Theory of Operation
The fuel injector is the main actuator in a modern fuel
injection systems. It is responsible for supplying the engine with fuel for
combustion. In order to obtain the near perfect air/fuel ratio in today’s
engines, the injector must meter and deliver a precise amount of fuel into
the intake runners. The correct fuel flow and spray pattern can only be
achieved, over a long period of time, through a well-maintained injector. In
modern OBD II systems, injectors are closely associated with misfire code
problems. There are many reasons why an injector could cause a misfire code.
A shorted injector coil that draws too much current, a bad injector driver,
an ECM that cuts pulsation to the injector due to an overheating problem to
keep the engine cooler and clogged injectors are all possible conditions
that will set those persistent misfire codes. In some cases, as in an
overheating engine, the problem is not the injector itself but some other
condition that causes the injector not to pulse and therefore create the
misfire.
The vast majority of fuel injectors are ground controlled.
This means that of the two wires going to the injector one is held at steady
12 – 14 volts while the other lead is pulsed to ground by the ECM. This type
of injector circuit is called negative trigger. There are, however, a
few (European) manufacturers that have used positive or battery voltage
trigger injector circuitry in the past. With positive injector trigger, the
positive side is the one being triggered by the ECM.
The component of the ECM that triggers the injector is
called the driver. Injector drivers fall into two categories, the
saturation and the
peak-and-hold type driver. The injector driver itself is nothing
more than a high current transistor and its main function is to switch the
injector on and off.
NOTE: To determine
if an injector circuit is ground or positive controlled, simply turn
the ignition key on and with a test light, probe for battery voltage
at either side of the injector connector. Steady battery voltage
seen at either side of the two-prong connector points to the circuit
being ground controlled.
NOTE: The above test will not
work on Chrysler vehicles. This manufacturer uses an ASD (automatic
shut down) relay, which activates the battery side of the injector
so long as the crank sensor is putting out a signal. If the engine
is at KOEO, the ASD relay is deactivated and the battery side of the
injector is held at ground.
A good number of late model ECMs are using the more advanced
microprocessors, with 32 bit processor systems on most of them. These
computer systems are capable of shutting down the injector driver in the
event of a short circuit, a severe misfire, or an overheating engine. The
Cadillac Northstar 4.6L engine was one of the first systems to employ such
an ECM. The system shuts
down the injectors intermittently in the event of an overheating engine
in order for it to work cooler and therefore protect the head gaskets. It is
important to determine if the misfire or lack of injector pulse is the
result of an ECM strategy to save the engine from damage or an actual
malfunction.
Fig 2 – Detailed analysis of a saturation type fuel injector
waveform.
The
1st point (inj. Turnon)
shows the ECM driver pulling the
battery voltage to ground. This action turns the injector ON. The
2nd point
in the waveform or the space between
the two vertical lines gives us the injector pulse duration. In this
case about 4.5 mS. Then 3rd point
is the injector turn-off. The vertical
lines at the injector turn-on and turn-off points, should be clean and well
defined. These lines show the condition of the ECM’s internal driver
transistor. The
4th point
is the inductive kick. This relatively
high voltage spike, resulting form the collapsing magnetic field around the
injector coil, is the main indicator of the general condition of the coil
itself. The voltage usually ranges between 55 and 90 volts, with 65 volts
being the norm. A low voltage inductive kick is a sure indication of an
electrical problem. As said
before shorted injector coil windings or any resistance in the injector
circuit will show up in the voltage waveform as a low voltage inductive
kick.
As a side note, always remember that in some systems this
inductive kick is clipped off. This is done through an internal ECM diode at
around 30 to 45 volts and
does not indicate a defective injector. In such systems, the upper part
of the spike is squared-off or flat, typical of a diode clipping action. The
5th and last point
of interest is the injector-pinttleclosing hump.
This hump is not present in all injector waveforms and with practice a
determination can be made as to which systems do show it.
The closing hump is an indicator of injector mechanical
condition. If it is placed too far up the position shown in fig. 2, then it
is a good sign of a dirty or clogged injector. If it is too far down, then
the injector valve spring is weak. With some experience a fair and accurate
determination can be made saving time and money.
INJECTOR DRIVERS
• The saturation driver is the more
common of the two types. This driver transistor usually works together with
a high impedance injector. High impedance injectors take their
name because of their higher internal resistance (usually form 12 to 20
Ohms). These injectors are mostly used in multi-port injection systems.
There are cases, however, in which low impedance injectors are used in
multi-port applications, but such cases are rare.
• The peak-and-hold driver is almost
always connected to a low
impedance injectors. This name is given because of their low
internal resistance (usually from 1 to 5 Ohms). The low impedance injector
is mostly used in TBI applications and generally requires a higher amount
current to operate.
The peak-and-hold injector gets its name from its waveform
characteristic. The actual current peaks at a certain level (4 to 6 Amps) so
as to open the injector and then levels off at about 1 Amp to keep the
injector open. Point (1 – A) is the injector turn-on. This is the
point at which the ECM driver transistor grounds the injector coil. At this
point the voltage goes low (grounded) and the current slopes up to about 4-6
Amps. The ECM does this to quickly open the injector. It takes a lot more
current to force an injector to open (break the inertia) than to leave it
open. This type of injector is used mainly in TBI applications, with it
being bigger and heavier. The current needed to break the injector pinttle
inertia is generally higher, hence the higher peak-current level. Point (
B ) is the injector peak duration. Injector peak times should never fall
bellow 1.5 mS. Injectors with shorted windings will tend to peak much faster
due to the low impedance of the windings. A range of 1.5 to 3 mS is normal.
Point (2 – C ) is the injector peak current/inductive voltage kick.
At this point the peak phase ends and the injector driver transistor goes
into the hold phase of the injector pulse. Peak current range from 4 to 6
Amps, with voltage values of around 60 to 90 volts. As in the saturation
type injector, a lower inductive kick is an indication of a problem with the
injector circuit or coil windings. Point (3 – D) is the injector-hold
time. The peak time plus the hold time is the actual ECM commanded injector
open time. Both duration times are taken into account. Normal hold current
is around 1 Amp. Point (4 – E) is the injector turn-off and turn-off
inductive kick. Voltage values here should be in the same range as point C
with a straight vertical line indicating good turn off ability by the
driver. At this point the injector pulse is over. Therefore, as said before,
ECM commanded injector pulse duration is from point 1 – A to 4 – E.
However, this is not the actual physical open-time, which is the time
between the two dark cursors. The reason is that the injector just does not
simply opens at the moment the ECM commands it to. It takes roughly ¾ of the
peak current to fully open the injector and this corresponds to the first
current hump (first cursor). Point F (second cursor) is the
injector-closing hump. This hump can only be seen on the voltage waveform.
Therefore, in order to make a determination as to whether the injector is
clogged (misfiring cylinder) the actual physical open time has to be
taken into account. With this in mind, a dual trace waveform capture of both
current and voltage is the first step to a thorough and sound injector
diagnostic. The reason is that the physical injector opening shows only
on the current waveform while the closing of it shows only on the
voltage waveform. Since today’s clamp-on Amp probes have come a long
way, this is not much of a problem. A complete scope hook-up can be done in
5 minutes or less.
Conditions that Affect
Operation
High impedance injectors in practical, real-life
applications usually draw a current of around 950 mA to 1.2 Amps, while it
is more common to see a low impedance injector draw as much as 6 Amps. Any
automotive actuator is always affected by conditions that place excessive
resistance in its particular circuit. A voltage feed relay with carbonized
or eroded contacts, a rusted or deteriorated injector connector, damage at
the injector wiring itself and an open injector driver at the ECM are all
examples of excessive injector circuit resistance problems. The same can
also be said about mechanical problems developing inside the injector
itself. A clogged, binding or corroded injector pinttle will cause a severe
misfire and engine performance will suffer.
Component Testing
A sound fuel injector testing procedure is an invaluable
asset in determining the cause of today’s injector related misfire codes. A
well thought out testing sequence can save money in diagnostic time. In this
last section, various techniques will be presented. It is up to each
individual technician to decide which or if all of them should be employed.
• Perform a current waveform capture. Locate the main power
feed fuse, injector wire or injector relay and clamp on with an Amp-probe.
Capture the current waveform for further analysis. Refer to the Fig
next.
• Locate the two injector leads and scope for a voltage
waveform using the scope’s second or B channel. Superimpose the two
waveforms together so as to be able to analyze the mechanical condition of
the injector. If the injector turnon point is too far up the current
waveform, it is a sign of injector pinttle binding. Dirty or corroded
injectors will make it harder for the pinttle to open and close. The same
holds true for the voltage signal. If the turn off point is too far down, it
is a sure indication that the injector pinttle is having a hard time closing
due to debris stuck inside the injector or a weak injector spring.
Fig 4 – Saturation type injector current waveform.
Fig 5 – Superimposed injector current and voltage waveforms
showing the actual injector mechanical open time.
Fig 6 – Injector signal ground analysis.
• Perform an injector ground waveform analysis. Connect the
scope’s negative lead to battery ground and the scope’s positive lead to the
switching side of the injector connector. A fully grounded injector signal
should be seen. The scope signal trace should go all the way down to 0 volts
indicating that the ECM is fully grounding the injector Point A shows
a fully grounded injector. This ground comes from the ECM itself. This is a
fast and easy way to determine how well the ECM is grounded. An ECM with a
poor ground would not be able to reach a full ground potential like the
waveform shown in Fig 6, at point A. Point B shows a
slightly upward curved line. This is normal and is caused by the expanding
magnetic field’s counter voltage being created in the injector coil core.
• Using a sensitive pressure/vacuum transducer, tap into the
fuel pressure regulator. Do so by simply connecting the transducer hose to
the fuel pressure regulator vacuum hose port. Remember to plug the
vacuum hose to prevent a vacuum leak. By picking up the small
fluctuations of the fuel pressure regulator diaphragm, an injector signal
pressure variation waveform can be acquired with the oscilloscope. Every
time each individual injector opens, a small pressure drop will be felt by
the sensitive pressure transducer through the fuel pressure regulator
diaphragm. It is important to point out that this technique does not
work with group fired injectors. In such cases, more than one injector is
firing at the same time and this renders the test useless. If any of
the injector pressure drop humps deviate much from one another, then proceed
to an injector balance test. Using either the scanner or an injector pulser,
actuate each injector for a specific period of time and record the pressure
drop for each injector. In practical applications a 1 psi pressure drop
difference between each injector is enough to cause an engine performance
problem or set a misfire code.
• Another way to test an injector electrically is by
applying a steady flow of current across its windings for 3 to 5 minutes, by
connecting it directly to the battery through an amp-meter. This type of
test will reveal an intermittently shorting injector. Simply apply power and
ground to the injector windings, by using jumper wires. At the same time,
monitor the current draw through the injector coils. During this test the
current flow will normally decrease, since the injector’s resistance also
increases due to heat. A 13 ohm injector will draw around 900 mA, when cold,
and about 730 mA at 16 ohms, when fully hot. Any sudden drops in current
value is indicative of open windings. On the other hand, if the current is
too high, then it is indicative of semi-shorted windings.
NOTE: When performing a pressure drop test,
a low quality mechanical fuel gauge should not be used. An electronic
pressure transducer is the best way to acquire the individual injector
pressure drop readings. It is also very helpful to plot a histo-graph of the
transducer’s pressure signal. By doing so an immediate diagnostic conclusion
can be reached by just glancing at the graphed pressure reading.
The correct use of the test equipment and the necessary
experience will help you in diagnosing injector circuits. A lot of practice
will dictate success.
