Mitsubishi Montero (1991+). Manual - part 319

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

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. 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.

NOTE:    Never apply battery voltage directly across a low resistance

         injector. This will cause injector damage from solenoid coil

         overheating.

         THE TWO WAYS INJECTOR CIRCUITS ARE WIRED

         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

NOTE:    Voltage controlled drivers are also known as "Saturated

         Switch" drivers. They typically require injector circuits

         with a total leg resistance of 12 ohms or more.

NOTE:    This example is based on a constant power/switched ground

         circuit.

     *   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 Point "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 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 at the top of Point "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