for a greater inductive kick. The opposite is also true. The less current flow or fewer windings means
less inductive kick. Typically you should see a minimum 35 volts.
If you see approximately 35 volts, it is because a zener diode is used with the driver to clamp the voltage. Make sure the beginning top of the
spike is squared off, indicating the zener dumped the remainder of the spike. If it is not squared, that indicates the spike is not strong enough to
make the zener fully dump, meaning there is a problem with a weak injector winding.
If a zener diode is not used in the computer, the spike from a good injector will be 60 or more volts.
At Point "E", notice that the trace is now just a few volts below system voltage and the injector is in the current limiting, or the "Hold" part of
the pattern. This line will either remain flat and stable as shown here, or will cycle up and down rapidly. Both are normal methods to limit
current flow. Any distortion may indicate shorted windings.
Point "F" is the actual turn-off point of the driver (and injector). To measure the millisecond on-time of the injector, measure between points
"C" an d "F". No t e t h at we u sed cu rso rs t o d o it for us; they are measuring a 2.56 mS on-time.
The top of Point "F" (second inductive kick) is created by the collapsing magnetic field caused by the final turn-off of the driver. This spike
should be like the spike on top of point "D".
Point "G" shows a slight hump. This is actually the mechanical injector pintle closing. Recall that moving an iron core through a magnetic field
will create a voltage surge. The pintle is the iron core here.
This pintle hump at Point "E" should occur near the end of the downward slope, and not afterwards. If it does occur after the slope has ended
and the voltage has stabilized, it is because the pintle is slightly sticking. Some older Nissan TBI systems suffered from this.
If you see more than one hump it is because of a distorted pintle or seat. This faulty condition is known as "pintle float".
It is important to realize that it takes a good digital storage oscilloscope or analog lab scope to see this pintle hump clearly. Unfortunately, it
cannot always be seen.

Fig. 3: Identifying Current Controlled Type Injector Pattern

CURRENT WAVEFORM SAMPLES
EXAMPLE #1 - VOLTAGE CONTROLLED DRIVER
The waveform pattern shown in Fig. 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 vo l t s, 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.
NOTE:This is GENERAL inform ation. This article is not intended to be specific to any unique situation or
individual vehicle configuration. For m odel-specific inform ation see appropriate articles where
available.
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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. 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. Fig. 6
. Notice that it limits
current flow to 750 milliamps.
The waveform shown in Fig. 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

Fig. 5: Injector Bank w/Excessive Current Flow
- Current Pattern

Fig. 6: Single Injector w/Normal Current Flow
- Current Pattern
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Fig. 7: Single Injector w/Excessive Current Flow
- Current Pattern
EXAMPLE #2 - VOLTAGE CONTROLLED DRIVER
This time we will look at a GM 3.1L V6 VIN [T]. Fig. 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 p o in t . " Fo r
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. 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. Fig. 10
shows a waveform of an individual injector at idle with the Lab Scope set on 200
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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. 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

Fig. 11: Single Injector w/Excessive Current Flow
- Current Pattern
EXAMPLE #4 - CURRENT CONTROLLED DRIVER
Example #4 is of a Ford 4.6L SEFI VIN [W]. See Fig. 12
for the known-good waveform pattern. This Ford system is different from the one
above in EXAMPLE #3
as it peaks at 900 milliamps and the actual opening of the injector (set point) is just below 600 milliamps.
This is offered as a comparison against the Ford pattern listed above, as they are both Ford SEFI injectors but with different operating ranges.
The point is that you should not make any broad assumptions for any manufacturer.

Fig. 12: Single Injector w/Normal Current Flow
- Current Pattern
EXAMPLE #5 - CURRENT CONTROLLED DRIVER
Th e kn o wn - go o d wa ve fo r m in F ig. Fig. 13
is from a Chrysler 3.0L V6 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.
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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. 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 that it shows a faulty injector. See Fig. 16
. Notice that the current went to almost 5
amps and stayed at 1 amp during the hold pattern. Excessive amounts of current flow from bad injectors are a common source of intermittent
computer shutdown. Using a current waveform pattern is the most accurate method of pinpointing this problem.

Fig. 15: Single Injector w/Normal Current Flow
- Current Pattern
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Fig. 16: Single Injector w/Excessive Current Flow
- Current Pattern
EXAMPLE #8 - CURRENT CONTROLLED DRIVER
This known-good CPI system waveform from a GM 4.3L V6 CPI VIN [W] peaks at 4 amps and holds at 1-amp. See Fig. 17
fo r wavefo rm.

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. Fig. 18
illustrates the 64 volt inductive kick on this engine,
indicating no clamping is occurring. The second pattern, Fig. Fig. 19
, was taken during hot idle, closed loop, and no load.
NOTE:This is GENERAL inform ation. This article is not intended to be specific to any unique situation or
individual vehicle configuration. For m odel-specific inform ation see appropriate articles where
available.
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Fig. 18: Injector Bank
- Known Good - Voltage Pattern

Fig. 19: Injector Bank
- Known Good - Voltage Pattern
EXAMPLE #2 - VOLTAGE CONTROLLED DRIVER
The known-good waveform pattern in Fig. Fig. 20
is from a GM 3.8L V6 PFI VIN [3]. It was taken during hot idle, closed loop and no load.
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Fig. 20: Injector Bank
- Known Good - Voltage Pattern
EXAMPLE #3 - VOLTAGE CONTROLLED DRIVER
This known-good waveform pattern, Fig. Fig. 21
, is from a GM 5.0L V8 TPI VIN [F]. It 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.
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