Electronic Fuel-Injection Output Devices

19-01 Analyze powertrain control module output signals to find failures.

To ensure an effective fuel-injection system, the PCM must be able to effect change on the engine if the inputs require it. Processing all of the inputs—the engine speed, fuel pressure, transmission temperature, driver inputs, and engine temperature—is very complicated (FIGURE 19-1). It requires a computer that has the necessary program and drivers for engine operation. These outputs cover fuel injector on time, ignition operation, idle speed control, transmission shifting, variable valve timing (VVT), displacement on demand, and other electrical components. All of these systems must work together and be precisely controlled to produce the desired effect.

FIGURE 19-1 The PCM/ECM is used to operate the fuel and ignition systems on the engine. It also takes inputs from other modules to change engine operation.

Output Signals

The output signals from an electronic control module (ECM) can be as simple as a controlled ground or as complex as a pulse-width modulation (PWM) of a fuel pump. The ECM has to decide, through its software programming, which signal to send to promote the proper operation of the engine. This does not include the computer data lines that are connected to the ECM. This networking function of the module allows one module to take input and provide output to other modules within the vehicle’s network (FIGURE 19-2).

FIGURE 19-2 The ECM must be able to operate the components that it is tasked with and communicate with the other modules.

Relays

Relays are used to control different components on the vehicle. A relay is a simple switch that uses low current to control a high-current circuit (FIGURE 19-3). A relay keeps current away from the ECM. The relay can keep current away from the ECM because it uses electromagnetic induction to control the operation of the switch (FIGURE 19-4). By not having a direct connection from the ECM to the component, the chances of voltage surges are very minimal. The way most relays are wired is through the ECM controlling the ground side of the coil. By controlling the ground side, it further reduces the possibility of surge or voltage spike when operating the component. Controlling components with a relay also allows the operation of the relay to be monitored based on the current draw of coil in the relay (FIGURE 19-5).

FIGURE 19-3 The relay allows the PCM to control the high-current relay without exposing the circuitry to potential voltage spikes.

FIGURE 19-4 Using an electromagnetic field to close the high-current switch allows no physical connection between the low-current control side and the high-current switch side.

FIGURE 19-5 The ECM can read the amount of current that the relay is drawing, which it can then use to determine whether the relay is operating correctly or not.

Solenoids

Solenoids are used to change electrical control into mechanical movement. The basic principles of a solenoid is electromagnetism (FIGURE 19-6). These solenoids are similar to relays in that they use power induced in a coil to create movement in another object. The use of a coil to control another object allows the controlling module to use low-amperage power, which does not stress the components very much; this power then controls the movement of a metal core that is attached to the piece that the module desires to move. For instance, the starter has a solenoid that moves the Bendix drive out into the flywheel to allow the starter to spin the engine over. In the starter solenoid case, once the solenoid has been deenergized, spring pressure will retract the Bendix drive away from the flywheel. Solenoids can be used for various situations that require physical movement or physically controlling a component. Understanding that they are just 12-volt components that create an electromagnet helps in diagnosing failures.

FIGURE 19-6 The solenoid is a simple coil that induces an electromagnetic field that moves a metal core. Anything attached to this metal core will also move, which allows an electrical input to be translated into mechanical movement.

Ignition Control

The purpose of the PCM is to control engine operation, including control of the ignition system. The PCM gets inputs from various position sensors on the engine that allow the PCM to control when the ignition events occur, which promotes efficient combustion. Because the PCM is a central computer that controls all aspects of engine operation, it can coordinate efficient combustion. Ignition control is done by using drivers to operate the ignition coils based on engine position (FIGURE 19-7). The drivers that are used in most PCMs are simple transistors that have no moving parts. These transistors operate rapidly, which makes them the ideal component to trigger the ignition coil event. The drawback is that the transistors will not handle high amperage that may be pulled by the components that they control. This is where using coils to induce voltage comes into play. These events must happen in sequence with the injection and mechanical events. By using the crankshaft position (CKP) sensor, camshaft position (CMP) sensor, and other engine sensors, the PCM can modulate the required components to promote the best combustion environment.

FIGURE 19-7 The PCM controls the ground side of the ignition coil to minimize the sparks generated by the control of the coil and .

Idle Speed Control Devices

The PCM also needs to control the idle speed of the engine to make sure that once it starts, it stays running when the vehicle comes to a stop or when acceleration is not requested. By using different types of idle control components, the PCM can precisely control how the engine idles; it can also increase or decrease engine rpm and compensate for an engine misfire if required. These devices either control the throttle blades or bypass the throttle blades completely.

Idle Air Control Motor

The idle air control (IAC) motor is used to bypass air behind the throttle blades to control the idle of the engine (FIGURE 19-8). This is a stepper motor—which is a specialized direct current (DC) motor that has a rotor operated by a series of coils that surround the rotor—that is commanded by the PCM to a particular position to allow a precalibrated amount of air to bypass the throttle blades. The engine is expecting this predetermined amount of air, to sustain the idle that the PCM is trying to maintain. The PCM controls this stepper motor by pulsing the outer magnet that then pushes and pulls the inner magnet to maintain the position that it desires (FIGURE 19-9). These pulses are bursts, which are considered “steps” to precisely control the position of the motor. These types of motors are used for various things on the engine or the vehicle that need to positioned in a particular orientation. Using this type of component to control the air flow around the throttle blades can help to stabilize the idle feature on the engine. The IAC motor is used on engines that still use a throttle cable to operate the throttle blades.

FIGURE 19-8 The IAC motor is used on a fuel-injected type of engine to control the idle circuit of airflow around the throttle blades.

FIGURE 19-9 The stepping of the IAC motor allows the idle to be smooth and consistent with the operation of the engine. The reaction of the motor to the control of the PCM is virtually instantaneous.

Electronically Controlled Throttle

Using an electric motor to operate the throttle allows the PCM to control the operation of the engine even further than it could before. The throttle actuator control (TAC) motor was developed to control the throttle with the PCM (FIGURE 19-10). Using a stepper motor, the PCM takes an input from the accelerator pedal position (APP) sensor and determines how much throttle is needed to make the engine operate in the way the driver is commanding it to. This allows the PCM to monitor other systems in the vehicle, like the antilock brake system (ABS), to adjust the throttle on the engine to make sure those systems are operating correctly. Along with a stepper motor, there are two throttle position sensors (TPSs) in the motor to verify that the operation of the throttle blades is as commanded. This is a fail-safe feature: If one TPS fails or has a skewed signal, the PCM can compare it to the other one. If the PCM does notice a failure of the motor or a TPS, it defaults the engine to idle and illuminates a reduced engine power message on the driver information system (FIGURE 19-11). Being able to the control the throttle away from the driver allows the ECMs on the vehicle to react quickly. For instance, if the vehicle is in a traction control situation, the PCM can reduce engine power by closing the throttle blades until the ABS sends a signal to the PCM that the wheels have regained traction. Electronic throttle control will become more apparent as autonomous operation of the vehicle becomes the new normal, because control of every aspect of engine operation will be needed to safely control the vehicle as it drives down the road.

FIGURE 19-10 The TAC motor is an electronic motor controlled by the PCM based on input from various sensors and modules that will command it to the position that is needed to operate the engine.

FIGURE 19-11 The reduced engine power message will accompany a vehicle that will not go over idle. This happens when there is a fault in the TAC system: The PCM will let the engine idle but not operate at any normal speed. This is a safety feature that indicates there is a fault in the system.

Fuel Trim Operation and Adjustment

One of the main operations of the PCM is to adjust the fuel trim to ensure efficient operation of the engine. The fuel trim is a calculated value derived from the oxygen sensor’s signals. The PCM uses the O₂ sensor’s input to determine whether the engine is operating rich or lean. The mission of the PCM is to keep the air-fuel mixture at 14.7:1, which is the optimum air-fuel ratio for gasoline. This produces the most power and allows the engine to run as efficiently as possible. Looking at the oxygen content in the exhaust gases allows the PCM to determine whether the combustion events in the cylinders are burning all of the fuel and air in the cylinder. The PCM calculates the needs of the engine and then displays them on the scan tool as short-term fuel trim (STFT) and long-term fuel trim (LTFT) (FIGURE 19-12). STFT quickly reacts to changes in the oxygen content of the exhaust to help keep the engine within the right operational parameters. This requires the PCM to adjust the fuel injector’s on time so that it either adds to or subtracts from the amount of fuel injected into the engine. The values that appear on the scan tool range from –25% to +25%. With a target of 0, on V-type engines each bank should be within ±5%. If the STFT is all the way at –25%, it is taking away fuel and the technician should suspect a fuel leak leaking internally in the intake system or a skewed O₂ sensor. If the STFT is at +25%, the PCM is adding a lot of fuel because it thinks the engine is lean. Look for a vacuum leak, a bad mass airflow (MAF) sensor, an exhaust leak before the O₂ sensor, and/or a possibly skewed O₂ sensor. The LTFT is a calculated number based on the operation of the engine throughout the closed-loop cycle and is kept in the memory of the PCM. This measurement is a more coarse adjustment that helps to keep the STFT close to the 0% mark.

FIGURE 19-12 The fuel trim calculations show what the PCM is doing to change the mixture within the cylinder to meet performance standards. This can include adding or removing fuel from the fueling strategy.