THEORY OF OPERATION
The following repair strategy should be used when diagnosing
a NO START- NO INJ. PULSE condition. The injector pulse is directly related to the engine speed sensor input (CRK,
PICK-UP, etc.). However, it is quite common to see a no injector pulse condition with good spark. EFI systems are
all different from one another. In this article, the differences will be explained among the different systems, and not
necessarily the different makes and models. Once a general understanding is reached, each make and model will fall within the
strategy explained here.
EFI SYSTEM OPERATION
There are three major types of fuel injection systems,
non-sequential, sequential, and direct fuel injection. The three systems perform the same function, which is to supply the
engine with the right amount of fuel at the right time.
• NON-SEQUENTIAL FUEL INJECTION system is found in older generation of
automobiles. A lot has been written on the subject. This article will provide you with a
practical extract on it. In this type of fuel injection, the ECM pulses the injectors in groups and on bigger engines the
injectors are divided in banks of two. Non-sequential injection does not have the precise fuel control that a sequential
system has. That is the reason why, for the most part, this system has been phased out in favor of the newer sequential
system. Non- sequential injection works by pulsating all or half the injectors at the same time. The fuel for the
particular cylinder stay in the intake manifold runners, until the cylinder intake stroke sucks it inside the combustion
chamber. Because of the imprecise operation, proper and exact fuel metering can not be attained by this system. Newer and
tougher emission laws demand a more precise and accurate fuel injection system.
• SEQUENTIAL
FUEL INJECTION system is chosen for newer and more precise OBD II systems.
Sequential fuel injection actually determines which and when each particular
cylinder needs the fuel pulse. Due of this level of control, sequential injection can be adjusted to the specific engine
operation in a much more precise manner. Also a few modes are possible that would otherwise be impossible with
the non-sequential system, like injection kill on a particular cylinder that is heavily misfiring. This is done to preserve
catalytic converter integrity. That is one of the reasons why on a new OBD II system a heavily misfiring vehicle
sometimes will not light up the CHECK ENG. LIGHT. Since on OBD II vehicles, the CEL only lights up if
the EMISSIONS go up 1 ˝ times the FTP (Federal Test Procedure) or maximum amount of emissions that can be
released to the atmosphere. Because the sequential injection is killing the misfiring cylinder’s fuel delivery
and no fuel is being burned in that particular cylinder, the total amount of exhaust emissions are not being raised
considerably even though the engine is misfiring.
• DIRECT fuel
injection is the future of fuel injection systems. It is currently used on
some models including hybrid electric/gasoline systems currently in production. Direct
injection has similarities to diesel injection. The system injects the fuel at the precise time directly to the combustion
chamber, allowing for very precise combustion. This type of system can operate at higher compression ratios because
only the air is being compressed. At the appropriate time, fuel is introduced to the combustion chamber at very high
pressures. Technologically speaking, it is a much more complicated system and will keep on developing as time goes
by.
NO INJECTION PULSE STRATEGY
NO FUEL PULSE TESTING
A no pulse condition is always related to one of the
following possibilities, and it is important not to assume the problem is related to any one component until the system is
thoroughly tested. The folowing steps take into account that the noinjector signal was verified using a noid-light or similar tool. It
is also important to note that noid-lights do not draw sufficient amperage (current) to adequately load the injector driver
circuit inside the ECM. Therefore, it is possible to have a perfectly pulsing noid-light and a defective injector driver
inside the ECM. Furthermore, some newer injector circuits use high impedance injectors that WILL NOT light a noid-light.
These systems can throw you off by making you think that there is no injector pulse. Always be mindful of some
of the drawbacks of using noid-lights. However, they are a quick and easy way of verifying injector pulse.
1. SCAN FOR CODES. Perform a scan test to verify any
engine speed sensor (crank or pick-up) codes. On some makes, it is possible
to verify crank operation by looking at the RPM signal from the scanner
while cranking the engine. This however does not work all the time and it is
not conclusive evidence of engine speed sensor malfunction. Some newer OBD
II systems will have an INJ FAULT PID to identify possible injector
circuit problems.
2. VOLTAGE SUPPLY. Second, check the voltage supply
or battery power going to the injectors. Most EFI systems out there work on
NEGATIVE TRIGGER, which means that the injectors are being
grounded or negatively triggered. This type of system supplies positive
voltage to one side of the injector, through a relay, and the ECM grounds
the other side. On very few systems, the operation is actually the opposite;
in which case the injectors are supplied a constant ground and the ECM
provides the positive pulse. To check for positive voltage at one side of
the injector, just turn on the ignition key and check one side of the
injection with a test light.
3. SWITCHED SIDE. Verify that there is continuity on
the wire between the switched side of the injector and the ECM. Injectors
are directly triggered by the ECM, so there has to be continuity between the
two. Injector harness connectors are a common source of problems, and tend
to corrode and cause contact problems. An injector pulse not reaching the
injector will look like a flat line on the scope (an open circuit) not to be
confused with a shorted injector. Remember that a flat lined injector
voltage waveform indicates an open circuit or that the injector signal is
not reaching it. Always verify that the injector pulse is being received.
Look for an injector pulse, both at the injector connector and if not at the
ECM connector.
4. SPARK. Verify that there is spark. By doing so, it is
possible to readily verify the pick-up coil or the crank sensor operation.
Some systems have no direct connection between the main engine speed input
sensor (crank or pick-up) and the ECM. With these systems the engine speed
input sensor goes from sensor (crank) to ignition module and then the module
will generate a DIST. REFERENCE signal that feeds the ECM. Either
way, if there is no spark as well as no injector pulse, then the problem is
related to an engine speed sensor (crank or pick-up) that is not providing
the ECM with an engine speed input.
5. ENGINE SPEED SIGNAL. Remove the ECM from its
holding brackets. Using a DSO and a wiring diagram, probe the appropriate
engine speed signal wires, crank the engine, and verify for proper signal.
It is important to understand that the main input for injection is the
engine speed signal. Regardless of how this signal reaches the ECM, without
it, the engine will not have injector pulse or ignition. It is the most
important signal in an EFI system. On engines where the crank signal goes to
the ignition module (ICM), the ICM will then generate a distributor REF
signal that the ECM will use for injection.
6. INJECTOR CURRENT. As a final step, a current
ramping test should be performed to ascertain the general health of the
injector. The only test that thoroughly checks the entire mechanical
integrity of an electric actuator (injector, solenoid, motor, etc) is a
current ramping test. Current ramping takes into account the circuit while
being loaded. It is only by loading a circuit that a definitive verification
can be obtained of a defective component. Current ramping is covered
elsewhere in this book. By following these steps and carefully analyzing the
wiring diagram pertaining to the particular vehicle in question, a proper
diagnostic conclusion can be reached. The important factor in diagnosing any
particular system is to know the system. It is always a good practice
to read and study the details about a system before devising a diagnostic
plan of attack.
NOTE: Chrysler vehicles have what is called
an AUTOMATIC SHUTDOWN RELAY. The ASD relay provides power to one side of the
injectors, ignition coil and fuel pump. This relay only works while the
engine is cranking or running. In other words, while the ECM is receiving an
RPM signal from the distributor or crank sensor. The ASD relay will not
provide power to the injectors with just the ignition key in the ON
position.
NOTE: It is a good idea to
develop the habit of performing a dual scope test on injector circuits. By
dual tracing the injector voltage and amperage (current), a much faster
diagnostic conclusion can be reached.
