NOTE: This article is intended for general information purposes only. This information may not apply to all makes and models.
* How a DVOM and noid light fall short of a lab scope.
* The two ways injector circuits can be wired, constant ground/switched power & constant power/switched ground.
* The two different pattern types you can use to diagnose with, voltage & current.
* All the valuable details injector patterns can reveal.
of injector patterns you encounter. Of course there are minor differences in each specific system, but that is where a waveform library helps out.
a primary voltage turn-on, firing line, spark line, etc.
The noid light is an excellent “quick and dirty” tool. It can usually be hooked to a fuel injector harness fast and the flashing light is easy to understand. It is a dependable way to identify a no- pulse situation.
* If the wrong one is used for the circuit being tested.
Beware: Just because a connector on a noid light fits the harness does not mean it is the right one.
* If an injector driver is weak or a minor voltage drop is present.
* A dimly flashing light when it should be normal.
* A normal flashing light when it should be dim.
current flow. This lower voltage can cause a dim flash on a noid light designed for full voltage.
* Circuits with current controlled injector drivers (e.g. “Peak and Hold”). Basically, this type of driver allows a quick burst of voltage/current to flow and then throttles it back significantly for the remainder of the pulse width duration. If a noid light was designed for the other type of driver (voltage controlled, e.g. “Saturated”), it will appear dim because it is expecting full voltage/current to flow for the entire duration of the pulse width.
If this is not clear, picture a battery with a lot of corrosion on the terminals. Say there is enough corrosion that the starter motor will not operate; it only clicks. Now imagine turning on the headlights (with the ignition in the RUN position). You find they light normally and are fully bright. This is the same idea as noid light: There is a problem, but enough amp flow exists to operate the headlights (“noid light”), but not the starter motor (“injector”).
* 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.
is used. You now know enough to identify the type of injector circuit. Label the noid light appropriately.
is only important that it flashes.
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.
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.
“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.
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.
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.
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.
using different scales. You can convert freely between them. See RELATIONSHIP BETWEEN DWELL & DUTY CYCLE READINGS TABLE.
only practical way to use a dwell/duty meter, but requires you to have known-good values to compare against.
* Injector mS on-time specification.
* Engine RPM when specification is valid.
* How many times the injectors fire per crankshaft revolution.
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.
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.
100 milliseconds, we know that the injector fires exactly one time.
.6; this equals 59.4 dwell.
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.
(1) – These are just some examples for your understanding.
(2) – Dwell meter on the six-cylinder scale.
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.
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.
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.
a high resistance injector without the solenoid resistor. See the left side of Fig. 1.
as the “peak” time, referring to the fact that current flow is allowed to “peak” (to open the injector).
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.
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.
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.
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.
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.
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.
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 see more than one hump it is because of a distorted pintle or seat. This faulty condition is known as “pintle float”.
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.
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.
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.
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.
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.
under the 12 ohm specification.
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.
200 milliamps per division. Notice the dimple in the rising edge. This dimple indicates the actual opening of the injector (set point) occurred at 400 milliamps and current peaked at 750 milliamps. This is a good specification for this engine.
Fig. 19, was taken during hot idle, closed loop, and no load.
These two known-good waveform patterns are from a Chrysler 3.
0L V6 VIN . The first waveform, Fig. 23, is a dual trace pattern that illustrates how Chrysler uses the rising edge of the engine speed signal to trigger the injectors. The second waveform, Fig. 24, was taken during hot idle, closed loop, and no load.
the injector’s inductive kick to 35-volts on this system. See Fig. 25.
These two known-good waveform patterns are from a GM 2.0L In- Line 4 VIN . Fig. 27 illustrates the 78 volt inductive spike that indicates a zener diode is not used. The second waveform, Fig. 28, was taken during hot idle, closed loop, and no load.