TURBOCHARGERS
Turbo models use a water-cooled turbocharger, mounted directly to exhaust manifold, with a wastegate assembly attached to rear of turbine housing. Turbocharger consists of a turbine/compressor assembly, oil supply system and wastegate. Other components include impellers, impeller shaft, bearings and impeller housings.
The safety valve of system is a pressure-actuated wastegate that prevents excessive intake boost pressure. The wastegate is controlled by the turbo control valve. This 3-port solenoid valve monitors boost pressure and prevents turbo lag. The control valve is activated by turbo control unit, which receives signals from throttle position sensor, fuel injection ECU and turbo pressure sensor.
If boost pressure exceeds safe limits, engine damage may result. The wastegate opens when exhaust pressure exceeds a predetermined limit and allows exhaust gases to by-pass compressor. Turbocharger operation requires a large quantity of clean oil to prevent bearing failure. Engine oil pressure provides constant lubrication to system.
At idle and light throttle, turbo engine operates like a standard engine. When more power is required, exhaust gases from exhaust manifold enter turbocharger's turbine housing and flow through turbine blades. Exhaust flow and turbine speed increase as throttle opens and RPM increases. Impeller turns with turbine and forces air into compressor housing and intake manifold. As impeller and turbine speed increases, boost pressure also increases.
COMPUTERIZED ENGINE CONTROLS
Thel 940 models with use a Bosch LH-Jetronic multiport fuel injection system with EZ116K electronic ignition system or Regina (Bendix) multiport fuel injection system with Rex-1 electronic ignition system. Both systems have self-diagnostic capabilities.
All 850 non-turbo models use a Bosch LH 3.2 multiport fuel injection system with a EZ129K electronic ignition system with knock sensor. 850 turbo models use a Motronic 4.3 injection and electronic ignition system. All 850 models are equipped with self-diagnostic capabilities.
960 models use a Motronic 1.8 injection and electronic ignition system. System has complete self-diagnostic capabilities.
All systems use an Electronic Control Unit (ECU) that receives input from engine monitoring sensors. These sensors include camshaft sensor (EZ129K and Motronic), coolant temperature sensor, air temperature sensor (Regina), pressure sensor (Regina), oxygen sensor, knock sensor (EZ129K, turbo, and Motronic), mass airflow meter/sensor (Bosch, LH 3.2, and Motronic), timing pick-up (Motronic) and throttle switch. ECU uses these input signals to control air/fuel mixture for emission control, fuel economy and good driveability. On 940 and 850 non-turbo models, ignition control is provided by a separate ignition ECU. On 960 and 850 turbo models, ignition and fuel controls are provided by a single ECU.
CONTROL UNITS
Electronic Control Units (ECUs) provide precise control of fuel, ignition and turbo operation (if equipped). Each system has self-diagnostic capabilities. 940 turbo and non-turbo models use separate Electronic Control Units (ECUs) for fuel injection and ignition control. Fuel injection control unit is located behind right front kick panel. Ignition control unit is located behind left side of instrument panel, near accelerator pedal.
960 models use a single ECU for ignition and fuel controls. ECU is located above and to the right of accelerator pedal.
850 non-turbo models use separate ECUs for fuel injection and ignition control. 850 turbo models use a single ECU for ignition and fuel controls. On all 850 models, ECU(s) is located in right front of engine compartment.
Note. Components are grouped into 2 categories. The first category is INPUT DEVICES, which are components that control or produce voltage signals monitored by control unit. The second category is OUTPUT SIGNALS, which are components controlled by control unit.
INPUT DEVICES
All vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine input usage on a specific model, see WIRING DIAGRAMS article in this section. Available input signals include
Acceleration Sensor (850 Turbo)
Acceleration sensor is mounted to firewall at right rear of engine compartment. Sensor consists of a piezoelectric vibration pick-up that detects vertical acceleration of vehicle, for example driving on a bumpy road. ECU uses this sensor signal to determine if irregularities in crankshaft rotation are due to cylinder misfiring or due to driving on an uneven road surface.
A/C Switch
Signals fuel injection ECU of A/C operation. This allows fuel injection ECU to control idle speed with idle valve.
Air Temperature Sensor
Information gathered from air temperature sensor is combined with information from pressure sensor to calculate intake air mass.
Coolant Temperature Sensor
Coolant temperature sensor is a negative temperature coefficient type, meaning its resistance lessens as temperature increases. Four cylinder sensor unit has 2 resistors. One resistor is connected to fuel injection ECU and the other resistor is connected to the ignition ECU.
Knock Sensor
Knock sensor detects knocking and sends signal to Motronic or ignition ECU. ECU is able to gradually retard ignition timing to each individual cylinder. If knocking does not stop, a signal is sent to Motronic or fuel injection ECU to enrich air/fuel mixture. 6-cylinder engines are equipped with 2 knock sensors.
Ignition Control Unit (940 & 850 Non-Turbo)
Ignition ECU serves as an input device for fuel injection calculation. Ignition ECU provides information on engine speed, crankshaft or camshaft position, knocking, etc.
Intake Air Temperature Sensor (940 Non-Turbo With Regina)
Air in intake manifold passes through a hole in sensor and affects a probe. Resistance of probe reduces with increasing temperature. With addition of signals from pressure sensor and temperature sensor for intake air, control unit can calculate volume of air inducted into engine.
Manifold Absolute Pressure Sensor (940 Non-Turbo With Regina)
Also called a pressure sensor, sensor measures both atmospheric and manifold absolute pressure. ECU calculates intake air mass from these inputs.
Mass Airflow Meter
This meter measures intake air mass. Measure sensor is a heated wire which is maintained at 250°F (120°C) warmer than intake air. Motronic or fuel injection ECU is able to calculate mass of intake air by measuring amount of current required to maintain wire temperature. When engine is turned off, any contaminants on wire is burned off by heating wire to greater than 1800°F (1000°C).
Mass Airflow Sensor (850 & 960)
This sensor uses a hot film, rather than a heated wire to measure intake air mass. Since working temperature is high at 338°F (170°C), and flow and temperature-sensitive resistances are mounted on side of hot film, a burn-off function is not required.
Heated Oxygen Sensor (HO2S)
Also known as a Lambda probe, this heated oxygen sensor generates an electrical signal proportional to air/fuel mixture. Motronic or fuel injection ECU uses this information to adjust amount of injected fuel. The 850 turbo uses 2 oxygen sensors.
Throttle Position (TP) Sensor
The TP sensor signals Motronic or ignition and fuel injection ECUs when throttle is fully closed or fully open.
OUTPUT SIGNALS
ECU processes information from input sensors and sends appropriate voltage control signals to control devices.
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 of each output component, refer to system indicated after component.
A/C Clutch
Information not available.
CHECK ENGINE Light
See CHECK ENGINE LIGHT under SELF-DIAGNOSTIC SYSTEM.
Cold Start Valve
See FUEL CONTROL under FUEL SYSTEM.
EGR Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump
See FUEL DELIVERY under FUEL SYSTEM.
Idle Valve
See IDLE SPEED under FUEL SYSTEM.
Ignition Control Unit
See IGNITION SYSTEM .
Heated Oxygen Sensor (HO2S) Control
See FUEL CONTROL under FUEL SYSTEM.
Power Transistor & Ignition Coil
See ELECTRONIC IGNITION under IGNITION SYSTEM.
940 and 960 models are equipped with 2 fuel pumps. These are a primary low pressure pump, located in fuel tank, and a high pressure in-line pump. The 850 is equipped with an in-tank fuel pump. On all models, fuel pump(s) are equipped with check valves to hold fuel pressure in system when ignition is off. Fuel from main pump is sent through an in-line fuel filter. Fuel is then sent to fuel pressure regulator where pressure is maintained at a constant pressure in relationship to manifold pressure. Excess fuel returns to fuel tank via a return line.
Fuel Pressure Regulator
Pressure regulator is a sealed unit which is divided by a diaphragm into 2 chambers (fuel and spring chambers). Fuel chamber receives fuel through inlet side from injector fuel rail. Spring chamber is connected to intake manifold vacuum. At idle, intake manifold vacuum is high. Diaphragm is pulled back by intake manifold vacuum. Any excessive fuel is returned to fuel tank. As throttle is depressed, intake manifold vacuum decreases. Regulator spring overcomes manifold vacuum, increasing fuel pressure.
FUEL CONTROL
ECU calculates base injection pulse width by processing signals from various engine sensors. On 940, 850 turbo and 960, information from crankshaft position sensor (RPM) is used to trigger timing of fuel injection. On 850 non-turbo, engine speed information is provided to fuel injection ECU via ignition ECU. On all models, during normal driving conditions, injection duration is regulated in reference to mass air meter/sensor, engine speed, oxygen content of exhaust gases and coolant temperature. Under full throttle conditions, a richer fuel mixture is provided for increased power and to reduce combustion heat in engine and catalytic converter.
Each injector incorporates a solenoid, plunger and needle valve which opens and closes an orifice. Control unit supplies current through auxiliary relay for a predetermined period, opening all injectors simultaneously to inject atomized fuel. Injection takes place twice per revolution while starter motor is running and once per revolution under normal driving conditions. Fuel is injected into intake manifold close to each intake valve.
Cold Start Valve (940)
Since fuel condenses on engine surfaces under extremely cold conditions, cold start valve compensates for this condition. Located further upstream of the engine than injectors, valve supplies fuel in more gaseous form than injectors. Operated directly by the ECU, rather than by a thermal timer, valve opens at about 5°F (-15°C) and stays open until an engine speed of about 900 RPM. Valve remains closed at engine speeds greater than 900 RPM.
HO2S operates only within a certain temperature range of about 545-1530°F (285-850°C). Sensor is electrically heated to enable it to reach operating temperature quickly, even at low exhaust gas temperatures.
IDLE SPEED
Engine idle speed is controlled by ECU depending upon engine operating conditions. ECU senses engine operating conditions and determines best idle speed. Idle speed is controlled by varying air passage inside idle valve.
Idle valve uses a solenoid or motor to control by-pass air. Signal from ECU determines idle speed by controlling amount of by-pass air.
ELECTRONIC IGNITION
On 940, electronic ignition ECU controls primary windings of ignition coil using signals from fuel injection ECU based on crankshaft position, engine speed and vehicle speed. Turbo models use a Hall Effect sensor, located in distributor, to determine crankshaft position and engine speed. Non-turbo models use a flywheel sensing permanent magnet generator to determine crankshaft position and engine speed.
On 850 non-turbo, distributor ignition ECU computes correct timing of each ignition pulse in response to signals from vehicle speed sensor (supplies information on engine speed and crankshaft position), camshaft position sensor (assists control module to determine rate of combustion in cylinders), knock sensors (supply signals indicating engine knock), fuel injection ECU (supplies information on throttle opening, engine load, and coolant temperature, and transmission ECM (supplies signals indicating reduction in torque required prior to gear shift).
On 850 turbo, ECU computes correct timing of each ignition pulse in response to signals from RPM sensor, Camshaft Position (CMP) sensor, Mass Airflow (MAF) sensor, Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, Knock Sensors (KS) and Transmission Control Module (TCM).
On 960, each cylinder is equipped with an individual ignition coil, eliminating the distributor. ECU computes instant at which each ignition coil must deliver its high voltage pulse. All 6 coils are controlled by 2 power units.
Ignition Timing Advance Control
Ignition timing is totally controlled by an ECU. Ignition timing is based on preprogrammed information and modified by inputs from engine sensors.
940 turbo has one knock sensor. 850 and 960 models have 2 knock sensors. On all models, knock sensor is fitted to cylinder block to sense detonation inside cylinders. When detonation is detected, ECU retards ignition timing in each cylinder individually until knocking stops. If knocking continues, ignition control unit signals turbo control unit to reduce boost pressure in stages.
FUEL EVAPORATIVE SYSTEM (EVAP)
Evaporative emissions system is designed to prevent fuel vapor from entering atmosphere. Fuel system is completely sealed and vented only through a carbon canister. System consists of pressure/vacuum relief fuel filler cap, rollover valve, charcoal canister, purge valve and various connecting hoses.
Fuel pressure/vacuum relief filler cap allows excessive tank pressure to vent. It also allows air into fuel tank if vacuum should become excessive due to a malfunction in fuel evaporation system. Fuel tank vapor is vented by a line through rollover valve to charcoal canister. Rollover valve is located in vent line close to fuel tank. Valve is designed to prevent fuel spillage if vehicle rolls over. Valve is open until vehicle is at a 45-degree angle or more from horizontal position.
Canister Purge Valve
Charcoal canister is filled with activated charcoal. Fuel vapor from tank is absorbed by charcoal when engine is not running. When engine is running faster than idle, canister purge valve opens and fuel vapor is drawn into engine and burned.
EGR System
EGR system operates by returning some exhaust gases to engine to be mixed with air/fuel mixture. This exhaust gas, which is basically inert at this stage, lowers combustion temperature. Reducing combustion temperature reduces amount of oxides of nitrogen (NOx) released into atmosphere.
When engine coolant temperature is less than 115°F (45°C), solenoid receives no current and EGR system is inactive. With engine at operating temperature, solenoid receives current from relay and opens vacuum line to EGR valve. EGR valve is opened completely by negative pressure. Even the slightest throttle opening opens idle switch. Time relay cuts current to solenoid, disconnecting EGR system for about 5 seconds and avoiding HC and particle build-up during acceleration from idle.
EGR Electronic Vacuum Regulator Valve (EVRV)
EGR valve is supplied with a vacuum control signal from connection in lower section of the EVRV. Vacuum in intake manifold is supplied to upper connection. EVRV stabilizes signal from intake manifold and converts control module signal into a modified vacuum signal for controlling EGR valve.
Note. CHECK ENGINE light is also known as Malfunction Indicator Light (MIL).
All models are equipped with a CHECK ENGINE light located on instrument panel. Light will illuminate when ignition switch is turned to ON position (bulb check) or when emission-related systems are malfunctioning during normal engine operation.