COMPUTERIZED ENGINE CONTROLS
The Motronic engine management system, which operates in conjunction with other computer-controlled systems, precisely controls fuel injection and ignition under variable operating conditions. These other computer-controlled systems, listed as follows, can influence engine management system operation depending on requirements.
Electronic Transmission Control (EGS), A/C System, Anti-Theft System (DWA), Anti-Lock Brake System/Traction Control (ABS/ASC), Instrument Cluster and On-Board Computer.
MOTRONIC CONTROL UNIT
An 88-pin control unit is used. See MOTRONIC CONTROL UNIT LOCATION table. The control unit uses the following input signals to determine optimum fuel injection and ignition timing: intake airflow, intake air temperature, throttle position, coolant temperature, engine speed, vehicle speed, crankshaft position, knock sensors and exhaust gas oxygen content.
The Motronic control unit can operate engine in a "limp mode," allowing vehicle operation despite component failure. The Motronic control unit also has adaptive capabilities to compensate for component wear and other factors, such as minor air (vacuum) leaks. Once a fault is recognized, it is stored in Motronic control unit memory as a fault code. The system automatically substitutes a fixed replacement value for the incorrect value caused by a defective component or circuit.
An air/fuel ratio of 14.7:1 is maintained under most driving conditions. Maximum engine RPM is limited by the Motronic control unit by eliminating power to fuel injectors. The Motronic control unit assumes control of the following functions.
Acceleration Enrichment, Catalytic Converter Protection, Cold Start Control, Dynamic Coasting Shutoff, Fuel Injection Control, Fuel Tank Vent (Evaporation) Control, Idle Speed Control, Ignition Timing and Anti-Knock Function, Heated Oxygen (Sensor) Control, Relay Controls and Self-Diagnostics.
| Application | Location |
|---|---|
| All Models | Inside Box On Right Rear Corner Of Engine Compartment, Behind Strut Tower |
MOTRONIC CONTROL UNIT LOCATION
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by control unit. The second category covers OUTPUT SIGNALS, which are components controlled by the control unit.
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS section. The available input signals include the following.
Note. Descriptions of the following input devices are based on information available from manufacturer. Complete information is not available from manufacturer.
Airflow Meter/Sensor
Airflow meter/sensor is located in intake passage between air filter and intake manifold. Air-flow meter includes a flap door (connected to a potentiometer) to detect airflow, and a temperature sensor. Airflow meter informs Motronic control unit of airflow rate.
Anti-Theft System (DWA)
When anti-theft system is armed (if equipped), the anti-theft control unit sends a voltage signal (greater than 10 volts) to the Motronic control unit, which will disable ignition and fuel injection systems.
Camshaft Position Sensor
Sensor is located at intake camshaft sprocket. An inductive pick-up detects position of intake camshaft and sends a signal to Motronic control unit. This signal is used to to determine ignition and fuel injection firing.
Crankshaft (Speed/Position Pulse) Sensor
Sensor is located on lower right front of engine. It supplies the Motronic control unit with crankshaft position information. When the Motronic control unit has determined optimum ignition timing (based on input from various sensors), information supplied by the crankshaft (speed/position pulse) sensor is used to signal ignition firing.
Engine (Coolant) Temperature Sensor
Sensor is located on top front of cylinder head, behind radiator hose. This component supplies coolant temperature information to Motronic control unit. The Motronic control unit then determines whether current operating condition is at cold or normal operating temperature. During cold operating conditions, air/fuel mixture is enriched by widening fuel injector pulse width. Extra rich conditions are maintained until normal operating temperature is reached.
Intake Air Temperature Sensor
The intake air temperature sensor is located in intake passage between air filter and intake manifold, and is part of the airflow sensor. The intake air temperature sensor informs the Motronic control unit of the ambient temperature of incoming air.
Knock Sensor
Information is not available from manufacturer.
Heated Oxygen (Sensor) Control
Oxygen content of exhaust gases is detected by the oxygen sensor(s) located in the exhaust manifold(s). This sensor converts the percentage of oxygen present in exhaust gases into an electrical signal that is transmitted to the Motronic control unit.
The Motronic control unit uses this information to determine air/fuel ratio, and adjusts injector timing (pulse width) to obtain a 14.7:1 air/fuel ratio. Oxygen sensor signal (closed loop mode) is also used for regulation of fuel tank (evaporative) control valve.
If fault code is set or if the oxygen sensor is malfunctioning, the Motronic control unit will operate engine at an oxygen sensor substitute value of.45 volt.
Throttle Position Sensor
The throttle valve potentiometer, or throttle position sensor, is used in place of a throttle position switch and has contacts for idle and wide open throttle. The potentiometer is not adjustable since the Motronic control unit determines idle speed ignition timing (idle speed switching point) on its own.
Vehicle (Road) Speed Sensor
Sensor information comes from speedometer sensor located in instrument cluster. Vehicle speed sensor sends information on vehicle speed to Motronic control unit.
OUTPUT SIGNALS
Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to system indicated after component.
A/C Compressor Shutoff
See A/C COMPRESSOR CLUTCH under MISCELLANEOUS CONTROLS.
A/C Switch & Compressor
See A/C COMPRESSOR CLUTCH under MISCELLANEOUS CONTROLS.
Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM .
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Tank Vent (Evaporation) Control
See EMISSION SYSTEMS .
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Idle Speed Actuator
See IDLE SPEED under FUEL SYSTEM.
Ignition Timing & Anti-Knock Function
See DIRECT IGNITION SYSTEM (DIS) under IGNITION SYSTEMS.
Ignition Timing Control
See IGNITION SYSTEMS .
Kickdown Prevention
See appropriate TRANSMISSION SERVICE & REPAIR article.
Master (Main) Relay
See FUEL DELIVERY under FUEL SYSTEM.
FUEL DELIVERY
Note. For locations of fuel delivery components, see MOTRONIC ENGINE MANAGEMENT SYSTEM RELAY LOCATIONS table.
Fuel Pump
All vehicles use an in-tank electric fuel pump, accessible through luggage compartment or rear bench seat. Fuel pump is activated by voltage supplied by fuel pump relay.
Fuel pump relay is powered through master (main) relay and positive battery junction point "B", and is grounded through Motronic control unit terminal No. 1.
The master (main) relay is powered through positive battery junction point "B", and supplies power to Motronic control unit, Heated Oxygen Sensor (HO2S), fuel pump relay and other computer-controlled systems.
Fuel Pressure Regulator
Pressure regulator maintains constant fuel pressure to injectors. An electric fuel pump provides fuel to pressure regulator. Pressure regulator is vacuum operated. As the throttle is depressed and manifold vacuum drops, pressure regulator increases fuel pressure to maintain constant flow to fuel injectors.
| Application | Location |
|---|---|
| 318i & 318is | (1) Behind Cover, Along Engine Compartment Firewall |
| 318ti | (2) |
| (1) First relay (viewed from left to right) after ground wire is fuel pump relay followed by master relay. (2) Information is not available from manufacturer. | |
| (1) | First relay (viewed from left to right) after ground wire is fuel pump relay followed by master relay. |
| (2) | Information is not available from manufacturer. |
MOTRONIC ENGINE MANAGEMENT SYSTEM RELAY LOCATIONS
FUEL CONTROL
Note. Information is not available from manufacturer for 318ti models.
Catalytic Converter Protection (318i & 318is)
To prevent overheating of the catalytic converter, the catalytic converter protection system enriches air/fuel mixture to reduce exhaust gas temperature.
Fuel Injectors (318i & 318is)
Each group of fuel injectors (No. 1 and 3, No. 2 and 4) is activated by Motronic control unit final stage (current amplifier), making it possible to divide injection cycle into groups of cylinders. This allows limited engine operation even when one group fails. Injectors operate under parallel or semi-sequential fuel injection modes.
With parallel injection, simultaneous activation of ALL fuel injectors occurs for each crankshaft revolution, and takes place only when the camshaft sensor has not sent output signals since starting the engine. If camshaft sensor signal is sent out later, the system switches over to semi-sequential fuel injection as soon as an engine speed of 2500 RPM is exceeded.
Under semi-sequential fuel injection, only one group of cylinders operates once every 720 crankshaft degrees from an engine speed of 600 RPM. Semi-sequential fuel injection timing can function only when the Motronic control unit receives a signal from the camshaft sensor.
Oxygen Sensor Heater Relay & Heating Resistor
A temperature of about 572°F (300°C) is needed for oxygen sensor operation. In order to quickly heat sensor, a heating resistor is included in oxygen sensor.
The Motronic control unit provides a ground path for the oxygen sensor heater relay, which then provides battery voltage to the oxygen sensor heating resistor in oxygen sensor. The oxygen sensor heater relay is activated when ignition is on. Relay is switched off when engine reaches certain speed and load.
Idle speed is kept constant by an idle speed actuator, which supplies the engine with the necessary amount of air. Idle speed control takes place during the period in which throttle potentiometer, or TPS, detects the idle setting. The pre-programmed idle speed values in the Motronic control unit are compared with the actual operating values, and corrected to compensate for component wear and other factors, such as minor air (vacuum) leaks.
When the Motronic engine management system detects engagement of a drive range (1, 2, 3 or "D"), idle speed is increased by the idle speed actuator to compensate for engine speed drop caused by engagement of torque converter. On A/C-equipped models, idle speed is also temporarily increased when the A/C system is switched on. Upon receiving the A/C compressor signal, the quantity of air required for idle speed is corrected.
Each cylinder has its own ignition coil (direct ignition system), eliminating the need for a distributor. The Motronic control unit activates each coil separately.
If the camshaft sensor has not sent output signals since starting the engine, ignition system enters double ignition mode. This mode activates all spark plugs for each crankshaft revolution. Switching from normal to double ignition, or vice versa, takes place immediately, independent of fuel injection operation.
Ignition timing is retarded (anti-knock function), depending on inputs from knock sensors, in order to prevent pre-ignition of the air/fuel mixture in the combustion chamber.
Fuel vapors are routed to the engine via an activated carbon canister. Installed between the carbon canister and the air manifold is a purge control valve. Valve restricts flow of air based on sensor inputs to the Motronic control unit.
Electrical activation of the purge control valve depends on engine speed and load. The vacuum line to the intake manifold is closed as long as the valve is supplied with voltage. When no power is applied to the valve, it can be opened by vacuum in intake manifold.
The fuel evaporation control cycle begins as soon as the oxygen control system is active (closed loop mode). Upon completion of the purge cycle, the valve is closed for about 30 seconds.
When engine is shut off, the valve remains closed for another 3 seconds to prevent engine run-on (dieseling). With the engine stationary and no power applied to purge control valve, a spring-loaded non-return valve is closed to ensure no fuel vapors flow into intake manifold.
An emission-related component or circuit failure will activate MIL on dashboard.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
In order to provide full engine power during acceleration, A/C compressor is switched off for up to 7 seconds during full throttle operation at speeds less than 5 MPH.
