COMPUTERIZED ENGINE CONTROLS
Computerized engine control system consists of a Powertrain Control Module (PCM), various sensors (inputs), control devices and actuators (outputs), and related wiring which links system together. PCM monitors system inputs and controls system outputs accordingly to obtain optimal fuel economy and engine performance, while maintaining acceptable exhaust emission levels.
In the event of an input or output failure, PCM will memorize and store information as a Diagnostic Trouble Code (DTC). DTCs can be used by technician to diagnose various driveability and emission related problems. DTCs are accessed through On-Board Diagnostics II (OBD II) Data Link Connector (DLC) with use of a Tech II or other OBD II compatible scan tool. DLC is located under instrument panel, accessible from driver's side.
PCM incorporates a fail-safe (limp-in) mode. If a fault occurs during vehicle operation, PCM will substitute a predetermined value and/or signal for continued operation. Driving performance will be affected, but vehicle may still be driven. When PCM detects a system fault, Malfunction Indicator Light (MIL) will flash or illuminate steadily to notify driver of a developing problem.
POWERTRAIN CONTROL MODULE
The Powertrain Control Module (PCM) is located under the glove box. PCM power is sent from main relay to PCM connector C2, terminals No. 4 and 15. PCM backup power is received from junction block to PCM connector C1, terminal No. 2.
Note. Components are grouped into 2 categories. First category is INPUT DEVICES , which are components that control or produce voltage signals monitored by PCM. Second category is OUTPUT SIGNALS , which are components controlled by PCM.
Camshaft Position Sensor
The Camshaft Position (CMP) sensor contains a signal generator (hall switch) and is located inside a distributor type housing, located on back of the cylinder head (2.0L) or left cylinder head (2.5L). Signal rotor, inside CMP sensor housing, is driven by camshaft. When sensor rotor turns, a magnetic flux is applied to hall element. Hall element generates a voltage proportional to the magnetic flux. Voltage signal is modified into a digital pulse and then sent to PCM. PCM uses this signal to control ignition timing and fuel injectors.
Crankshaft Position Sensor
The Crankshaft Position (CKP) sensor is mounted on engine block, behind flexplate (2.0L) or near crankshaft pulley (2.5L). CKP sensor consists of a pick-up coil and magnet. CKP sensor generates an AC voltage signal as signal rotor on crankshaft timing belt pulley rotates. This signal is sent to PCM and is used to monitor misfire conditions.
Electric Load Idle-Up Signal
Signal is sent to PCM from high electrical load components (headlights, clearance lights, heater fan, etc.) and high accessory load components (Power Steering Pressure (PSP) switch and A/C compressor clutch) as one or more are turned on. PCM uses this input to control operation of idle control system to prevent rough idle or stalling.
Electronic Brake Control Module Signal
Signal is sent from the Electronic Brake Control Module (EBCM) to PCM when ABS is active. PCM uses this input to modify fuel delivery and IAC valve operation during ABS control.
Engine Coolant Temperature Sensor
The Engine Coolant Temperature (ECT) sensor is located at rear of cylinder head, in coolant housing (2.0L), at rear of left cylinder head (2.5L). ECT sensor thermistor changes resistance with respect to temperature. High temperature causes low resistance. Low temperature causes high resistance. A reference voltage (supplied and monitored by PCM) is modified by sensor resistance. PCM uses this information to control operation of various devices.
Fuel Level Sensor
Fuel level sensor is located in fuel tank. Sensor sends a signal to PCM and fuel gauge in instrument cluster. PCM uses fuel level sensor signal as one of the monitoring conditions for detecting EVAP control system DTCs. PCM also uses fuel level sensor input to control EVAP tank pressure control solenoid vacuum valve. If fuel level is greater than specified, PCM will operate EVAP tank pressure control solenoid vacuum valve to prevent fuel from flowing into EVAP charcoal canister from fuel tank.
Fuel Tank Pressure Sensor
Fuel tank pressure sensor is installed on top of fuel tank. Fuel tank pressure sensor senses fuel vapor pressure in fuel tank and compares it with barometric pressure. PCM will convert pressure reading into a voltage signal. Fuel tank pressure sensor receives a 5-volt reference from PCM. As fuel tank pressure changes, resistance of sensor changes and sends a voltage signal back to PCM. PCM uses this signal to determine if a leak exists in EVAP system.
Heated Oxygen Sensor No. 1
Heated Oxygen Sensor No. 1 (HO2S1) is mounted in exhaust manifold in front of catalytic converter. HO2S1 produces 0.1-0.9 volt under normal operating temperatures greater than 600°F (316°C). PCM uses HO2S1 voltage to determine exhaust gas oxygen concentration during engine operation. Low voltage indicates a lean exhaust mixture and a higher voltage indicates a rich exhaust mixture. HO2S1 works similar to a non-heated sensor, except HO2S1 is heated to operating temperatures to allow system to quickly enter closed loop operation.
Heated Oxygen Sensor No. 2
Heated Oxygen Sensor No. 2 (HO2S2) is mounted just after catalytic converter. HO2S2 operates similar to HO2S1 and is used by PCM to monitor catalyst efficiency. HO2S2 is normal when its activity appears lazy or inactive, indicating converter is functioning properly.
Intake Air Temperature Sensor
The Intake Air Temperature (IAT) sensor is located on side of air cleaner housing. IAT sensor thermistor changes resistance with respect to intake air temperature. High temperature causes low resistance. Low temperature causes high resistance. A reference voltage (supplied and monitored by PCM) is modified by sensor resistance. PCM uses this information to help control fuel injector(s), ignition timing and EGR operation.
Knock Sensor (2.0L)
The Knock Sensor (KS) is a piezoelectric element, located in center of the block and under the intake manifold. KS sends a variable AC voltage signal to PCM depending on engine detonation. PCM will then retard spark timing until engine knock is reduced.
Manifold Absolute Pressure Sensor
The Manifold Absolute Pressure (MAP) sensor is connected to PCM by a 3-wire harness and to engine by a manifold vacuum hose. MAP sensor has an internal mechanical resistor which varies resistance based on changes in engine load (manifold vacuum). As internal resistance varies, return voltage signal to PCM varies. PCM interprets this voltage change as changes in engine load and uses this signal to help determine control of fuel injectors and other various sensors and switches.
Mass Airflow Sensor
The Mass Airflow (MAF) sensor is located in air duct between air cleaner and throttle body. MAF sensor consists of a thermal resistor, metering duct and control circuit. Sensor uses thermal resistor to detect amount of air drawn into engine and sends information to PCM as a current signal. PCM uses this signal to determine engine requirements and control fuel injectors.
Power Steering Pressure Switch
The Power Steering Pressure (PSP) switch signals PCM when vehicle needs power steering assist. When steering wheel is turned, pressure will close PSP switch. PCM uses this signal to tell IAC valve to raise idle speed before load can cause a poor idling condition. PSP switch is located in power steering pump housing.
System Voltage Compensation
Fuel injector is driven by PCM. There is a delay from when PCM signals fuel injector on to when injector actually opens and provides fuel. This delay is known as Ineffective Injection Time, and is part of the normal event of electrical flow through fuel injector solenoid coil. Ineffective injection time varies with system voltage. PCM will take voltage readings and compute required offset necessary for proper timing of fuel injector signal.
Throttle Position Sensor
PCM supplies Throttle Position (TP) sensor with a 5-volt reference signal. Sensor is a potentiometer (variable resistor). The TP sensor sends PCM a potentiometer output signal corresponding to throttle valve position. PCM uses this signal to control the following
- Idle Air Control (IAC) valve operation.
- EGR valve operation.
- Ignition timing.
- Fuel delivery.
- OBD II system diagnostics.
Transaxle Range Switch Signal (A/T Only)
A Transaxle Range (TR) signal is sent to PCM from Park/Neutral Position (PNP) switch when shift lever is in Park or Neutral. PCM uses this signal to control fuel injection timing and IAC valve operation.
Vehicle Speed Sensor
The Vehicle Speed Sensor (VSS) is located in output shaft of transmission (2WD) or transfer case (4WD). VSS contains a driven gear that triggers a hall switch. PCM receives a digital signal from VSS and used the signal to assist in controlling transmission and engine operation.
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 the system indicated after component.
A/C Condenser Fan Relay
A/C condenser fan motor is operated by the A/C condenser fan relay. A/C condenser fan relay is controlled by the PCM. PCM turns on relay when A/C system is operating, ECT sensor indicates that engine coolant temperature is greater than 230°F (110°C), and PCM detects an ECT sensor malfunction.
A/C Cutout Signal
PCM can prevent engagement of A/C compressor clutch through the A/C compressor control module. PCM provides a ground signal to A/C compressor control module that enables module to operate A/C compressor clutch relay. PCM will allow operation of A/C compressor clutch except when PCM did not receive A/C request input (idle-up signal) from A/C compressor control module, Engine is operating in Wide Open Throttle (WOT), ECT sensor indicates that engine coolant temperature is greater than 230°F (110°C), and engine speed is less than 500 RPM or greater than 6500 RPM.
EGR Valve
See EXHAUST GAS RECIRCULATION under EMISSION SYSTEMS.
EVAP Canister Purge Valve
See FUEL EVAPORATION SYSTEM under EMISSION SYSTEMS.
Fuel Cutoff System
See FUEL CONTROL under FUEL SYSTEM.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Heated Oxygen Sensor Heater
See FUEL CONTROL under FUEL SYSTEM.
Idle Air Control System
See IDLE SPEED under FUEL SYSTEM.
Ignition Control System
See IGNITION TIMING CONTROL SYSTEM under IGNITION SYSTEM.
Main Relay
Main relay provides power to most engine control system components. Relay is controlled by the PCM. PCM provides an electrical ground for main relay coil when ignition is on. Main relay is located in junction block, behind left kick panel.
Malfunction Indicator Light (MIL)
See SELF-DIAGNOSTIC SYSTEM .
Fuel Pump
Electric fuel pump is located in fuel tank and is part of fuel sender assembly. Fuel pump receives power when PCM provides ground to fuel pump relay. When power is supplied to fuel pump, motor and impeller inside pump turn. This causes a pressure difference to occur between both sides of impeller. When fuel is drawn through fuel pump inlet port, pressure is increased and discharged through outlet port.
The fuel pump also incorporates a relief valve to prevent excessive rise of discharge pressure and a pressure control valve to keep residual pressure in fuel feed line when fuel pump is not activated.
Fuel pump relay is energized based upon ignition switch and RPM inputs. Power for fuel pump is supplied by main relay, which receives its power from FI fuse (15-amp). Main relay is energized for 3 seconds by PCM when ignition is turned on. When engine is running, PCM maintains relay operation. Fuel pump relay is located in junction block, behind left kick panel.
Fuel Pressure Regulator
Fuel pressure regulator is a spring/vacuum operated diaphragm-type relief valve which maintains a constant regulated fuel pressure under all vehicle operating modes. When manifold vacuum is high (low fuel requirements), diaphragm is drawn in counteracting spring pressure, routing excess fuel back to fuel tank. When manifold vacuum decreases (engine load), spring pressure overcomes vacuum, closing off fuel tank return line. This maintains appropriate pressure and volume to injector(s) under different operating conditions.
Fuel Injector
Sequential Fuel Injection (SFI) is used, which incorporates fuel injectors mounted between fuel rail assembly intake manifold. SFI system injects fuel into each cylinder head intake port. When solenoid coil of fuel injector is energized by PCM, coil becomes an electromagnet. This lifts injector plunger, allowing fuel under pressure to be injected into cylinder head. Since fuel pressure is relatively constant, air/fuel mixture is controlled by injector pulse width (ON time). PCM determines proper pulse width based upon input signals received from various sensors and switches.
Fuel injector triggering and timing is determined by PCM based upon ignition signals from Camshaft Position (CMP) sensor. Since PCM interprets CMP sensor signal as an indication of spark presence, fuel injector triggering will cease if CMP sensor signal is lost.
The fuel cutoff system will stop fuel injection during deceleration to prevent excess fuel build-up during periods when oxygen is insufficient for combustion (i.e., closed throttle, high RPM). Fuel cutoff system will also deactivate injectors when engine speed exceeds 6800 RPM. This is done to prevent engine damage due to excessive engine speed. As engine speed drops to less than 6500 RPM, injection will resume.
An electrical heating element is located inside oxygen sensor to bring oxygen sensor up to operating temperature quickly. When vehicle is cold, PCM grounds oxygen sensor heating element circuit. Power to sensor is provided when ignition is on.
A drop in system voltage directly affects pulse width of injectors. As system voltage drops, pulse width decreases. This causes a leaner air/fuel mixture than desired. To compensate for this, PCM monitors system voltage. If system voltage drops, PCM will increase injector pulse width.
Idle Air Control (IAC) Valve
The Idle Air Control (IAC) valve is located on underside of throttle body. IAC valve enables PCM to easily control engine idle speed by precisely metering engine's air intake at closed throttle. An air passage by-passing the throttle valve is provided to route intake air directly into intake manifold. PCM-energized IAC valve regulates airflow through this passage. IAC valve contains an engine coolant passage to enable IAC valve to operate more efficiently at cold temperatures.
Air is allowed to pass through IAC valve when it is energized. Solenoid portion of valve is energized whenever idle speed drops to less than desired RPM due to engine load (i.e., electrical, A/C, P/S, automatic transmission in Drive, etc.). IAC valve is also energized each time engine is started and during periods of deceleration (to compensate for rich mixtures caused by a fully closed throttle). Duration of IAC valve operation is dependent on coolant temperature.
ELECTRONIC IGNITION SYSTEM
The electronic ignition system consists of 4 ignition coils (2.0L) or 6 ignition coils (2.5L) controlled by the PCM. Each coil is directly mounted to spark plug. Each ignition coil assembly has a built-in ignition module (ignitor) that control current flow in primary coil winding. The secondary coil voltage travels directly to spark plugs. PCM utilizes reference pulses from Camshaft Position (CMP) sensor to determine engine speed. PCM cannot operate ignition system or fuel injectors without engine speed signal from CMP sensor. Power for ignition coils is provided from IG fuse (20-amp), located in junction block.
Ignition timing is calculated by PCM based on current engine status. PCM provides 3 modes of ignition timing control. Each ignition timing mode provides the most suitable spark advance for optimal engine performance. The 3 modes are as follows
- Initial Timing Control (At Engine Start) Initial Timing Control ignition advance provides better starting performance during start-up when engine speed is less than 500 RPM. PCM set initial ignition timing advance to 5 degrees BTDC.
- Basic Timing Control (After Engine Start) After engine is running, ignition timing advance is determined by combining Basic Timing Control and Compensating Timing Control. Basic Timing Control ignition advance is based on engine coolant temperature, engine speed and intake air volume. Ignition timing advance value calculated from the Engine Coolant Temperature (ECT) , Camshaft Position (CMP) and Mass Airflow (MAF) sensor inputs are added to the 5 degrees BTDC initial advance and then modified by the Compensating Timing Control value.
- Compensating Timing Control Compensating Timing Control is added to Basic Timing Control value and varies according to engine and vehicle conditions. Conditions utilized for determining Compensating Timing Control value are: ECT, Intake Air Temperature (IAT), Knock Sensor (KS) input and engine load.
PCM cannot operate ignition system without engine speed signal from CMP sensor. PCM controls ignition timing by controlling ignition coils. Ignition timing is controlled by PCM and is adjustable. See IGNITION TIMING in appropriate ON-VEHICLE ADJUSTMENTS article.
EMISSION SYSTEMS
For additional information, see appropriate EMISSION APPLICATIONS and VACUUM DIAGRAMS articles.
EXHAUST GAS RECIRCULATION
To lower oxides of nitrogen (NOx) exhaust gas emissions, an Exhaust Gas Recirculation (EGR) system is used. EGR system introduces exhaust gases into intake system. Exhaust gases are noncombustible gases which, when combined with incoming air/fuel mixture, lower peak combustion chamber temperatures. A ported with pressure transducer type EGR control system is used.
Exhaust Gas Recirculation Valve
The Exhaust Gas Recirculation (EGR) valve is a stepper motor design. Stepper motor controls the opening of exhaust gas passage to intake manifold. PCM uses inputs from ECT, TP, VSS, and MAP sensors for proper operation of EGR valve. EGR valve is closed under heavy load when demand for power is high. EGR valve is also closed at closed throttle operation to ensure smooth idle and stable engine operation. During low and medium engine loads, EGR valve is usually open. PCM calculates optimum opening of EGR valve by combining preprogrammed engine calibrations with various sensor inputs.
FUEL EVAPORATION SYSTEM
Note. For fuel evaporation system component identification, refer to illustration. (Scheme 1)
Charcoal Canister
Vapors generated in fuel tank pass through tank pressure control valve and enter charcoal canister where charcoal absorbs and stores fuel vapors. Canister is purged or cleaned by air drawn through filter at bottom of canister and into intake manifold through canister purge valve and purge line. Vacuum is applied to canister purge valve to open it when engine is at normal operating temperature and is running at a specified RPM.
Canister Purge Valve
PCM controls canister purge valve according to signals from various sensors. When PCM signals canister purge valve, fuel vapors flow from charcoal canister into combustion chambers for burning.
Tank Pressure Control Valve
Fuel tank pressure or vacuum is controlled by tank pressure control valve. When fuel tank pressure reaches a specified value, tank pressure control valve opens and allows fuel vapors to enter charcoal canister. When engine is running and fuel level is lower or higher than specified value, PCM turns on EVAP tank pressure control solenoid vacuum valve and vacuum is applied to diaphragm of tank pressure control valve. With vacuum applied, tank pressure control valve will open and equalize pressure between fuel tank and charcoal canister.
Scheme 1
POSITIVE CRANKCASE VENTILATION
The Positive Crankcase Ventilation (PCV) system draws crankcase blow-by gases (hydrocarbons) into air induction system rather than allowing them to escape to atmosphere. Crankcase gases are mixed with air/fuel mixture and burned in combustion chamber. PCV system consists of a PCV valve, air inlet filter or separator (if equipped), and crankcase vent (breather) or air inlet tube. The PCV valve is the primary control of blow-by gas and meters blow-by gas according to manifold vacuum signal. To maintain idle quality, PCV valve restricts flow of blow-by gas when intake manifold vacuum is high.
MALFUNCTION INDICATOR LIGHT
Note. The Malfunction Indicator Light (MIL) may also be referred to as CHECK ENGINE or SERVICE ENGINE SOON light.
The MIL is located on instrument cluster. MIL will illuminate when ignition switch is turned to ON position (bulb check) and engine is not running. Light should not flash at this time and should turn off when engine is started. When a system fault occurs, MIL will remain illuminated to inform driver a problem exists. See appropriate SELF-DIAGNOSTICS article.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/C IDLE-UP SIGNAL
A/C idle-up signal is sent from A/C compressor control module to PCM. PCM uses this signal to detect when A/C is operating and sends a signal to Idle Air Control (IAC) valve to increase idle speed.