Engine Controls - Theory & Operation BMW Z3 E36

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.

COMPUTERIZED ENGINE CONTROLS

Engine management system, which operates in conjunction with other computer-controlled systems, controls fuel injection and ignition under variable operating conditions. Engine management system controls air/fuel mixture and exhaust emissions by adjusting fuel injection and ignition timing. Electronic Control Unit (ECU) receives and processes sensor input and sends output signals to adjust injector pulse widths and ignition timing.

ELECTRONIC CONTROL UNIT (ECU)

An 88-pin ECU is used. (Scheme 1) ECU 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.

ECU can operate engine in a limp mode, allowing vehicle operation despite component failure. ECU also has adaptive capabilities to compensate for component wear and other factors, such as minor vacuum leaks. Once a fault is recognized, it is stored in ECU memory as a Diagnostic Trouble Code (DTC). 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 ECU by eliminating power to fuel injectors. The ECU assumes control of following functions.

4-Cylinder

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.

6-Cylinder

A/C Compressor Control, Acceleration Enrichment, Cold Start Control, Deceleration Fuel Cut, Fuel Injection Control, Fuel Tank Vent (Evaporation) Control, Idle Speed Control, Ignition Timing Control and Anti-Knock Function, Ignition Timing/Fuel Injection Intervention Via DWA and On-Board Computer, Oxygen (Sensor) Control, Relay Controls and Self-Diagnostics.

V8

A/C Compressor Control, Acceleration Enrichment, Cold Start Control, Deceleration Fuel Cut, Fuel Injection Control, Fuel Tank Vent (Evaporation) Control, Idle Speed Control, Ignition Timing Control and Anti-Knock Function, Ignition Timing/Fuel Injection Intervention Via DWA and On-Board Computer, Oxygen (Sensor) Control, Rear Window Defogger, Relay Controls and Self-Diagnostics.

Scheme 1

DME ECU CONTROL UNIT LOCATION

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by ECU. The second category covers OUTPUT SIGNALS, which are components controlled by ECU.

INPUT DEVICES

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine input device usage for a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following.

Note. Descriptions of following input devices are based on information available from manufacturer. Complete information is not available from manufacturer.

Airflow Meter/Sensor (4-Cylinder)

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 ECU of airflow rate.

Anti-Theft System (DWA)

When anti-theft system (if equipped) is armed, anti-theft ECU sends a voltage signal (greater than 10 volts) to ECU, which disables 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 ECU. This signal is used to determine ignition and fuel injection firing.

Crankshaft (Speed/Position Pulse) Sensor

Sensor is located on lower right front of engine. It supplies ECU with crankshaft position information. When ECU has determined optimum ignition timing (based on input from various sensors), information supplied by 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 ECU. The ECU 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.

Hot-Wire/Hot-Film Airflow Meter (6-Cylinder & V8)

Sensor is located in intake passage between air filter and intake manifold. Sensor informs ECU of airflow rate. Hot-wire airflow meter is used on 6-cylinder engines and uses a heating element to burn off deposits from sensor. Hot-film airflow meter is used on V8 engines and does not require a heating element.

Intake Air Temperature Sensor (6-Cylinder & V8)

Intake air temperature sensor is located in intake passage in intake manifold. The intake air temperature sensor informs ECU of ambient temperature of incoming air.

Intake Air Temperature Sensor (4-Cylinder)

The intake air temperature sensor is located in intake passage between air filter and intake manifold and is part of airflow sensor. The intake air temperature sensor informs ECU of 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 oxygen sensor(s) located in exhaust manifold(s). This sensor converts percentage of oxygen present in exhaust gases into an electrical signal that is transmitted to ECU.

The ECU 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 oxygen sensor is malfunctioning, ECU will operate engine at an oxygen sensor substitute value of.45 volt.

Throttle Position (TP) Sensor

TP 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 ECU determines idle speed ignition timing (idle speed switching point) on its own.

Vehicle Speed Sensor (VSS)

Sensor information comes from speedometer sensor located in instrument cluster. Vehicle speed sensor sends information on vehicle speed to ECU.

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 section.

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 ECU terminal No. 1.

Master Relay (DME Relay)

Master relay is powered through positive battery junction point "B" and supplies power to ECU, 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 throttle is depressed and manifold vacuum drops, pressure regulator increases fuel pressure to maintain constant flow to fuel injectors.

Catalytic Converter Protection (4-Cylinder)

To prevent overheating of catalytic converter, catalytic converter protection system enriches air/fuel mixture to reduce exhaust gas temperature.

Cold Start Control (6-Cylinder & V8)

During engine start-up, an increased quantity of fuel is injected according to coolant temperature and engine speed. If an engine re-start is attempted within one minute, complete initial quantity is no longer injected.

During engine warm-up, injected fuel quantity is reduced according to coolant temperature and engine speed in order to avoid a rich mixture. After warm-up to an engine coolant temperature of at least 158°F (70°C), injector timing (pulse width) is adapted to engine speed and load.

Fuel Injectors (4-Cylinder)

Each group of fuel injectors (No. 1 and 3, No. 2 and 4) is activated by ECU 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 camshaft sensor has not sent output signals since starting engine. If camshaft sensor signal is sent out later, 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 ECU receives a signal from camshaft sensor.

Fuel Injectors (6-Cylinder & V8)

Each fuel injector is independently controlled by ECU. This enables ECU to precisely meter injected fuel quantity and provides a faster response time during engine load changes. After engine has started (at about 600 RPM), fuel is injected into each specific cylinder at intervals of 120 degrees (crankshaft).

The ECU can also turn off fuel injectors as part of its catalytic converter protection function. See IGNITION TIMING CONTROL under IGNITION SYSTEMS for additional information.

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 ECU provides a ground path for oxygen sensor heater relay, which then provides battery voltage to 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 Control Valve

Idle speed is kept constant by an idle speed control valve, which supplies engine with necessary amount of air. Idle speed control takes place during period in which throttle potentiometer, or TPS, detects idle setting. The pre-programmed idle speed values in ECU are compared with actual operating values and corrected to compensate for component wear and other factors, such as minor air (vacuum) leaks.

When engine management system detects engagement of a drive range (1, 2, 3 or "D"), idle speed is increased by idle speed control valve to compensate for engine speed drop caused by engagement of torque converter. On A/C-equipped models, idle speed is also temporarily increased when A/C system is switched on. Upon receiving A/C compressor signal, quantity of air required for idle speed is corrected.

Ignition Timing & Anti-Knock Function (4-Cylinder)

Each cylinder has its own ignition coil (direct ignition system), eliminating need for a distributor. The ECU activates each coil separately.

If camshaft sensor has not sent output signals since starting 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 air/fuel mixture in combustion chamber.

(6-Cylinder & V8)

Ignition timing is controlled for each cylinder by means of a separate ignition coil, eliminating unnecessary high tension distributor. This ignition system is referred to by manufacturer as a coil ignition system with static high tension distribution.

For each cylinder, an output stage-controlled ignition coil is provided, which routes secondary voltage (up to 32,000 volts) via spark plug connector to spark plug. This configuration permits independent control of ignition timing.

By eliminating distributor, effective range of ignition timing control is increased by about 10 degrees, to a maximum of 59 degrees per cylinder. A camshaft sensor is used to maintain correct firing order. Ignition timing is retarded (anti-knock function), depending on inputs from knock sensors, in order to prevent pre-ignition of air/fuel mixture in combustion chamber.

Catalytic Converter Protection Function (6-Cylinder & V8)

If malfunctions on primary side of ignition system are detected by ECU, fuel injector(s) for that cylinder(s) will be turned off. This effectively prevents rich exhaust gas mixtures from reaching catalytic converter.

All Models

Fuel vapors are routed to engine via an activated carbon canister. Installed between carbon canister and air manifold is a purge control valve. Valve restricts flow of air based on sensor inputs to ECU.

Electrical activation of purge control valve depends on engine speed and load. Vacuum line to intake manifold is closed as long as valve is supplied with voltage. When no power is applied to valve, it can be opened by vacuum in intake manifold.

The fuel evaporation control cycle begins as soon as oxygen control system is active (closed loop mode). Upon completion of purge cycle, valve is closed for about 30 seconds. When engine is shut off, valve remains closed for another 3 seconds to prevent engine run-on.

SELF-DIAGNOSTIC FEATURE

ECU has a self-diagnostic feature which detects malfunctions in emission-related components and stores Diagnostic Trouble Codes (DTCs). ECU provides a substitute value if a failure occurs in engine (coolant) temperature sensor, intake air temperature sensor, airflow meter or exhaust gas oxygen sensor. These substitute values are canceled when normal engine operation is resumed. To aid in trouble shooting, stored fault codes and sensor values can be monitored via ECU and actuated components.

Vehicles are equipped with an OBD-II self-diagnostic system which utilizes a generic scan tool connected to in-vehicle Data Link Connector (DLC). In addition, vehicles are also equipped with a BMW self-diagnostic system which accesses DTCs using special BMW hardware and software connected to underhood BMW engine diagnostic connector socket. See TESTS W/CODES article.

An emission-related component or circuit failure will activate MIL on instrument cluster.

MISCELLANEOUS CONTROLS

Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.

A/C Compressor Shutoff (4-Cylinder)

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.

A/C Compressor Shutoff (Except 4-Cylinder)

During acceleration, normally closed A/C compressor relay temporarily switches off A/C compressor clutch on A/C compressor. During start-off phase (full engine load and vehicle speed less than 5 MPH), A/C compressor relay is activated by ECU for 10 seconds.

See also:
• TESTS W/CODES
• A/C COMPRESSOR CLUTCH