Mitsubishi Montero (1998+). Manual - part 358

WAVEFORMS - INJECTOR PATTERN TUTORIAL 

1998 Mitsubishi Montero

         GENERAL INFORMATION

         Waveforms - Injector Pattern Tutorial

         * PLEASE READ THIS FIRST *

NOTE:    This article is intended for general information purposes

         only. This information may not apply to all makes and models.

         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

light is easy to 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:

      *  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

noid light 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.

NOTE:    Some noid lights can meet both the second and third

         categories simultaneously.

         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 enough to 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.

         Since DVOMs update their display roughly two to five times a

second, all measurements in between are averaged. Because a potential

voltage drop is visible for such a small amount of time, it gets

"averaged out", causing you to miss it.

         Only a DVOM that has a "min-max" function that checks EVERY

MILLISECOND will catch this fault consistently (if used in that mode).

The Fluke 87 among others has this capability.

         A "min-max" DVOM with a lower frequency of checking (100

millisecond) can miss the fault because it will probably check when

the injector is not on. This is especially true with current

controlled driver circuits. The Fluke 88, among others fall into this

category.

         Outside of using a Fluke 87 (or equivalent) in the 1 mS "min-

max" mode, the only way to catch a voltage drop fault is with a lab

scope. You will be able to see a voltage drop as it happens.

         One final note. It is important to be aware that an injector

circuit with a solenoid resistor will always show a voltage drop when

the circuit is energized. This is somewhat obvious and normal; it is a

designed-in voltage drop. What can be unexpected is what we already

covered--a voltage drop disappears when the circuit is unloaded. The

unloaded injector circuit will show normal battery voltage at the

injector. Remember this and do not get confused.

         Checking Injector On-Time With Built-In Function

         Several DVOMs have a feature that allows them to measure

injector on-time (mS pulse width). While they are accurate and fast to

hookup, they have three limitations you should be aware of:

      *  They only work on voltage controlled injector drivers (e.g

         "Saturated Switch"), NOT on current controlled injector

          drivers (e.g. "Peak & Hold").

      *  A few unusual conditions can cause inaccurate readings.

      *  Varying engine speeds can result in inaccurate readings.

         Regarding the first limitation, DVOMs need a well-defined

injector pulse in order to determine when the injector turns ON and

OFF. Voltage controlled drivers provide this because of their simple

switch-like operation. They completely close the circuit for the

entire duration of the pulse. This is easy for the DVOM to interpret.

         The other type of driver, the current controlled type, start

off well by completely closing the circuit (until the injector pintle

opens), but then they throttle back the voltage/current for the

duration of the pulse. The DVOM understands the beginning of the pulse