Mitsubishi Montero (1998+). Manual - part 361

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

Fig. 5:  Injector Bank w/Excessive Current Flow - Current Pattern

Fig. 6:  Single Injector w/Normal Current Flow - Current Pattern

Fig. 7:  Single Injector w/Excessive Current Flow - Current Pattern

         EXAMPLE #2 - VOLTAGE CONTROLLED DRIVER

         This time we will look at a GM 3.1L V6 VIN [T].  Fig. 8 shows

the 1, 3, 5 (odd) injector bank with the current waveform indicating

about a 2.6 amp draw at idle. This pattern, taken from a known good

vehicle, correctly stays at or below the maximum 2.6 amps current

range. Ideally, the current for each bank should be very close in

comparison.

         Notice the small dimple on the current flow’s rising edge.

This is the actual injector opening or what engineers refer to as the

"set point." For good idle quality, the set point should be uniform

between the banks.

         When discussing Ohm’s Law as it pertains to this parallel

circuit, consider that each injector has specified resistance of 12.2

ohms. Since all three injectors are in parallel the total resistance

of this parallel circuit drops to 4.1 ohms. Fourteen volts divided by

four ohms would pull a maximum of 3.4 amps on this bank of injectors.

However, as we discussed in EXAMPLE #1 above, other factors knock this

value down to roughly the 2.6 amp neighborhood.

         Now we are going to take a look at the even bank of

injectors; injectors 2, 4, and 6. See Fig. 9. Notice this bank peaked

at 1.7 amps at idle as compared to the 2.6 amps peak of the odd bank (

Fig. 8). Current flow between even and odd injectors banks is not

uniform, yet it is not causing a driveability problem. That is because

it is still under the maximum amperage we figured out earlier. But be

aware this vehicle could develop a problem if the amperage flow

increases any more.

         Checking the resistance of this even injector group with a

DVOM yielded 6.2 ohms, while the odd injector group in the previous

example read 4.1 ohms.

Fig. 8:  Injector Odd Bank w/Normal Current Flow - Current Pattern