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
All models use Port Fuel Injection (PFI) system, called Multi-Point Injection (MPI) by manufacturer. This is a computerized emission, fuel and ignition system. Electronic Control Unit (ECU), determines at which point each injector supplies fuel. PFI system controls air/fuel ratio, ignition timing, Idle Speed Control (ISC), fuel pressure and purge control.
CONTROL UNIT
ECU receives and processes signals from data sensors and switches to control fuel delivery. Frequency and duration of injection (fuel delivery time) is controlled by ECU. Fuel delivery time is modified for operating conditions such as cold starting, altitude, acceleration and deceleration.
Note. Components are grouped into 2 categories. First category covers INPUT DEVICES, which control or produce voltage signals monitored by ECU. Second category covers OUTPUT SIGNALS, which are components controlled by ECU.
INPUT DEVICES
ECU determines control of various output devices based upon signals received from input devices. These devices can include sensors and switches, or simple monitored circuits, such as a RPM reference signal from ignition coil.
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 the ENGINE PERFORMANCE Section. Available input signals include following
Air Conditioner Switch
When air conditioner is turned on, a signal is sent to ECU. ECU then adjusts Idle Speed Control (ISC) servo to maintain optimum idle speed.
Airflow Sensor (AFS)
Karmen vortex-type AFS is mounted inside air cleaner assembly. AFS measures airflow rate through air cleaner and sends a proportionate electrical signal to ECU. ECU uses this signal to determine basic fuel injection duration.
Atmospheric (Barometric) Pressure Sensor
Pressure sensor, installed on airflow sensor, senses atmospheric pressure and converts it to voltage, which is sent to ECU. ECU computes altitude and corrects air/fuel ratio and ignition timing.
Battery Voltage
Battery voltage is sensed at 2 different ECU terminals. One terminal is ignition switch coupled. See IGNITION SWITCH. Other terminal is battery back-up power.
Coolant Temperature Sensor (CTS)
CTS converts coolant temperature to an electrical signal for use by ECU. ECU uses coolant temperature information for controlling fuel enrichment when engine is cold.
EGR Temperature Sensor (California)
Located on EGR valve, EGR temperature sensor is a thermistor that relays a variable resistance signal to ECU. When EGR malfunction occurs, EGR temperature decreases and ECU illuminates CHECK ENGINE (malfunction indicator) light.
Idle Switch
Idle switch is mounted at tip of Idle Speed Control (ISC) servo. When throttle valve is closed, switch is activated. When throttle valve is in any other position, switch is deactivated. This is used by ECU for controlling fuel delivery time during deceleration and also as an idle speed adjusting device.
Ignition Switch
Switch signals ECU when turned on, and then relays an ignition pulse when turned to START position.
Ignition Timing Adjustment Terminal
Terminal is used for adjusting timing and basic idle speed. When terminal is grounded, ECU cancels ignition timing and idle speed control then sets timing and idle speed control servos in adjustment mode.
Intake Air Temperature Sensor
Air temperature sensor is located on airflow sensor. Resistor-based sensor measures temperature of incoming air and supplies air density information to ECU. ECU uses air temperature sensor information for controlling fuel delivery.
Inhibitor Switch (Automatic Transmission Only)
Inhibitor switch senses whether select lever is in Neutral, Park or Drive. Based on this signal, ECU measures automatic transmission load and drives Idle Speed Control (ISC) servo to maintain optimum idle speed.
Motor Position Sensor (MPS)
Motor position sensor is installed in Idle Speed Control (ISC) servo. It is a variable resistor-type sensor. MPS senses ISC servo plunger position and sends a signal to ECU. ECU then controls throttle valve opening and idle speed.
Oxygen (O2) Sensor
O2 sensor is located in exhaust system. Output voltage of O2 sensor varies with oxygen content in exhaust gas stream. O2 sensor signal to ECU is used to control fuel delivery time.
Power Steering Oil Pressure Switch
Power steering oil pressure switch detects power steering oil pressure. When oil pressure rises, switch signals ECU to prevent engine speed drop due to power steering load.
Top Dead Center (TDC) Sensor & Crankshaft Angle Sensor (CAS)
These sensors are a metal disc and unit assembly. Disc has 4 slits, 90 degrees apart (crankshaft angle) and one additional slit (No. 1 cylinder) located within 4 slits. When crankshaft rotates, slits in disc are optically read by unit assembly. ECU uses TDC sensor, CAS and airflow sensor signals to determine fuel injector spray sequence and fuel delivery time.
Throttle Position Sensor (TPS)
TPS is a rotary resistor, attached to throttle body. Sensor signals ECU of changes in throttle valve position. This information is used by ECU to control idle speed and fuel delivery time.
Vehicle Speed Sensor (VSS)
VSS uses a reed switch. VSS in speedometer converts speedometer gear revolution (vehicle speed) into pulse signals which are sent 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.
ECU processes information from input sensors and sends appropriate voltage control signals to following engine controls
A/C Clutch
See MISCELLANEOUS CONTROLS.
CHECK ENGINE Light
See SELF-DIAGNOSTIC SYSTEM.
EGR Control Solenoid Valve
See EXHAUST GAS RECIRCULATION (EGR) CONTROL under EMISSION SYSTEMS.
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump Relay
See FUEL DELIVERY under FUEL SYSTEM.
Idle Speed Control
See IDLE SPEED under FUEL SYSTEM.
Power Transistor
See IGNITION SYSTEM.
Purge Control Solenoid
See EVAPORATIVE CONTROL under EMISSION SYSTEMS.
Self-Diagnostics
See SELF-DIAGNOSTIC SYSTEM.
FUEL DELIVERY
Fuel is fed through in-tank filter by an electric fuel pump located in fuel tank. Passing through a second filter, fuel is maintained at a constant pressure at injectors by fuel pump and pressure regulator.
Fuel Pump
Located in fuel tank, fuel pump consists of an impeller driven by a DC motor. Pump has an internal check valve to maintain system pressure and a relief valve to protect fuel pressure circuit.
Fuel Pump Relay (Control Relay)
Relay is located at front of driver's console. This multipurpose relay switches power to the following vehicle sensors and actuators: airflow sensor, crank angle sensor, idle speed control, injectors and fuel pump. When ignition switch is turned to ON position, ECU energizes coils controlling injectors, airflow sensor and idle speed control. When ignition switch is turned to START position, ECU energizes coils (through inhibitor switch on automatic transmission) to supply power to fuel pump. Relay failure will cause a no-start condition.
Fuel Pressure Regulator
Fuel pressure regulator, located on fuel delivery pipe, maintains a constant pressure at injectors during all engine operating conditions. Fuel flows from top of fuel injectors to pressure regulator. Regulator is a diaphragm-operated relief valve with pressure applied by intake manifold vacuum. If this pressure becomes less than fuel pressure, valve opens, causing excess fuel to return to fuel tank by return lines.
Fuel is supplied to engine through electronically pulsed (timed) injector valves located on fuel delivery pipe. ECU controls amount of fuel metered through injectors based on information received from input sensors.
Idle Speed Control (ISC)
During start mode, ECU controls throttle position in accordance with coolant temperature sensor input.
After starting with idle switch activated (throttle closed), fast idle speed is controlled by coolant temperature input to ECU and engine RPM feedback to ECU. When idle switch is deactivated (throttle open), ISC motor moves to a preset position in accordance with coolant temperature sensor input.
Idle control increases idle speed when power steering oil pressure switch or A/C switch is turned on or when transmission is shifted from Neutral to Drive position (A/T). During deceleration, ISC motor delays closing of throttle valve to its normal idling position.
Elantra & 2.0L Sonata
DIS is a molded dual-coil system that supplies energy for ignition at high speeds. A distributor is not used, Electronic Control Unit (ECU) directly activates power transistor for ignition timing control.
When ignition is turned on, battery voltage is applied to ignition coil primary winding. As crank angle sensor shaft rotates, ignition signals are transmitted from ECU to power transistor. These signals activate power transistor, causing ignition coil primary winding current to either repeatedly flow from ignition coil negative terminal through power transistor to ground, or to be interrupted. This action produces high voltage in secondary winding of ignition coil. From ignition coil, secondary winding current flows through spark plug to ground, creating ignition in appropriate cylinder.
Excel, Scoupe & 3.0L Sonata
EIS consists of a distributor, spark plugs, ignition coil, power transistor, Electronic Control Unit (ECU), high tension cables and wiring. ECU detects engine operating conditions from various sensor signals, and regulates ignition timing based on these signals.
With ignition on, battery voltage is applied to ignition coil primary winding. As distributor shaft rotates, ignition signals are transmitted from ECU to power transistor. These signals activate power transistor, causing ignition coil primary winding current to either repeatedly flow from ignition coil negative terminal through power transistor to ground, or to be interrupted. This action produces high voltage in secondary winding of ignition coil. From coil, secondary winding current flows through distributor and spark plug to ground, creating ignition in appropriate cylinder.
Ignition timing control system is composed of various sensors, Electronic Control Unit (ECU) and power transistor. Depending on engine operating conditions, ignition timing control system functions to regulate ignition timing and current flow time of primary current, assuring good ignition performance.On-off switching of primary current flowing in ignition coil is performed by power transistor, which in turn is regulated by ECU.
Ignition timing control system uses Electronic Control Unit (ECU). ECU controls timing and firing of cylinders based on signals from various engine sensors. ECU activates power transistors so ignition occurs, taking operating conditions of engine into consideration. Optimum ignition timing control is determined by making preset corrections for engine coolant temperature, intake air temperature and other conditions of ignition advance angle, which are preset according to engine operating conditions.
California
ECU controls EGR operation by activating EGR control solenoid valve according to engine load. When engine is cold, ECU signals EGR control solenoid valve to deactivate EGR.
California models are equipped with an EGR temperature sensor. When EGR malfunction occurs, EGR temperature decreases and ECU illuminates CHECK ENGINE (malfunction indicator) light.
Except California
To lower oxides of nitrogen (NOx) emissions, a non-computer-controlled exhaust gas recirculation system is used. EGR operation is controlled by throttle body ported vacuum. Vacuum is routed through a thermovalve to prevent low temperature EGR operation.
Spring pressure holds EGR closed during no/low vacuum conditions (engine idling or wide open throttle). When vacuum increases and overcomes spring pressure of EGR control valve, EGR pintle is lifted and allows exhaust gases to flow into intake manifold.
EGR Control Solenoid Valve (California)
Vacuum applied to EGR control valve is controlled by solenoid valve through control relay.
Thermovalve (Except California)
Thermovalve senses coolant temperature to prevent EGR operation below a preset temperature. At temperatures less than 140°F (60°C), thermovalve is open and vacuum is vented (no vacuum is supplied to EGR valve). At temperatures greater than 140°F (60°C), thermovalve closes (vacuum is supplied to EGR valve) and EGR operates.
EVAPORATIVE CONTROL
Fuel evaporation system prevents fuel vapor from entering atmosphere. System consists of a vacuum relief filler cap, overfill limiter (2-way valve), fuel check valve, charcoal canister, purge control valve and connecting lines and hoses.
When ignition switch is in OFF position, fuel vapors are vented into charcoal canister. When engine is at normal operating temperature and engine speed is greater than idle, thermovalve will open and allow vacuum to open purge control valve. Canister vapors are then drawn into intake manifold for burning through purge control valve. Purge control valve is kept closed during idle and engine warm-up to reduce HC and CO emissions.
PCV VALVE
Positive Crankcase Ventilation (PCV) valve operates in closed crankcase ventilation system, which consists of PCV valve and ventilation hoses.
PCV valve is a one-way check valve, located in valve cover. When engine is running, manifold vacuum pulls PCV valve open allowing crankcase fumes to enter intake manifold. If engine backfires through intake manifold, PCV valve closes to prevent crankcase combustion.
SELF-DIAGNOSTIC SYSTEM
Self-diagnostic system monitors all input signals from each sensor. If an abnormal input signal occurs, that item is stored by ECU and given a code number. There are 15 codes that can be read using a voltmeter or Scan tester. Self-diagnostic connector is located in fuse box on left kick panel. ECU long term memory is not erased when ignition is turned off. ECU memory will erase if battery or ECU is disconnected.
CHECK ENGINE LIGHT (CALIFORNIA)
CHECK ENGINE (malfunction indicator) light will illuminate when key is turned to ON position. It remains on for several seconds after engine has started. Self-diagnostic system monitors all input signals from each sensor. If an abnormal input signal occurs, malfunction indicator light will illuminate and a code will be stored in ECU memory. For additional information, see TESTS W/CODES article in the ENGINE PERFORMANCE Section.
Note. ECU diagnostic memory is kept alive by direct power supply from battery. Memory is not erased by turning ignition off. Memory will erase if battery or ECU is disconnected.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
When A/C switch is turned on while engine is at idle, ISC servo is commanded to increase idle speed. To prevent A/C compressor clutch switching on before idle speed has increased, ECU briefly opens A/C relay circuit.