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 the manufacturer. This is a computerized emission, fuel and ignition system. The Electronic Control Unit (ECU), determines at which point each injector supplies fuel. The PFI system controls air/fuel ratio, ignition timing, Idle Speed Control (ISC), fuel pressure and purge control.
CONTROL UNIT
The ECU receives and processes signals from data sensors and switches to control fuel delivery. The frequency and duration of injection (fuel delivery time) is controlled by ECU. Fuel delivery time is modified for such operating conditions as cold starting, altitude, acceleration and deceleration.
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the ECU. The second category covers OUTPUT SIGNALS, which are components controlled by the ECU.
INPUT DEVICES
The ECU determines the 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 the ignition coil.
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input usage on a specific model, see appropriate wiring diagram in WIRING DIAGRAMS article. The available input signals include the following
Air Conditioner Switch
When air conditioner is turned on, a signal is sent to ECU. The ECU then adjusts Idle Speed Control (ISC) servo to maintain optimum idle speed.
Airflow Sensor
Airflow sensor (Karmen Vortex type) is mounted inside air cleaner assembly. Sensor measures airflow rate through air cleaner and sends a proportionate electrical signal to ECU. The ECU uses this signal to determine basic fuel injection duration.
Atmospheric (Barometric) Pressure Sensor
Pressure sensor is installed on airflow sensor. It senses atmospheric pressure and converts it to voltage which is sent to ECU. The 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. The other terminal is battery backup power.
Coolant Temperature Sensor (CTS)
Coolant temperature sensor converts coolant temperature to an electrical signal for use by ECU. The ECU uses coolant temperature information for controlling fuel enrichment when engine is cold.
Engine Speed Sensor
Engine speed signal is received from ignition coil. Electrical signals from ignition coil are sent to ECU where time between signals is used to calculate engine speed. This information is used by ECU for controlling air/fuel ratio and idle speed.
EGR Temperature Sensor (California)
Located on EGR valve, the EGR temperature sensor is a thermistor that relays a variable resistance signal to the ECU. When EGR malfunction occurs, EGR temperature decreases and the ECU illuminates the CHECK ENGINE (malfunction indicator) light.
Idle Switch
Idle switch is mounted at tip of Idle Speed Control (ISC) servo. When throttle valve is closed, the 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
Signals the ECU when turned on then relays an ignition pulse when turned to START position.
Ignition Timing Adjustment Terminal
Used for adjusting timing and basic idle speed. When terminal is grounded, ECU cancels ignition timing and idle speed control and sets timing and idle speed control servos in adjustment mode.
Intake Air Temperature Sensor
Air temperature sensor is located on the airflow sensor. The resistor-based sensor measures temperature of incoming air and supplies air density information to ECU. The 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, the 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. The MPS senses ISC servo plunger position and sends a signal to ECU. The ECU then controls throttle valve opening and idle speed.
Oxygen Sensor
Oxygen (O2) sensor is located in exhaust system. Output voltage of oxygen sensor varies with oxygen content in exhaust gas stream. Oxygen 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.
TDC Sensor & Crankshaft Angle Sensor
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 inside the 4 slits. Disc is attached to distributor shaft. When shaft rotates, slits in disc are optically read by unit assembly. The ECU uses No. 1 TDC sensor signal to determine fuel injection cylinder. The ECU uses crankshaft angle sensor signal to determine fuel injection timing for one stroke of engine revolution.
Throttle Position Sensor
Throttle Position Sensor (TPS) is a rotary resistor, attached to throttle body. The sensor signals ECU of changes in throttle valve position. This information is used by the ECU to control idle speed and fuel delivery time.
Vehicle Speed Sensor
The vehicle speed sensor uses a reed switch. Speed sensor 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 the system indicated in brackets after component.
The ECU processes information from the input sensors and sends appropriate voltage control signals to the following engine controls
- A/C Clutch - See MISCELLANEOUS CONTROLS.
- Check Engine Light - See SELF DIAGNOSTIC SYSTEM
- EGR Control Solenoid Valve - See EMISSION SYSTEMS.
- Fuel Injectors - See FUEL SYSTEM.
- Fuel Pump Relay - See FUEL SYSTEM.
- Idle Speed Control Servo - See FUEL SYSTEM.
- Power Transistor - See IGNITION SYSTEM.
- Purge Control Solenoid - See EMISSION SYSTEM.
- Self-Diagnostics - See SELF DIAGNOSTIC SYSTEM.
FUEL DELIVERY
Fuel is fed through the in-tank filter by an electric fuel pump located in the fuel tank. Passing through a second filter, fuel is maintained at a constant pressure at the injectors by the 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 the fuel pressure circuit.
Fuel Pump Relay (Control Relay)
Located at front of driver's console. This multi-purpose relay switches power to vehicle sensors and actuators including; airflow sensor, crank angle sensor, idle speed control, injectors and the 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 the injectors throughout 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 the fuel tank by return lines.
Fuel Injectors
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 upon information received from input sensors.
Idle Speed Control (ISC)
During start mode, the ECU controls the 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 the 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 (automatic transmission only). During deceleration ISC motor delays closing of throttle valve to its normal idling position.
Fast Idle Air Control Valve
Fast idle air valve is a wax-actuated valve. Engine coolant circulates around wax pellet. As temperature increases the air valve is forced out, decreasing by-pass airflow rate. Valve is closed completely when coolant temperature reaches 122°F (50°C), then idle air control is taken over by ECU and Idle Speed Control (ISC) servo.
ELECTRONIC IGNITION SYSTEM
The ignition system consists of a distributor containing a crank-angle sensor, rotor plate, Light Emitting Diode (LED), photo diode and the wave forming circuit, an ignition coil equipped with a power transistor and ECU. The crank-angle signal and reference signal detected by crank-angle sensor are sent to ECU.
Power Transistor
The control unit determines optimum ignition timing from these signals and other engine operating parameters. It then transmits an ignition signal to ignition coil power transistor. The power transistor amplifies this ignition signal and causes primary current to trigger in ignition coil.
Ignition Timing Advance Control
The ignition timing control system provides cold timing control, high altitude control and detonation control. When coolant temperature is low or the vehicle is above 3937 feet (1120 meters), ignition timing is advanced a fixed amount (5 degrees at crankshaft).
Federal
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 the spring pressure of the EGR control valve, EGR pintle is lifted and allows exhaust gases to flow into intake manifold.
California
The ECU controls the EGR operation by activating the EGR control solenoid valve according to engine load. When engine is cold, the 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 the ECU illuminates the CHECK ENGINE (malfunction indicator) light.
EGR Control Solenoid Valve (California)
Pressure applied to EGR control valve is controlled by solenoid valve through the control relay.
Thermovalve (Federal)
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 so no vacuum is supplied to EGR valve. At temperatures greater than 140°F (60°C), thermovalve closes and EGR operation takes place.
EVAPORATIVE CONTROL
Fuel evaporation system prevents fuel vapor from entering the 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.
Purge Control Solenoid
When ignition switch is in OFF position, fuel vapors are vented into charcoal canister. When engine is running above idle and warmed to normal operating temperature, thermovalve will open and allow vacuum to open purge control valve. Canister vapors are then drawn into intake manifold through purge control valve for burning. Purge control valve is kept closed during idle and engine warm-up to reduce HC and CO emissions.
HIGH ALTITUDE CONTROL (HAC)
When atmospheric (barometric) pressure sensor determines the vehicle is above 3937 feet, ECU advances ignition timing a fixed amount (5 degrees BTDC). Injector ON time is also adjusted through normal O2 sensor feedback loop.
PCV VALVE
Positive Crankcase Ventilation (PCV) valve operates in the closed crankcase ventilation system. Closed crankcase ventilation system 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-DIAGNOSTICS
The self-diagnostics system monitors all input signals from each sensor. If an abnormal input signal occurs, that item is stored by the ECU and given a code number. There are 13 (Sonata V6) codes or 14 (all others) 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)
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, the malfunction indicator light will illuminate and a code will be stored in ECU memory. For additional information, see TESTS W/CODES article.
Note. ECU diagnostic memory is kept alive by direct power supply from the 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.
A/C CLUTCH
When A/C switch is turned while engine is at idle, the ISC servo is commanded to increase idle speed. To prevent A/C compressor clutch switching on before idle speed has increased, the ECU briefly opens the A/C relay circuit.
On automatic transmission models, during acceleration when throttle valve opening is 65 degrees or more, A/C power relay circuit is cut-off for approximately 5 seconds to aid acceleration.