Mitsubishi Montero (1991+). Manual - part 318

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

but it cannot figure out the throttling action. In other words, it

cannot distinguish the throttling from an open circuit (de-energized)

condition.

         Yet current controlled injectors will still yield a

millisecond on-time reading on these DVOMs. You will find it is also

always the same, regardless of the operating conditions. This is

because it is only measuring the initial completely-closed circuit on-

time, which always takes the same amount of time (to lift the injector

pintle off its seat). So even though you get a reading, it is useless.

         The second limitation is that a few erratic conditions can

cause inaccurate readings. This is because of a DVOM’s slow display

rate; roughly two to five times a second. As we covered earlier,

measurements in between display updates get averaged. So conditions

like skipped injector pulses or intermittent long/short injector

pulses tend to get "averaged out", which will cause you to miss

important details.

         The last limitation is that varying engine speeds can result

in inaccurate readings. This is caused by the quickly shifting

injector on-time as the engine load varies, or the RPM moves from a

state of acceleration to stabilization, or similar situations. It too

is caused by the averaging of all measurements in between DVOM display

periods. You can avoid this by checking on-time when there are no RPM

or load changes.

         A lab scope allows you to overcome each one of these

limitations.

         Checking Injector On-Time With Dwell Or Duty

         If no tool is available to directly measure injector

millisecond on-time measurement, some techs use a simple DVOM dwell or

duty cycle functions as a replacement.

         While this is an approach of last resort, it does provide

benefits. We will discuss the strengths and weaknesses in a moment,

but first we will look at how a duty cycle meter and dwell meter work.

         How A Duty Cycle Meter and Dwell Meter Work

         All readings are obtained by comparing how long something has

been OFF to how long it has been ON in a fixed time period. A dwell

meter and duty cycle meter actually come up with the same answers

using different scales. You can convert freely between them. See

RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE .

         The DVOM display updates roughly one time a second, although

some DVOMs can be a little faster or slower. All measurements during

this update period are tallied inside the DVOM as ON time or OFF time,

and then the total ratio is displayed as either a percentage (duty

cycle) or degrees (dwell meter).

         For example, let’s say a DVOM had an update rate of exactly 1

second (1000 milliseconds). Let’s also say that it has been

measuring/tallying an injector circuit that had been ON a total of 250

mS out of the 1000 mS. That is a ratio of one-quarter, which would be

displayed as 25% duty cycle or 15  dwell (six-cylinder scale). Note

that most duty cycle meters can reverse the readings by selecting the

positive or negative slope to trigger on. If this reading were

reversed, a duty cycle meter would display 75%.

         Strengths of Dwell/Duty Meter

         The obvious strength of a dwell/duty meter is that you can

compare injector on-time against a known-good reading. This is the

only practical way to use a dwell/duty meter, but requires you to have

known-good values to compare against.

         Another strength is that you can roughly convert injector mS

on-time into dwell reading with some computations.

         A final strength is that because the meter averages

everything together it does not miss anything (though this is also a

severe weakness that we will look at later). If an injector has a

fault where it occasionally skips a pulse, the meter registers it and

the reading changes accordingly.

         Let’s go back to figuring out dwell/duty readings by using

injector on-time specification. This is not generally practical, but

we will cover it for completeness. You NEED to know three things:

      *  Injector mS on-time specification.

      *  Engine RPM when specification is valid.

      *  How many times the injectors fire per crankshaft revolution.

         The first two are self-explanatory. The last one may require

some research into whether it is a bank-fire type that injects every

360  of crankshaft rotation, a bank-fire that injects every 720 , or

an SFI that injects every 720 . Many manufacturers do not release this

data so you may have to figure it out yourself with a frequency meter.

         Here are the four complete steps to convert millisecond on-

time:

         1) Determine the injector pulse width and RPM it was obtained

at. Let’s say the specification is for one millisecond of on-time at a

hot idle of 600 RPM.

         2) Determine injector firing method for the complete 4 stroke

cycle. Let’s say this is a 360  bank-fired, meaning an injector fires

each and every crankshaft revolution.

         3) Determine how many times the injector will fire at the

specified engine speed (600 RPM) in a fixed time period. We will use

100 milliseconds because it is easy to use.

         Six hundred crankshaft Revolutions Per Minute (RPM) divided

by 60 seconds equals 10 revolutions per second.

         Multiplying 10 times .100 yields one; the crankshaft turns

one time in 100 milliseconds. With exactly one crankshaft rotation in

100 milliseconds, we know that the injector fires exactly one time.

         4) Determine the ratio of injector on-time vs. off-time in

the fixed time period, then figure duty cycle and/or dwell. The

injector fires one time for a total of one millisecond in any given

100 millisecond period.

         One hundred minus one equals 99. We have a 99% duty cycle. If

we wanted to know the dwell (on 6 cylinder scale), multiple 99% times

.6; this equals 59.4  dwell.

         Weaknesses of Dwell/Duty Meter

         The weaknesses are significant. First, there is no one-to-one

correspondence to actual mS on-time. No manufacturer releases

dwell/duty data, and it is time-consuming to convert the mS on-time

readings. Besides, there can be a large degree of error because the

conversion forces you to assume that the injector(s) are always firing

at the same rate for the same period of time. This can be a dangerous

assumption.

         Second, all level of detail is lost in the averaging process.

This is the primary weakness. You cannot see the details you need to

make a confident diagnosis.

         Here is one example. Imagine a vehicle that has a faulty

injector driver that occasionally skips an injector pulse. Every

skipped pulse means that that cylinder does not fire, thus unburned O2

gets pushed into the exhaust and passes the O2 sensor. The O2 sensor

indicates lean, so the computer fattens up the mixture to compensate

for the supposed "lean" condition.

         A connected dwell/duty meter would see the fattened pulse

width but would also see the skipped pulses. It would tally both and

likely come back with a reading that indicated the "pulse width" was

within specification because the rich mixture and missing pulses

offset each other.

         This situation is not a far-fetched scenario. Some early GM