No Injector Pulse... Well then what? It'aint my fault.

The injector drover is a wonder of technology. Always switching, always sensing the input from the ECM, but in many cases it also gives up or simply never gets the command from it. Detecting why is a mainstay of auto diagnostics and covered here in detail...

 

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.

NOTEChrysler 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.

 

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SRS-Airbag Repair Guide

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copyright 2011 Mandy Concepcion, Automotive Diagnostics and Publishing