COMPUTERIZED ENGINE CONTROLS
Fuel is metered to intake system through individual injectors, mounted in intake manifold next to intake valves, one for each cylinder. Electronic Control Module (ECM) monitors data from various sensors and controls such functions as fuel injector pulse width ("on" time), ignition timing, and emission control devices.
ELECTRONIC CONTROL MODULE (ECM)
If a system malfunction occurs, a back-up system within ECM controls fuel and ignition system functions according to preprogrammed values. This allows vehicle to be driven, but performance may not be optimal. A self-diagnostic function allows ECM to store Diagnostic Trouble Codes (DTCs) in its memory. If a system fault occurs, Malfunction Indicator Light (MIL) will illuminate to inform driver of system problems and the appropriate DTC will be stored in memory. For further self-diagnostic system information, see appropriate TESTS W/CODES article.
Note. Components are grouped into 2 categories. First category is INPUT DEVICES, which are components that control or produce signals monitored by ECM. Second category is OUTPUT SIGNALS, which are components controlled by ECM.
INPUT DEVICES
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 appropriate WIRING DIAGRAMS article. Available input signals include the following
A/C Switch
Signals ECM of A/C operation.
Mass Airflow (MAF) Sensor
Hot-wire type airflow sensor uses heat transfer between incoming air and a heating resistor located in air intake to convert quantity of airflow into engine to an electrical signal.
Intake Air Temperature (IAT) Sensor
ECM applies reference voltage to a thermistor installed on air cleaner housing. Signal voltage increases and decreases with variations in air temperature.
Barometric Pressure (BARO) Sensor
Sensor is incorporated into ECM. ECM uses this signal to compensate for variations in altitude, which affect air/fuel mixture ratios.
Camshaft Position (CMP) Sensor
Sensor is located on camshaft support on left cylinder bank. Based on signals received from CMP sensor, ECM identifies cylinder No. 1. ECM then uses these signals to trigger ignition system and fuel injectors. This device may also be referred to as cam angle sensor.
Crankshaft Position (CKP) Sensor
Sensor is located on oil pump, at front center of cylinder block. Based on signals received from CKP sensor, ECM determines crankshaft angle and triggers ignition system and fuel injectors. This device may also be referred to as a crank angle sensor.
Engine Coolant Temperature (ECT) Sensor
ECM supplies reference voltage signal to thermistor. Resistance of ECT sensor changes with variations in coolant temperature, causing signal voltage to increase or decrease.
Inhibitor Switch (A/T)
Switch sends transmission gear position signal to ECM.
Knock Sensor (KS)
Installed on cylinder block, this sensor responds to cylinder block vibrations resulting from detonation. If detonation occurs, KS generates a signal to ECM. From this signal, ECM retards spark timing until detonation stops. This device may also be referred to as a detonation sensor.
Neutral Position Switch
Switch sends transmission gear position signal to ECM.
Oxygen Sensor (O2S)
Sensor generates voltage according to exhaust gas oxygen content. Signal voltage increases when oxygen content is low (rich), and decreases when oxygen content is high (lean). ECM controls air/fuel ratio based on voltage generated.
Throttle Position (TP) Sensor
TP sensor consists of a potentiometer and an idle switch. TP sensor sends ECM a signal corresponding to throttle valve position. Idle switch signal occurs when throttle is near idle position. ECM uses these signals to control air/fuel ratio during acceleration, deceleration, and idle.
Vehicle Speed Sensor (VSS)
ECM supplies voltage to one side of a reed switch located in speedometer. Speedometer cable revolution opens and closes reed switch, providing ECM with continuity/no continuity input. ECM interprets this signal as vehicle speed.
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 of each output component, refer to indicated system.
Idle Air Control (IAC) Valve
See IDLE SPEED under FUEL SYSTEM.
Canister Purge Control Valve (CPCV)
See FUEL EVAPORATIVE SYSTEM under EMISSION SYSTEMS.
Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM.
EGR Control Valve
See EXHAUST GAS RECIRCULATION (EGR) under EMISSION SYSTEMS.
Fuel Injector(s)
See FUEL CONTROL (SFI) under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Power Transistor
See IGNITION SYSTEMS.
Fuel Pump
Impeller-type pump is used. On all models, fuel pump is located in fuel tank. On all models, fuel pump pressurizes fuel through in-line filter, to fuel rail. Fuel pump receives battery power through fuel pump relay.
ECM energizes fuel pump relay based on inputs from ignition switch and ignition coil. During cranking, cranking circuit supplies current to energize fuel pump relay. After engine starts and key is released to RUN position, ECM provides fuel pump relay ground. This activates fuel pump.
Fuel Pressure Regulator
Regulator maintains constant fuel system pressure by bleeding excess fuel at injector rail back to fuel tank. Intake manifold vacuum acts on regulator diaphragm to control position of bleed-off valve in regulator.
Fuel Injectors
Fuel is supplied to engine through injector valves located near intake valve ports. ECM controls injector pulse width, which controls quantity of fuel metered through injectors.
Injectors are pulsed sequentially in spark plug firing order. Constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the ECM to control pulse width.
Fast Idle Control Device (FICD) Solenoid
FICD solenoid is mounted in-line with fresh air intake duct and intake manifold. When A/C request is present, FICD solenoid opens and allows fresh air to by-pass throttle valve, increasing idle speed for the extra engine load.
Idle Air Control (IAC) Solenoid Valve
The IAC solenoid valve combines an air cut valve, a duty valve, an intake air passage and a coolant passage.
Air cut valve contains a bimetallic strip which responds to coolant temperature, and a duty control valve which is operated by a signal sent from ECM.
When engine coolant temperature is low, air cut valve is fully opened by the action of the bimetallic strip so that air flow required for low engine coolant temperatures is maintained.
ECM controls the duty control valve to bring operating engine speed as close to preset idle speed as possible.
Idle Speed Control (ISC) Valve
Valve is mounted on air filter housing, in-line between air filter housing and intake manifold. At idle, ECM opens and closes solenoid valve. This allows fresh air to by-pass throttle valve through solenoid valve, into intake manifold.
Power transistor acts as primary current switching device for ignition system. When ECM sends signal to power transistor base, primary current flows through ignition coil.
Legacy
Distributorless ignition system is controlled by ECM. System consists of 2 ignition coils and a power transistor assembly. Power transistor assembly consists of 2 transistors which control primary current through each ignition coil. One transistor controls coil which fires cylinders No. 1 and 2. The other transistor controls ignition coil which fires cylinders No. 3 and 4.
Although each coil fires 2 plugs simultaneously, ignition takes place in only one cylinder, since the other cylinder is on its exhaust stroke when plug fires. Based on input from crankshaft and camshaft angle sensors, ECM sends signals to appropriate power transistor, which turns ignition coil on and off.
When engine knock occurs, the KS generates a signal to ECM. ECM retards spark timing until engine knocking stops, then gradually advances spark timing.
Ignition Timing Advance Control
On all models, ignition timing advance is controlled by ECM. Based on sensor input signals, ECM adjusts ignition timing to preprogrammed advance and retard specifications.
EXHAUST GAS RECIRCULATION (EGR)
EGR lowers oxides of nitrogen (NOx) exhaust emissions by admitting exhaust gases back into intake system. Exhaust gases lower peak combustion temperatures, which lowers NOx emissions.
EGR valve diaphragm receives operating vacuum through EGR control valve. ECM controls operation of EGR control valve.
Canister Purge Control Solenoid (CPCS) Valve
On models with Canister Purge Control Valve (CPCV), valve is located in vacuum signal line between CPCV and ported vacuum source. See CANISTER PURGE CONTROL VALVE (CPCV). Under appropriate conditions, ECM energizes CPCS valve. This allows ported vacuum signal to CPCV.
On models without CPCV, CPCS solenoid valve is located in purge line between canister and ported vacuum source. Under appropriate conditions, ECM energizes CPCS valve, allowing ported vacuum to purge canister.
Located on top of canister, this vacuum-controlled valve opens and closes purge line between intake manifold and canister. CPCV valve remains closed at idle because ported vacuum activates its control diaphragm. When vacuum activates control diaphragm, purge line opens, and stored vapors are drawn into intake manifold.
POSITIVE CRANKCASE VENTILATION (PCV)
PCV system draws crankcase blow-by, vapors, and gases into combustion system rather than allowing them to escape into atmosphere. Crankcase gases mix with air/fuel mixture, and are burned in combustion chamber. When engine is running, manifold vacuum pulls PCV valve open, allowing crankcase fumes to enter intake manifold. If engine backfires, PCV valve is forced closed, stopping flow of gases. This prevents fumes from igniting in crankcase.
All vehicles are equipped with MIL on instrument panel. Light comes on when ignition switch is turned on (bulb check), and when system malfunctions occur. For additional information, see appropriate TESTS W/CODES article.