Mitsubishi Montero (1991+). Manual - part 320

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

         If you see more than one hump it is because of a distorted

pintle or seat. This faulty condition is known as "pintle float".

         It is important to realize that it takes a good digital

storage oscilloscope or analog lab scope to see this pintle hump

clearly. Unfortunately, it cannot always be seen.

Fig. 2:  Identifying Voltage Controlled Type Injector Pattern

         INTERPRETING A CURRENT CONTROLLED PATTERN

NOTE:    Current controlled drivers are also known as "Peak and Hold"

         drivers. They typically require injector circuits

         with a total leg resistance with less than 12 ohm.

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

         circuit.

     *   See Fig. 3 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 could 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.

         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.

         Right after Point "C", something interesting happens. Notice

the trace starts a normal upward bend. This slight inductive rise is

created by the effects of counter voltage and is normal. This is

because the low circuit resistance allowed a fast build-up of the

magnetic field, which in turn created the counter voltage.

         Point "D" is the start of the current limiting, also known as

the "Hold" time. Before this point, the driver had allowed the current

to free-flow ("Peak") just to get the injector pintle open. By the

time point "D" occurs, the injector pintle has already opened and the

computer has just significantly throttled the current back. It does

this by only allowing a few volts through to maintain the minimum

current required to keep the pintle open.

         The height of the voltage spike seen at the top of 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.

         If you 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 there

is a problem with a weak injector winding.

         If a zener diode is not used in the computer, the spike from

a good injector will be 60 or more volts.

         At Point "E", notice that the trace is now just a few volts

below system voltage and the injector is in the current limiting, or

the "Hold" part of the pattern. This line will either remain flat and

stable as shown here, or will cycle up and down rapidly. Both are

normal methods to limit current flow. Any distortion may indicate

shorted windings.

         Point "F" is the actual turn-off point of the driver (and

injector). To measure the millisecond on-time of the injector, measure

between points "C" and "F". Note that we used cursors to do it for us;

they are measuring a 2.56 mS on-time.

         The top of Point "F" (second inductive kick) is created by

the collapsing magnetic field caused by the final turn-off of the

driver. This spike should be like the spike on top of point "D".

         Point "G" shows a 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. Some older Nissan TBI systems suffered

from this.

         If you see more than one hump it is because of a distorted

pintle or seat. This faulty condition is known as "pintle float".

         It is important to realize that it takes a good digital

storage oscilloscope or analog lab scope to see this pintle hump

clearly. Unfortunately, it cannot always be seen.

Fig. 3:  Identifying Current Controlled Type Injector Pattern

         CURRENT WAVEFORM SAMPLES

         EXAMPLE #1 - VOLTAGE CONTROLLED DRIVER

         The waveform pattern shown in Fig. 4 indicate a normal

current waveform from a Ford 3.0L V6 VIN [U] engine. This voltage

controlled type circuit pulses the injectors in groups of three

injectors. Injectors No. 1, 3, and 5 are pulsed together and cylinders

2, 4, and 6 are pulsed together. The specification for an acceptable

bank resistance is 4.4 ohms. Using Ohm’s Law and assuming a hot run

voltage of 14 volts, we determine that the bank would draw a current

of 3.2 amps.

         However this is not the case because as the injector windings

become saturated, counter voltage is created which impedes the current

flow. This, coupled with the inherent resistance of the driver’s

transistor, impedes the current flow even more. So, what is a known

good value for a dynamic current draw on a voltage controlled bank of

injectors? The waveform pattern shown below indicates a good parallel

injector current flow of 2 amps. See Fig. 4.

         Note that if just one injector has a resistance problem and

partially shorts, the entire parallel bank that it belongs to will

draw more current. This can damage the injector driver.

         The waveform pattern in Fig. 5 indicates this type of problem

with too much current flow. This is on other bank of injectors of the

same vehicle; the even side. Notice the Lab Scope is set on a one amp

per division scale. As you can see, the current is at an unacceptable

2.5 amps.

         It is easy to find out which individual injector is at fault.

All you need to do is inductively clamp onto each individual injector

and compare them. To obtain a known-good value to compare against, we

used the good bank to capture the waveform in  Fig. 6. Notice that it

limits current flow to 750 milliamps.

         The waveform shown in Fig. 7 illustrates the problem injector

we found. This waveform indicates an unacceptable current draw of just

over one amp as compared to the 750 milliamp draw of the known-good

injector. A subsequent check with a DVOM found 8.2 ohms, which is

under the 12 ohm specification.

Fig. 4:  Injector Bank w/Normal Current Flow - Current Pattern