1991 FORD FESTIVA ignition

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GENERAL INFORMATION
How To Use The Engine Perform ance Section - 1989 & Newer Models
* PLEASE READ THIS FIRST *
HOW TO USE THE ENGINE PERFORMANCE SECTION
Congratulations, you have purchased the most advanced automotive repair and service information available. This information can help you, as
a professional automotive technician, to maintain top vehicle performance, and correct driveability problems on today's high-tech vehicles.
For your convenience and ease in use, all of our engine performance service and repair information is consistently organized by manufacturer,
using a progressive diagnostic/workflow approach. Due to the differences in how each manufacturer approaches diagnosis and repair, once
started and inside of an article, that manufacturer may drive the workflow in a direction other than what is outlined here.
The progressive diagnostic/workflow of our data is as follows:
APPLICATION to identify vehicle and system usage.
EMISSION APPLICATION to identify emission system usage.
SPECIFICATIONS to quickly find an engine performance service specification.
ADJUSTMENTS to perform engine performance related routine adjustments.
THEORY & OPERATION to familiarize yourself with new systems and technologies.
BASIC DIAGNOSTIC PROCEDURES located under TESTING & DIAGNOSTICS, also referred to as BASIC TESTING, is used for
performing a basic vehicle inspection and is also the starting point for diagnosis of a "no-start" condition.
SELF-DIAGNOSTICS located under TESTING & DIAGNOSTICS, also referred to as TESTS W/CODES, is where manufacturer
specific procedures for retrieving, identifying and diagnosing DTCs (trouble codes) retained in a control modules memory are located.
TROUBLE SHOOTING - NO CODES located under TESTING & DIAGNOSTICS, also referred to as TESTS W/O CODES, is where
an engine performance problem that does not set a DTC can be potentially isolated through either a SYMPTOM or INTERMITTENTS
duplication procedure.
SYSTEM & COMPONENT TESTING located under TESTING & DIAGNOSTICS, also referred to as SYSTEM/COMPONENT
TESTS, once directed to this article, specific system and component tests can be performed to help isolate faulty component/system
prior to replacement.
PIN VOLTAGE CHARTS provide supplemental information to help determine correct control module input and output signals. Pin
charts may also be referred to as PID charts by some manufacturers.
SENSOR RANGE CHARTS help determine if a sensor is out of calibration. In some cases an out-of-calibration sensor will not set a
DTC (trouble code), resulting in difficult to diagnose driveability symptoms.
VACUUM DIAGRAMS help determine correct routing of vacuum hoses when reinstalling components or performing emission
inspections.
REMOVE, OVERHAUL & INSTALL provides procedures necessary for removing and installing engine performance related
components.
WIRING DIAGRAMS can be used to identify circuits, terminals, wire colors and components referenced in testing procedures. NEW
COLOR WIRING DIAGRAMS (system diagrams) provide an easy method of identifying and tracing circuits.
APPLICATION
INTRODUCTION/ENGINE/VIN ID
Here you will find out how to identify an engine by its Vehicle Identification Number (VIN). The manufacturer's MODEL COVERAGE chart
lists each model and engine option, the fuel system, ignition system and engine code. Engine serial number locations are also shown here, as
well as the VIN code breakdown. Using model lookup in conjunction with VIN and engine ID will identify application information necessary
for servicing vehicle and ordering parts.
EMISSION APPLICATIONS
EMISSION APPLICATION TABLES
Here you will find a chart listing what emission control devices apply to each model. This can be helpful when performing government-
required emissions inspections. For quick reference, major emission systems and devices are listed in bold type in the emission table. Sub
components are listed in light type.
SPECIFICATIONS
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
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SERVICE & ADJUSTMENT SPECIFICATIONS
If you want a specification quickly, this is the place to look. Instead of hunting through a long article, we've separated out the important
specifications and arranged them into easy-to-use tables in a centralized location. You can find valuable information like spark plug wire
resistance, valve clearance, timing, firing orders, etc.
ADJUSTMENT
ON-VEHICLE ADJUSTMENTS
The ON-VEHICLE ADJUSTMENTS article contains the type of information that was previously thought of as TUNE-UP information.
Procedures for checking and adjusting valves, base ignition timing and idle speed are found in this section. Use this section in conjunction with
SERVICE & ADJUSTMENT SPECIFICATIONS for performing routine maintenance. Also, if you have a driveability problem, ensure all on-
vehicle adjustments are within specification before attempting further diagnosis.
THEORY & OPERATION
This article covers basic THEORY & OPERATION of engine performance-related systems and components. Before diagnosing vehicles or new
systems with which you are not completely familiar, read this article.
TESTING & DIAGNOSTICS
BASIC DIAGNOSTIC PROCEDURES/BASIC TESTING
The procedures listed in this article can help you avoid skipping a simple step early, like checking base timing, which could be costly in both
time and money later. This is also a potential starting point for diagnosis of a "no-start" condition. If all systems check out okay here, proceed
to SELF-DIAGNOSTICS/TESTS W/CODES or TROUBLE SHOOTING - NO CODES/TESTS W/O CODES article.
SELF-DIAGNOSTICS/TESTS W/CODES
Use this information to retrieve and interpret Diagnostic Trouble Codes (DTCs) accessed from the vehicle's self-diagnostic system. Once
information is retrieved, manufacturer diagnostic procedures are given to help pinpoint and repair computer system/component faults. Also
included are steps for clearing trouble codes once these faults are repaired. If there is a driveability symptom with no trouble codes set,
proceed to TROUBLE SHOOTING - NO CODES/TESTS W/O CODES article.
TROUBLE SHOOTING - NO CODES/TESTS W/O CODES
This is where to go when you have a problem that does not set a trouble code. It can help determine cause of problem using driveability
symptoms and intermittent testing procedures. Procedures in this information should lead you to a specific component or system test.
SYSTEM & COMPONENT TESTING
Here you will find various tests for engine performance systems and their components, such as air induction (turbochargers and superchargers),
fuel control, ignition control and emission systems.
PIN VOLTAGE CHARTS
These are supplied (when available from manufacturer) to quicken the diagnostic process. By checking pin voltages at the Powertrain Control
Module (PCM), you can determine if the PCM is receiving and/or transmitting proper voltage signals. Pin charts may also be referred to as PID
charts by some manufacturers.
SENSOR RANGE CHARTS
SENSOR OPERATING RANGE CHARTS
These are supplied (when available from manufacturer) to determine if a sensor is out of calibration. An out-of-calibration sensor may not set a
trouble code, but it may cause driveability problems.
VACUUM DIAGRAMS
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
NOTE:T his article is generic in nature and all inform ation does not apply to all vehicles. For vehicle specific
inform ation, see the appropriate articles in the ENGINE PERFORMANCE category.
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GENERAL INFORMATION
Parasitic Load Explanation & T est Procedures
* PLEASE READ THIS FIRST *
GENERAL INFORMATION
The term Parasitic Load refers to electrical devices that continue to use or draw current after the ignition switch is turned to OFF position. This
small amount of continuous battery draw is expressed in milliamps (mA). On Chrysler vehicles, a typical Parasitic Load should be no more
than 30 milliamps (0.030 amps). On Ford Motor Co. and General Motors vehicles produced after 1980, a typical Parasitic Load should be no
more than 50 milliamps (0.050 amps).
Vehicles produced since 1980 have memory devices that draw current with ignition off for as long as 20 minutes before shutting down the
Parasitic Drain. When Parasitic Load exceeds normal specifications, the vehicle may exhibit dead battery and no-start condition.
Follow test procedure for checking Parasitic Loads to completion. A brief overview of a suggested test procedure is included along with some
typical Parasitic Load specifications. Refer to GENERAL MOTORS PARASITIC LOAD TABLE chart.
TESTING FOR PARASITIC LOAD
The battery circuit must be opened to connect test switch (shunt) and ammeter into the circuit. When a battery cable is removed, timer circuits
within the vehicle computer are interrupted and immediately begin to discharge. If in doubt about the condition of the ammeter fuse, test it
with an ohmmeter prior to beginning test. An open fuse will show the same reading (00.00) as no parasitic drain. Begin test sequence with the
meter installed and on the 10-amp scale. Select lower scale to read parasitic draw.
CHRYSLER IGNITION OFF DRAW (IOD) TEST
To test for excessive IOD, verify that all electrical accessories are OFF. Turn off all lights, remove ignition key, and close all doors and decklid.
If the vehicle is equipped with electronic accessories (illuminated entry, automatic load leveler, body computer, or high line radio), allow the
system to automatically shut off (time out), up to 3 minutes.
1. Raise the hood and disconnect both battery cables, negative first.
2. Reconnect the negative cable and connect a typical 12-volt test light (low wattage bulb) between the positive cable clamp and the
positive battery post. Remove the engine compartment lamp bulb. If the test light does not light, proceed to step 3
. If the test light does
light, proceed to step, 4
. The test light will indicate IOD greater than 3 amps. After higher amperage IOD has been corrected, proceed to
step 3
.
3. ith 12-volt test light still connected (not lit), connect an ammeter (milliampere scale) between the positive cable clamp and the positive
battery post, disconnect test light, refer to instructions provided with ammeter being used. A reading of 30 milliamperes or less indicates
normal electrical draw. If ammeter reads more than 30 milliamperes, excessive IOD must be corrected.
4. Locate the fuse panel and remove fuses or circuit breakers one at a time, and observe ammeter after each fuse or circuit breaker is
removed. If test light goes out and the reading drops below 30 milliamperes when a certain fuse or circuit breaker is removed, that circuit
may have a defect.
5. If IOD is detected after all fuses and circuit breakers have been removed, disconnect the 60-way connector at the Single Module Engine
Control (SMEC), located outboard of the battery.
6. If excessive IOD is detected after all fused circuits and SMEC have been verified, disconnect the B+ terminal from the alternat o r. If
reading drops below 30 milliamperes, reinstall all fuses and circuit breakers, reconnect B+ terminal at alternator, reconnect battery, and
perform alternator diagnostics.
7. Install engine compartment lamp bulb.
TEST PROCEDURE USING TEST SWITCH
1. Turn ignition off. Remove negative battery terminal cable. Install Disconnect Tool (J-38758) test switch male end to negative battery
cable. Turn test switch knob to OFF position (current through meter). Install negative battery cable to the female end of test switch. 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.
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.
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.
CAUT ION: Always turn ignition off when connecting or disconnecting battery cables, battery chargers or jum per
cables. DO NOT turn test switch to OFF position (which causes current to run through am m eter or
vehicle electrical system ).
NOTE:Mem ory functions of various accessories m ust be reset after the battery is reconnected.
CAUT ION: IOD greater than 3 am ps m ay dam age m illam pm eter.
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2. Turn test switch knob to ON position (current through switch). Road test vehicle with vehicle accessories on (radio, air conditioner, etc).
After road test, turn ignition switch to LOCKED position and remove key. Connect ammeter terminals to test switch terminals. See Fig.
1. Select 10-amp scale.
3. Turn off all electrical accessories. Turn off interior lights, underhood lamp, trunk light, illuminated entry, etc. To avoid damaging
ammeter or obtaining a false meter reading, all accessories must be off before turning test switch knob to OFF position.
4. Turn test switch knob to OFF position to allow current to flow through ammeter. If meter reads wrong polarity, turn test switch to ON
position and reverse leads. Turn test switch to OFF position. Observe current reading. If reading is less than 2 amps, turn test switch to
ON position to keep electrical circuits powered-up.
5. Select low amp scale. Switch lead to the correct meter position. Turn test switch to OFF position and compare results to normal current
draw. See GENERAL MOTORS PARASITIC LOAD TABLE (MILLIAMPS)
. If current draw is unusually high for the vehicle's
overall electrical system, remove system fuses one at a time until current draw returns to normal.
6. Turn test switch to ON position each time door is opened or fuse is removed. Turn switch to OFF position to read current draw va l u e
through meter. When the cause of excessive current drain has been located and repaired, remove test switch and reconnect negative
battery cable to the negative battery terminal.
INTERMITTENT PARASITIC LOAD PROBLEMS
Intermittent parasitic load can occur because of a memory device that does not power down with ignition off. With an intermittent parasitic
load, battery draw can be greater than 1.0 amp.
To find an intermittent problem requires that an ammeter and Disconnect Tool (J-38758) test switch be connected and left in the circuit. See
Fig. 1
. Road test vehicle. After road test, turn ignition off and remove key.
Monitor the milliamps scale for 15-20 minutes after ignition is turned off. This allows monitoring memory devices to determine if they time out
and stop drawing memory current. The test switch is needed to protect ammeter when the vehicle is started.

Fig. 1: Connecting Kent
-Moore Disconnect Tool (J-38758)
Courtesy of GENERAL MOTORS CORP.
GENERAL MOTORS PARASITIC LOAD
ComponentNormal DrawMaximum DrawTime-Out (Minutes)
Anti-Theft System0.41.0.....
Auto Door Lock1.01.0.....
Body Control Module3.612.420
Central Processing System1.62.720
Electronic Control Module5.610.0.....
Electronic Level Control2.03.320
Heated Windshield Module0.30.4.....
HVAC Power Module1.01.0.....
Illuminated Entry1.01.01
Light Control Module0.51.0.....
Oil Level Module0.10.1.....
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GENERAL INFORMATION
Waveform s - Injector Pattern T utorial
* PLEASE READ THIS FIRST *
PURPOSE OF THIS ARTICLE
Learning how to interpret injector drive patterns from a Lab Scope can be like learning ignition patterns all over again. This article exists to
ease you into becoming a skilled injector pattern interpreter.
You will learn:
How a DVOM and noid light fall short of a lab scope.
The two types of injector driver circuits, voltage controlled & current controlled.
The two ways injector circuits can be wired, constant ground/switched power & constant power/switched ground.
The two different pattern types you can use to diagnose with, voltage & current.
All the valuable details injector patterns can reveal.
SCOPE OF THIS ARTICLE
This is NOT a manufacturer specific article. All different types of systems are covered here, regardless of the specific year/make/model/engine.
The reason for such broad coverage is because there are only a few basic ways to operate a solenoid-type injector. By understanding the
fundamental principles, you will understand all the major points of injector patterns you encounter. Of course there are minor differences in
each specific system, but that is where a waveform library helps out.
If this is confusing, consider a secondary ignition pattern. Even though there are many different implementations, each still has a primary
voltage turn-on, firing line, spark line, etc.
If specific waveforms are available in On Demand for the engine and vehicle you are working on, you will find them in the Engine Performance
section under the Engine Performance category.
IS A LAB SCOPE NECESSARY?
INTRODUCTION
You probably have several tools at your disposal to diagnose injector circuits. But you might have questioned "Is a lab scope necessary to do a
thorough job, or will a set of noid lights and a multifunction DVOM do just as well?"
In the following text, we are going to look at what noid lights and DVOMs do best, do not do very well, and when they can mislead you. As
you might suspect, the lab scope, with its ability to look inside an active circuit, comes to the rescue by answering for the deficiencies of these
other tools.
OVERVIEW OF NOID LIGHT
The noid light is an excellent "quick and dirty" tool. It can usually be hooked to a fuel injector harness fast and the flashing l igh t is e a sy t o
understand. It is a dependable way to identify a no-pulse situation.
However, a noid light can be very deceptive in two cases:
If the wrong one is used for the circuit being tested. Beware: Just because a connector on a noid light fits the harness does not mean it is
the right one.
If an injector driver is weak or a minor voltage drop is present.
Use the Right Noid Light
In the following text we will look at what can happen if the wrong noid light is used, why there are different types of noid lights (besides
differences with connectors), how to identify the types of noid lights, and how to know the right type to use.
First, let's discuss what can happen if the incorrect type of noid light is used. You might see:
A dimly flashing light when it should be normal.
A normal flashing light when it should be dim.
A noid light will flash dim if used on a lower voltage circuit than it was designed for. A normally operating circuit would appear
underpowered, which could be misinterpreted as the cause of a fuel starvation problem.
Here are the two circuit types that could cause this problem: 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.
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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Circuits with external injector resistors. Used predominately on some Asian & European systems, they are used to reduce the available
voltage to an injector in order to limit the current flow. This lower voltage can cause a dim flash on a noid light designed for full voltage.
Circuits with current controlled injector drivers (e.g. "Peak and Hold"). Basically, this type of driver allows a quick burst of
voltage/current to flow and then throttles it back significantly for the remainder of the pulse width duration. If a noid light was designed
for the other type of driver (voltage controlled, e.g. "Saturated"), it will appear dim because it is expecting full voltage/current to flow
for the entire duration of the pulse width.
Let's move to the other situation where a noid light flashes normally when it should be dim. This could occur if a more sensitive n o id l igh t is
used on a higher voltage/amperage circuit that was weakened enough to cause problems (but not outright broken). A circuit with an actual
problem would thus appear normal.
Let's look at why. A noid light does not come close to consuming as much amperage as an injector solenoid. If there is a partial driver failure
or a minor voltage drop in the injector circuit, there can be adequate amperage to fully operate the noid light BUT NOT ENOUGH TO
OPERATE THE INJECTOR.
If this is not clear, picture a battery with a lot of corrosion on the terminals. Say there is enough corrosion that the starter motor will not
operate; it only clicks. Now imagine turning on the headlights (with the ignition in the RUN position). You find they light normally and are
fully bright. This is the same idea as noid light: There is a problem, but enough amp flow exists to operate the headlights ("noid light"), but not
the starter motor ("injector").
How do you identify and avoid all these situations? By using the correct type of noid light. This requires that you understanding the types of
injector circuits that your noid lights are designed for. There are three. They are:
Systems with a voltage controlled injector driver. Another way to say it: The noid light is designed for a circuit with a "high" resistance
injector (generally 12 ohms or above).
Systems with a current controlled injector driver. Another way to say it: The noid light is designed for a circuit with a low resistance
injector (generally less than 12 ohms) without an external injector resistor.
Systems with a voltage controlled injector driver and an external injector resistor. Another way of saying it: The noid light is designed
for a circuit with a low resistance injector (generally less than 12 ohms) and an external injector resistor.
If you are not sure which type of circuit your noid light is designed for, plug it into a known good car and check out the results. If it flashes
normally during cranking, determine the circuit type by finding out injector resistance and if an external injector resistor is used. You now
know enough to identify the type of injector circuit. Label the noid light appropriately.
Next time you need to use a noid light for diagnosis, determine what type of injector circuit you are dealing with and select the appropriate
noid light.
Of course, if you suspect a no-pulse condition you could plug in any one whose connector fit without fear of misdiagnosis. This is because it is
unimportant if the flashing light is dim or bright. It is only important that it flashes.
In any cases of doubt regarding the use of a noid light, a lab scope will overcome all inherent weaknesses.
OVERVIEW OF DVOM
A DVOM is typically used to check injector resistance and available voltage at the injector. Some techs also use it check injector on-time
either with a built-in feature or by using the dwell/duty function.
There are situations where the DVOM performs these checks dependably, and other situations where it can deceive you. It is important to be
aware of these strengths and weaknesses. We will cover the topics above in the following text.
Checking Injector Resistance
If a short in an injector coil winding is constant, an ohmmeter will accurately identify the lower resistance. The same is true with an open
winding. Unfortunately, an intermittent short is an exception. A faulty injector with an intermittent short will show "good" if the ohmmeter
cannot force the short to occur during testing.
Alcohol in fuel typically causes an intermittent short, happening only when the injector coil is hot and loaded by a current high e n o u gh t o
jump the air gap between two bare windings or to break down any oxides that may have formed between them.
When you measure resistance with an ohmmeter, you are only applying a small current of a few milliamps. This is nowhere near enough to
load the coil sufficiently to detect most problems. As a result, most resistance checks identify intermittently shorted injectors as being normal.
There are two methods to get around this limitation. The first is to purchase an tool that checks injector coil windings under full load. The
Kent-Moore J-39021 is such a tool, though there are others. The Kent-Moore costs around $240 at the time of this writing and works on many
different manufacturer's systems.
The second method is to use a lab scope. Remember, a lab scope allows you to see the regular operation of a circuit in real time. If an injector
is having an short or intermittent short, the lab scope will show it.
Checking Available Voltage At the Injector
Verifying a fuel injector has the proper voltage to operate correctly is good diagnostic technique. Finding an open circuit on the feed circuit
like a broken wire or connector is an accurate check with a DVOM. Unfortunately, finding an intermittent or excessive resistance problem with
a DVOM is unreliable.
Let's explore this drawback. Remember that a voltage drop due to excessive resistance will only occur when a circuit is operating? Since the
injector circuit is only operating for a few milliseconds at a time, a DVOM will only see a potential fault for a few milliseconds. The remaining
90+% of the time the unloaded injector circuit will show normal battery voltage. NOTE:Som e noid lights can m eet both the second and third categories sim ultaneously.
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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. 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 times by increasing injector pulse width accordingly.

Fig. 1: Injector Driver Types
- Current and Voltage
CURRENT CONTROLLED CIRCUIT ("PEAK & HOLD")
The current controlled driver inside the computer is more complex than a voltage controlled driver because as the name implies, it has to limit
current flow in addition to its ON-OFF switching function. Recall, this driver typically requires injector circuits with a total leg resistance of
less than 12 ohms.
Once the driver is turned ON, it will not limit current flow until enough time has passed for the injector pintle to open. This period is preset by
the particular manufacturer/system based on the amount of current flow needed to open their injector. This is typically between two and six
amps. Some manufacturers refer to this as the "peak" time, referring to the fact that current flow is allowed to "peak" (to open the injector).
Once the injector pintle is open, the amp flow is considerably reduced for the rest of the pulse duration to protect the injector from
overheating. This is okay because very little amperage is needed to hold the injector open, typically in the area of one amp or less. Some
manufacturers refer to this as the "hold" time, meaning that just enough current is allowed through the circuit to "hold" the already-open
injector open.
There are a couple methods of reducing the current. The most common trims back the available voltage for the circuit, similar to turning down
a light at home with a dimmer.
The other method involves repeatedly cycling the circuit ON-OFF. It does this so fast that the magnetic field never collapses and the pintle
stays open, but the current is still significantly reduced. See the right side of Fig. Fig. 1
for an illustration.
The advantage to the current controlled driver circuit is the short time period from when the driver transistor goes ON to when the injector
actually opens. This is a function of the speed with which current flow reaches its peak due to the low circuit resistance. Also, the injector
closes faster when the driver turns OFF because of the lower holding current.
THE TWO WAYS INJECTOR CIRCUITS ARE WIRED
NOTE:Never apply battery voltage directly across a low resistance injector. T his will cause injector dam age
from solenoid coil overheating.
NOTE:Never apply battery voltage directly across a low resistance injector. T his will cause injector dam age
from solenoid coil overheating.
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Like other circuits, injector circuits can be wired in one of two fundamental directions. The first method is to steadily power the injectors and
have the computer driver switch the ground side of the circuit. Conversely, the injectors can be steadily grounded while the driver switches the
power side of the circuit.
There is no performance benefit to either method. Voltage controlled and current controlled drivers have been successfully implemented both
ways.
However, 95% percent of the systems are wired so the driver controls the ground side of the circuit. Only a handful of systems use the drivers
on the power side of the circuit. Some examples of the latter are the 1970's Cadillac EFI system, early Jeep 4.0 EFI (Renix system), and
Chrysler 1984-87 TBI.
INTERPRETING INJECTOR WAVEFORMS
INTERPRETING A VOLTAGE CONTROLLED PATTERN
See Fig. 2 for pattern that the following text describes.
Point "A" is where system voltage is supplied to the injector. A good hot run voltage is usually 13.5 or more volts. This point, commonly
known as open circuit voltage, is critical because the injector will not get sufficient current saturation if there is a voltage shortfall. To obtain a
good look at this precise point, you will need to shift your Lab Scope to five volts per division.
You will find that some systems have slight voltage fluctuations here. This can occur if the injector feed wire is also used to power up other
cycling components, like the ignition coil(s). Slight voltage fluctuations are normal and are no reason for concern. Major voltage fluctuations
are a different story, however. Major voltage shifts on the injector feed line will create injector performance problems. Look for excessive
resistance problems in the feed circuit if you see big shifts and repair as necessary.
Note that circuits with external injector resistors will not be any different because the resistor does not affect open circuit voltage.
Point "B" is where the driver completes the circuit to ground. This point of the waveform should be a clean square point straight down with no
rounded edges. It is during this period that current saturation of the injector windings is taking place and the driver is heavily stressed. Weak
drivers will distort this vertical line.
Point "C" represents the voltage drop across the injector windings. Point "C" should come very close to the ground reference point, but not
quite touch. This is because the driver has a small amount of inherent resistance. Any significant offset from ground is an indication of a
resistance problem on the ground circuit that needs repaired. You might miss this fault if you do not use the negative battery post for your Lab
Scope hook-up, so it is HIGHLY recommended that you use the battery as your hook-up.
The points between "B" and "D" represent the time in milliseconds that the injector is being energized or held open. This line at Po int "C"
should remain flat. Any distortion or upward bend indicates a ground problem, short problem, or a weak driver. Alert readers will catch that
this is exactly opposite of the current controlled type drivers (explained in the next section), because they bend upwards at this point.
How come the difference? Because of the total circuit resistance. Voltage controlled driver circuits have a high resistance of 12+ ohms that
slows the building of the magnetic field in the injector. Hence, no counter voltage is built up and the line remains flat.
On the other hand, the current controlled driver circuit has low resistance which allows for a rapid magnetic field build-up. This causes a
slight inductive rise (created by the effects of counter voltage) and hence, the upward bend. You should not see that here with voltage
controlled circuits.
Point "D" represents the electrical condition of the injector windings. The height of this voltage spike (inductive kick) is proportional to the
number of windings and the current flow through them. The more current flow and greater number of windings, the more potential fo r 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
min imu m 3 5 vo l t s at t h e t o p o f Po in t "D".
If you do 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 the injector has a weak winding.
If a zener diode is not used in the computer, the spike from a good injector will be 60 or more volts.
Point "E" brings us to a very interesting section. As you can see, the voltage dissipates back to supply value after the peak of the inductive kick.
Notice the 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 because of a faulty injector 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.
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.
NOTE:Voltage controlled drivers are also known as "Saturated Switch" drivers. T hey typically require injector
circuits with a total leg resistance of 12 ohm s or m ore.
NOTE:T his exam ple is based on a constant power/switched ground circuit.
Page 6 of 19 MITCHELL 1 ARTICLE - GENERAL INFORMATION Waveforms - Injector Pattern Tutorial
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