Mitsubishi Montero (1998+). Manual - part 359

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

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

times by increasing injector pulse width accordingly.

NOTE:    Never apply battery voltage directly across a low resistance

         injector. This will cause injector damage from solenoid coil

         overheating.

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