See Figure 1
The TBI system is used on 1988 238 and 318 engines. The system is controlled by a pre-programmed digital computer known as the Single Module Engine Controller. The SMEC controls ignition timing, air/fuel ratio, emission control devices, charging system and idle speed. The SMEC constantly varies all settings to meet changing operating conditions.
Various sensors provide the input necessary for the logic module or 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 backlite, air conditioning, air conditioning clutch switches, and an electronic idle switch.
All inputs to the logic module or 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 logic module or 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.
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.
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
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 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, (electric backlite, 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.
The fuel pump used in this system is a positive displacement, roller vane immersible pump with a permanent magnet electric motor. The fuel is drawn in through a filter sock and pushed through the electric motor to the outlet. The pump contains two check valves. One valve is used to relieve internal fuel pump pressure and regulate maximum pump output. The other check valve, located near the pump outlet, restricts fuel movement in either direction when the pump is not operational. Voltage to operate the pump is supplied through the auto shutdown relay (ASD).