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.
RESONANCE FLAPS
Intake system is divided by 2 resonance flaps. This allows the natural occurrence of intake air oscillations to draw more air into the engine.
Resonance flaps are activated by Digital Motor Electronics (DME) control unit via a resonance flap solenoid, which opens resonance flaps with the help of a diaphragm valve when engine speed is greater than 5500 RPM. Solenoid is connected with a vacuum supply tank.
Solenoid and diaphragm valve will close when engine speed drops to less than 5400 RPM. To avoid seizure of resonance flaps (from deposits in intake), solenoid is activated briefly each time ignition is turned on. This causes resonance flaps to move from closed to open position, and back to closed.
Note. The aluminum intake runners have been changed to plastic. To reduce noise, a muffler has also been fitted between the idle speed stabilizer and the intake system.
THROTTLE VALVE ASSEMBLY
To improve engine pickup and transition behavior during slow acceleration, movement of throttle valve assembly has been modified so that smaller diameter throttle valve plate moves in the open direction first. Large diameter throttle valve plate is also opened via a drag arm, after first throttle valve is opened approximately 5 degrees. In addition, idle speed switch must open before small throttle valve plate moves.
Note. A single-flap throttle valve assembly is used in conjunction with the new plastic intake runners. Because of these changes, the control and venting lines, the air box, and the hoses for the venturi tube are also modified.
COMPUTERIZED ENGINE CONTROLS
All models are equipped with Digital Motor Electronics (DME) engine management system. (Scheme 1) The DME system uses various sensors to monitor intake air volume, engine speed, crankshaft position, intake air temperature and throttle position. Signals from these sensors, along with the start and oxygen (O2) sensor signals, are sent to the DME control unit. The control unit uses input from these sensors to determine correct fuel amount and ignition timing.
Scheme 1
DME CONTROL UNIT
A 55-pin DME control unit monitors and controls all engine management functions. (Scheme 2) The DME control unit is located under driver's seat. The DME control unit has the ability to store fault codes related to fuel injection and ignition system. Detected faults remain stored for at least 50 engine starts.
On 1990 models, the DME control unit harness connector includes 2-pin and 3-pin harness connector leads that must be bridged to battery positive (B+) or ground, depending on the state (California) and whether vehicle is equipped with a manual or automatic (Tiptronic) transmission. The process of bridging these leads is known as control unit coding.
All 1991-92 models are equipped with a modified DME control unit (Part No. 964 618 124 00). Control unit coding distinctions are only made between manual and automatic transmissions. The state (California) coding feature has been deleted. The modified control unit also has the ability to activate CHECK ENGINE light.
Scheme 2
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the 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 input device usage, see WIRING DIAGRAMS article in this section. The available input signals include the following
Airflow Sensor
Sensor is located in the air stream of airflow meter and supplies air volume information to DME control unit. The control unit uses this and other information to regulate fuel injection rate.
Airflow meter incorporates an airflow measuring plate. Airflow plate is connected to a potentiometer and opens when engine draws in air. Potentiometer informs control unit of the engine load by transmitting an electrical signal determined by the position of measuring flap. Potentiometer also signals control unit when fuel enrichment is necessary to prevent loss of engine power during sudden acceleration or deceleration.
Detonation (Knock) Sensors
Engine is fitted with 2 knock sensor brackets, each with a knock sensor attached. These brackets unite No. 1 and 3 cylinders, and No. 4 and 6 cylinders. If engine knock is detected, ignition timing for pertinent cylinder is retarded 3 degrees. If engine knock continues, ignition will continue to be retarded in 3-degree decrements (up to 9 degrees maximum). When engine knock stops, ignition timing is returned in small steps to its optimal value.
Engine Speed/Reference Mark Sensor
Speed sensor is mounted on an adjustable bracket with reference mark. Sensor measures engine speed by monitoring teeth on flywheel signal ring gear. Sensor sends 2 voltage pulses to the DME control unit as each tooth passes.
Reference mark sensor is located on crankcase flange. Sensor detects crankshaft position in relation to TDC and sends signal to control unit. Sensor is triggered by reference mark sender ring gear on flywheel.
Engine Temperature Sensor
Sensor is located in cylinder head, near No. 3 cylinder. Sensor supplies information on engine temperature to control air/fuel ratio (as engine temperature varies during cold start) and spark timing.
Hall Effect Sensor
Hall Effect sensor is installed in the double-ignition distributor. Hall Effect sensor identifies ignition timing of No. 1 cylinder and appropriately assigns signals of both knock sensors to firing order.
Idle Speed Switch
Switch is used to sense closed throttle position and is mounted on throttle housing, opposite full throttle switch. Idle speed switch signals DME control unit when to control idle stabilization and coasting fuel cutoff.
Intake Air Temperature Sensor
Sensor is located in air stream of airflow meter and supplies incoming air temperature information to control unit. The DME control unit uses this, along with other information, to regulate fuel injection rate.
Full Throttle Switch
The full throttle switch is mounted on throttle housing. Full throttle switch signals DME control unit of optimum power demands.
Note. The oxygen (O2) sensor has been modified to work with the new DME control unit. The new O2 sensor may ONLY be used with the new DME control unit. The O2 sensors CANNOT be identified by physical appearance. The part number must be used to properly distinguish the NEW sensor (Part No. 258 003 024) from the OLD sensor (Part No. 258 003 084).
Oxygen (O2) Sensor
The Oxygen (O2) sensor is mounted in the engine exhaust stream, in front of the catalytic converter. The O2 sensor supplies a low voltage signal (less than 0.5 volt) when fuel mixture is lean (high oxygen), and a higher voltage (up to one volt) when fuel mixture is rich (low oxygen). This rich-to-lean fluctuating signal is used by the DME control unit in calculations of air/fuel mixture (fuel injector) control.
The O2 sensor has 3 wires: 2 for heater element (power and ground), and a single wire for O2 sensor signal. The O2 sensor heating element allows sensor to reach operating temperature faster. Heating element is energized through the fuel pump/DME relay when ignition is turned on. Connector from sensor to wiring harness are located near flywheel engine speed/reference mark sensor connector.
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 indicated system after components.
A/C Compressor Control
See A/C COMPRESSOR CONTROLS under MISCELLANEOUS CONTROLS.
CHECK ENGINE Light
Refer to CHECK ENGINE LIGHT under SELF-DIAGNOSTIC SYSTEM.
DME (Fuel Pump) Relay
See FUEL DELIVERY under FUEL SYSTEM.
Double Ignition
See IGNITION TIMING CONTROL SYSTEM under IGNITION SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Tank Vent Solenoid
See EVAPORATIVE EMISSIONS SYSTEM under EMISSION SYSTEMS.
Idle Speed Control
See IDLE SPEED CONTROL under FUEL SYSTEM.
Resonance Flap Solenoid
See RESONANCE FLAPS under AIR INDUCTION SYSTEM.
Fuel Pressure Regulator
Pressure regulator is located at end of injection collection line. (Scheme 1) Pressure regulator maintains constant fuel pressure to fuel injectors.
Fuel Pump & Fuel Pump Relay
Electric fuel pump is located in fuel tank and maintains a constant fuel supply to injection system. (Scheme 3) Power for fuel pump is supplied by the DME relay. Fuel pump portion of DME relay is energized by the DME control unit.
Scheme 3
FUEL CONTROL
The DME control unit meters the amount of fuel distributed to each cylinder by sequential control of fuel injectors. The amount of fuel reaching each cylinder is controlled separately, once for each working (compression) stroke, according to firing order. Injection timing can be changed depending on engine temperature, engine speed and load, or other engine operating conditions.
A fuel rail links fuel pressure regulator with fuel injectors. Each cylinder is provided with a solenoid-operated injector, which sprays fuel toward backside of each inlet valve. The DME control determines the length of time each injector is open. The injector's on time determines amount of fuel delivered.
All models use an air regulating valve (idle speed positioner) to provide auxiliary air valve operation and idle speed stabilization. Air regulating valve is located near throttle body and is controlled by DME control unit. When necessary, air regulating valve will adjust air by-pass opening to maintain both cold and warm engine idle speeds.
Air Regulating Valve
Valve controls air by-pass opening around throttle. When necessary, control unit operates air regulating valve to stabilize idle speed.
IGNITION TIMING CONTROL SYSTEM
The engine speed/reference mark sensor sends a signal to DME control unit. The control unit uses this signal to fire coils for secondary ignition. Based on information received from data sensors, spark control allows DME control unit to determine the exact instant that ignition is required.
Detonation Retard Operation
See DETONATION (KNOCK) SENSORS under INPUT DEVICES.
Double-Ignition System
Two spark plugs per cylinder ignite air/fuel mixture. With this system, burn time of air/fuel mixture is shorter because of shorter spark travel. There is also less tendency for detonation, making it possible to increase compression ratio and improve thermodynamic efficiency in engine. Ignition must be triggered about 6 degrees later because of shorter spark travel and burn time.
Ignition triggering signal goes from Digital Motor Electronic (DME) control unit to 2 ignition control units installed on left side in engine compartment.
In each ignition final stage, there is activation of a transistor, which interrupts secondary circuit of both ignition coils (also installed on left side of engine compartment). In this manner, an ignition spark is created by both ignition coils for each cylinder in sequence.
A Hall Effect sensor, installed in the double-ignition distributor, identifies cylinder No. 1 (ignition TDC). (Scheme 4) Hall signal is required by DME for assignment of both knock sensor signals, as well as for sequential fuel injection control. Each distributor rotor is controlled by centrifugal weights. This ensures alignment between distributor rotor and corresponding ignition contact in distributor cover.
Double-Ign. Dist., Hall Effect Sensor & Centrifugal Weights. Scheme 4
EVAPORATIVE EMISSIONS SYSTEM
Fuel evaporation control system is designed to prevent fuel vapors from escaping into atmosphere. (Scheme 5) System consists of a non-vented fuel tank filler cap, expansion tank, evaporation tank, charcoal canister, fuel tank vent solenoid, purge diaphragm valve, restrictor and connecting lines and hoses.
Expanded fuel, caused by high ambient temperatures, is collected in expansion tank. Liquid fuel is returned to main tank by venting action as fuel is used from main tank.
Fuel vapor passes through a vent line to charcoal canister where it is stored. A second vent line connects canister to a purge control diaphragm valve. Purge control diaphragm valve is closed when engine is not running. When engine is started, manifold vacuum is applied to top fitting of purge diaphragm valve. This opens an internal passage in diaphragm valve, allowing vapors stored in charcoal canister to be drawn into the intake manifold for burning during the combustion process. Purge line also contains a fuel tank vent solenoid which is controlled by the DME control unit.
Scheme 5
SELF-DIAGNOSTIC SYSTEM
The DME control unit has the ability to store fault codes related to fuel injection and ignition system. Detected faults remain stored for at least 50 engine starts. If positive battery cable or DME control unit connector is disconnected, the fault code memory and system adaptation will be cleared. For additional information, see the TESTS W/CODES article in this section.
All models are equipped with a CHECK ENGINE light, which comes on if a component related to fuel injection and ignition system fails. The CHECK ENGINE light is installed in oil temperature/pressure gauge cluster. For more information, see TESTS W/CODES article in this section.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/C COMPRESSOR CONTROLS
Specific information is not available from manufacturer. The DME control unit, however, receives input and sends output signals over terminals No. 40 (A/C compressor), 41 (A/C switch) and 50 (A/C-heating system regulation).
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