See Figure 1
The Electronic Fuel Injection System is a computer regulated single point fuel injection system that provides precise air/fuel ratio for all driving conditions. At the center of this system is a Single Module Engine Controller (SMEC) that regulates ignition timing, air/fuel ratio, emission control devices, idle speed and cooling fan and charging system. This component has the ability to update and revise its programming to meet changing operating conditions.
Various sensors provide the input necessary for the SMEC to correctly regulate the fuel flow at the fuel injector. These include the manifold absolute pressure, throttle position, oxygen sensor, coolant temperature, charge temperature, vehicle speed (distance) sensors and throttle body temperature. In addition to the sensors, various switches also provide important information. These include the neutral-safety, heated rear window, air conditioning, air conditioning clutch switches, and an electronic idle switch.
All inputs to the SMEC are converted into signals sent to the power module. These signals cause the power module to change either the fuel flow at the injector or ignition timing or both.
The SMEC tests many of its own input and output circuits. If a fault is found in a major system this information is stored in the SMEC. Information on this fault can be displayed to a technician by means of the instrument panel power loss (check engine) lamp or by connecting a diagnostic read out and reading a numbered display code which directly relates to a specific fault.
COMPONENTS & OPERATION
The power module contains the circuits necessary to power the ignition coil and the fuel injector. These are high current devices and their power supply has been isolated to minimize any "electrical noise" reaching the SMEC. The power module also energizes the Automatic Shut Down (ASD) relay which activates the fuel pump, ignition coil, and the power module itself. The module also receives a signal from the distributor and sends this signal to the logic module. In the event of no distributor signal, the ASD relay is not activated and power is shut off from the fuel pump and ignition coil. The power module contains a voltage converter which reduces battery voltage to a regulated 8.0 volt output. This 8.0 volt output powers the distributor and also powers the SMEC.Single Module Engine Controller (SMEC)
The SMEC contains the circuits necessary to drive the ignition coil, fuel injector, and the alternator field. These are high current devices and have been isolated to minimize any electrical noise in the passenger compartment.
The Automatic Shut Down (ASD) relay is mounted externally, but is turned on and off by the SMEC. Distributor pick-up signal goes to the SMEC. In the event of no distributor signal, the ASD relay is not activated and power is shut off from the fuel injector and ignition coil. The SMEC contains a voltage convertor which converts battery voltage to a regulated 8.0 volt output. This 8.0 volt output powers the distributor pick-up. The internal 5 volt supply which, in turn, powers the MAP sensor and TPS.
The SMEC is a digital computer containing a microprocessor. The module receives input signals from various switches and sensors. It then computes the fuel injector pulse width, spark advance, ignition coil dwell, idle speed, purge and cooling fan turn on and alternator charge rate.
The SMEC tests many of its own input and output circuits. If a fault is found in a major system, this information is stored in the SMEC. Information on this fault can be displayed to a technician by means of the instrument panel check engine lamp or by connecting the diagnostic read out tool C-4805 and reading a numbered display code which directly relates to a general fault.Manifold Absolute Pressure (MAP) Sensor
The Manifold Absolute Pressure (MAP) sensor is a device which monitors manifold vacuum. It is connected to a vacuum nipple on the throttle body and electrically to the logic module or SMEC. The sensor transmits information on manifold vacuum conditions and barometric pressure to the logic module or SMEC. The MAP sensor data on engine load is used with data from other sensors to determine the correct air/fuel mixture.Oxygen Sensor (O 2 Sensor)
See Figures 2, 3, 4 and 5
The oxygen sensor (O 2 sensor) is a device which produces an electrical voltage when exposed to the oxygen present in the exhaust gases. The sensor is mounted in the exhaust manifold. The oxygen sensor is electrically heated internally for faster switching when the engine is running. When there is a large amount of oxygen present (lean mixture), the sensor produces a low voltage. When there is a lesser amount present (rich mixture) it produces a higher voltage. By monitoring the oxygen content and converting it to electrical voltage, the sensor acts as a rich-lean switch. The voltage is transmitted to the SMEC. The SMEC signals the power module to trigger the fuel injector. The injector changes the mixture.Coolant Temperature Sensor
The coolant temperature sensor is a device that monitors coolant temperature (which is the same as engine operating temperature). It is mounted in the thermostat housing. This sensor provides data on engine operating temperature to the SMEC. This allows the SMEC to demand slightly richer air/fuel mixtures and higher idle speeds until normal operating temperatures are reached. This sensor is also used for cooling fan control.TESTING
- With the key in the OFF position, detach the wire connector from the coolant sensor.
- Connect one lead of an ohmmeter to one terminal on the sensor, and the other lead to the sensor's remaining connector. The ohmmeter should read 700-1,000 ohms at 200°F (93°C), and 7,000-13,000 ohms at 70°F (21°C).
Various switches provide information to the SMEC. These include the neutral safety, electric rear window heater, air conditioning, air conditioning clutch, and brake light switches. If one or more of these switches is sensed as being in the ON position, the SMEC signals the automatic idle speed motor to increase idle speed to a scheduled rpm.
With the air conditioning on and the throttle blade above a specific angle, the wide open throttle cut-out relay prevents the air conditioning clutch from engaging until the throttle blade is below this angle.Power Loss/Limited (Check Engine) Lamp
The power loss (check engine) lamp comes on each time the ignition key is turned on and stays on for a few seconds as a bulb test. If the SMEC receives an incorrect signal or no signal from either the coolant temperature sensor, manifold absolute pressure sensor, or the throttle position sensor, the lamp on the instrument panel is illuminated. This is a warning that the SMEC has gone into limp-in mode in an attempt to keep the system operational.
The lamp can also be used to display fault codes. Cycle the ignition switch on, off, on, off, on, within five seconds and any fault code stored in the memory will be displayed.Exhaust Gas Recirculation Solenoid
The EGR solenoid is operated by the SMEC. When engine temperature is below 70°F (21°C), the module energizes the solenoid by grounding it. This closes the solenoid and prevents ported vacuum from reaching the EGR valve. When the prescribed temperature is reached, the module will turn off the ground for the solenoid de-energizing it. Once the solenoid is de-energized, ported vacuum from the throttle body will pass through to the EGR valve. At idle and wide open throttle the solenoid is energized which prevents EGR operation.Purge Solenoid
The purge solenoid is controlled by the SMEC. When engine temperature is below 145°F (61°C) the module grounds the purge solenoid, energizing it. This prevents vacuum from reaching the charcoal canister valve. When this temperature is reached, the module de-energizes the solenoid by turning the ground off. Once this occurs, vacuum will flow to the canister purge valve and purge fuel vapors through the throttle body.Air Conditioning Cutout Relay
The air conditioning cutout relay is connected, in series, electrically with the A/C damped pressure switch, the A/C switch and, on some models, the A/C fan relay. This relay is in the energized, closed (on), position during engine operation. When the module senses low idle speeds and wide open throttle through the throttle position sensor, it will de-energize the relay, open its contacts and prevent air conditioning clutch engagement.Throttle Body
The throttle body assembly replaces a conventional carburetor and is mounted on top of the intake manifold. The throttle body houses the fuel injector, pressure regulator, throttle position sensor, automatic idle speed motor and throttle body temperature sensor. Air flow through the throttle body is controlled by a cable operated throttle blade located in the base of the throttle body. The throttle body itself provides the chamber for metering atomizing and distributing fuel throughout the air entering the engine.Fuel Injector
The fuel injector is an electric solenoid driven by the power module, but controlled by the SMEC. The SMEC, based on ambient, mechanical, and sensor input, determines when and how long the power module should operate the injector. When an electric current is supplied to the injector, a spring loaded ball is lifted from its seat. This allows fuel to flow through six spray orifices and deflects off the sharp edge of the injector nozzle. This action causes the fuel to form a 45° cone shaped spray pattern before entering the air stream in the throttle body.Fuel Pressure Regulator
See Figures 6 and 7
The pressure regulator is a mechanical device located downstream of the fuel injector on the throttle body. Its function is to maintain a constant 14.5 psi (100 kPa) across the fuel injector tip. The regulator uses a spring loaded rubber diaphragm to uncover a fuel return port. When the fuel pump becomes operational, fuel flows past the injector into the regulator, and is restricted from flowing any further by the blocked return port. When fuel pressure reaches the predetermined setting, it pushes on the diaphragm, compressing the spring, and uncovers the fuel return port. The diaphragm and spring will constantly move from an open to closed position to keep the fuel pressure constant.Throttle Position Sensor (TPS)
The Throttle Position Sensor (TPS) is an electric resistor which is activated by the movement of the throttle shaft. It is mounted on the throttle body and senses the angle of the throttle blade opening. The voltage that the sensor produces increases or decreases according to the throttle blade opening. This voltage is transmitted to the SMEC, where it is used along with data from other sensors to adjust the air/fuel ratio to varying conditions and during acceleration, deceleration, idle, and wide open throttle operations.Automatic Idle Speed (AIS) Motor
The Automatic Idle Speed (AIS) motor is operated by the SMEC. Data from the throttle position sensor, speed sensor, coolant temperature sensor, and various switch operations, (heated rear window, air conditioning, safety/neutral, brake) are used by the module to adjust engine idle to an optimum during all idle conditions. The AIS adjusts the air portion of the air/fuel mixture through an air bypass on the back of the throttle body. Basic (no load) idle is determined by the minimum air flow through the throttle body. The AIS opens or closes off the air bypass as an increase or decrease is needed due to engine loads or ambient conditions. The module senses an air/fuel change and increases or decreases fuel proportionally to change engine idle. Deceleration die out is also prevented by increasing engine idle when the throttle is closed quickly after a driving (speed) condition.Throttle Body Temperature Sensor
The throttle body temperature sensor is a device that monitors throttle body temperature which is the same as fuel temperature. It is mounted in the throttle body. This sensor provides information on fuel temperature which allows the SMEC to provide the correct air fuel mixture for a hot restart condition.