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
The engine control system uses an Electronic Control Unit (ECU) with a built-in microprocessor. Programs for calculation of fuel injection duration, ignition timing and idle speed are stored inside the ECU and are matched with various engine condition data relayed by input sensors. (Scheme 3)& 4. The ECU uses these data signals from various sensors in the vehicle and makes calculations with stored programs to determine fuel injection duration, ignition timing and idle speed, and outputs control signals to respective actuators that control engine operation.
The engine ECU and transmission ECU are integrated into one control unit called the Engine & Transmission ECU. (Scheme 1)and (Scheme 2).
The ECU is also equipped with a self-diagnostic function. Trouble codes are set by the malfunction of engine sensors or circuits, and are stored in the ECU memory. A CHECK ENGINE light on the instrument panel will come on if a trouble code is stored.
Scheme 1
Scheme 2
CONTROL UNIT
The Electronic Control Unit (ECU) is a microprocessor that controls all engine and transmission management functions. ECU receives signals from sensors, switches, ignition system and starting system. The ECU is located under center section of dash panel on ES250 and below glove box on LS400.
Fail-Safe Function
If the ECU determines there is a malfunction from signals received (if any), it will continue engine operation, using its own data or stop the engine.
High Engine Speed Cut-Off
To prevent engine overrun, fuel injection is stopped if engine speed is greater than 6500 RPM. Fuel injection is resumed when engine speed falls below this level.
Circuit Opening Relay
Circuit opening relay controls fuel pump circuit. ECU receives input signal at STA terminal (ST terminal on LS400) when engine is cranking. This same starter signal is also applied to terminal STA (or ST) of circuit opening relay. Starter signal energizes circuit opening relay during cranking, which in turn activates fuel pump (through fuel pump relay on LS400). On ES250, when airflow meter senses airflow to engine, fuel pump switch in airflow meter provides an alternate relay ground. On LS400, circuit opening relay is grounded by ECU through FC terminal. Circuit opening relay is located next to ECU.
EFI Main Relay
EFI main relay supplies battery voltage to terminals +B and +B1 of ECU. The EFI fuse supplies constant battery voltage to EFI main relay.
Scheme 3
Scheme 4
Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the control unit. The second category covers OUTPUT SIGNALS, which are components controlled by the control unit.
INPUT DEVICES
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 the appropriate L-WIRING DIAGRAMS article in this section. The available input signals include the following
Airflow Meter (ES250)
Airflow meter measures airflow volume through the airflow meter. Airflow meter converts intake air readings into a voltage signal by means of a variable resistor (potentiometer). Signal is sent to ECU for controlling fuel injection duration and spark advance system.
Airflow Meter (LS400)
Airflow volume is measured by a Karman-Vortex airflow meter. This input signal is sent to the ECU.
Air Temperature Sensor
Air temperature sensor measures intake air temperature. Signal is sent to ECU for controlling fuel injection duration. Sensor is mounted in the airflow meter.
Cold Start Injector Time Switch
This switch, installed in the cooling system, is used to determine cold start injector ON time for cold engine starting.
Coolant Temperature Sensor
Monitors coolant temperature. Sensor has built-in thermistor whose resistance varies according to engine temperature. Coolant temperature signal is input to ECU at TWH terminal. ECU uses sensor signal for controlling fuel injection duration, electronic automatic transmission, spark advance system, idle speed control system and EGR system.
Cam Position Sensor (LS400)
Two sensors are used which work as a pair. These sensors consists of a signal plate which is fixed to each camshaft timing pulley and sensors which are fitted to the distributor housing. From signals generated by these sensors, the ECU can detect when No. 6 and No. 1 pistons are near their TDC position.
Clutch Start Switch
Relays information to the ECU as to clutch pedal position. The ECU uses this information to control starter, cold start injector and idle operation.
Crankshaft Angle Sensor (ES250)
Two sensors are used which work as a pair. These sensors are mounted inside the distributor. From signals generated by these sensors, the ECU can detect when No. 4 and No. 1 pistons are near their TDC position.
Engine Speed (RPM) Sensor
Located inside the distributor on ES250 and near crankshaft pulley on LS400. From signals generated by these sensors, the ECU can detect actual crankshaft angle and engine speed.
Exhaust Gas Temperature Sensor (A/T)
Sensor determines EGR gas temperature and sends signal to ECU.
Knock Sensor
Knock sensors are installed on the left and right banks of the cylinder block (LS400) or on cylinder block (ES250). A piezoelectric ceramic element is incorporated into the sensor.
If knocking develops in the engine, this piezoelectric element, by resonating with the knocking vibration, generates a voltage which corresponds with the knocking strength and sends a signal to the ECU. ECU uses this signal to retard ignition timing to prevent knocking.
Neutral Start Switch
Relays information to the ECU as to the transmission gear position. The ECU uses this information to control starter, cold start injector and idle operation.
Oxygen Sensor
Oxygen sensors are installed in the exhaust system and monitor oxygen content of exhaust gases. A signal is sent to the ECU and is used to determine fuel injection duration.
ES250 uses 2 oxygen sensors. The main sensor is installed on the right bank exhaust manifold where the exhaust gas from the left bank and right bank merges. The sub-oxygen sensor is installed on the exhaust pipe behind the converter. Its construction and function are the same as the main oxygen sensor except that the main oxygen sensor is equipped with a heater.
LS400 uses 4 oxygen sensors. The sensors are installed with one in front of and one after each front catalytic converter. The oxygen sensors are identical in construction and function, except the main oxygen sensors have heaters.
Throttle Position Sensor
Throttle position sensor is mounted on the throttle body. This sensor converts throttle opening angle into a voltage and sends it to the ECU as the throttle position signal.
Vehicle Speed Sensor
On ES250, vehicle speed sensor is located on back of instrument cluster. On LS400, vehicle speed sensor is located on transmission. Sensor is used to monitor vehicle speed. Vehicle speed information is used by the ECU for fuel injection, cruise control and electronic transmission control.
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.
A/C Clutch
See MISCELLANEOUS CONTROLS .
CHECK ENGINE Light
See SELF-DIAGNOSTIC SYSTEM .
Electronic Spark Advance (ESA)
See IGNITION SYSTEM .
Exhaust Gas Sensor Heater Control
See EMISSION SYSTEMS .
Fuel Injectors
See FUEL CONTROL under FUEL SYSTEM.
Fuel Pump Control
See FUEL DELIVERY under FUEL SYSTEM.
Fuel Pump Speed Control
See FUEL CONTROL under FUEL SYSTEM.
Idle Speed Control (ISC) Valve
See IDLE SPEED under FUEL SYSTEM.
Fuel Pressure Regulator Control Solenoid Valve
See FUEL DELIVERY under FUEL SYSTEM.
Self-Diagnostics
See SELF-DIAGNOSTIC SYSTEM .
Electronically Controlled Transmission/Transaxle
See TRANSMISSION/TRANSAXLE under MISCELLANEOUS CONTROLS.
Fuel Pump
An in-tank, turbine fuel pump with low discharge pulsations is used. The pump assembly consists of a motor portion and pump portion. A check valve and relief valve are also incorporated into pump assembly.
Fuel Pressure Regulator
The pressure regulator controls the amount of fuel that is returned to the fuel tank through return pipe, thus adjusting fuel pressure to the injectors. The ECU controls the vacuum signal to the fuel pressure regulator by using a Vacuum Switching Valve (VSV).
Pulsation Damper
The fuel pulsation damper eliminates pressure surges during vehicle operation.
Cold Start Injector
All models use a cold start injector. This device delivers additional fuel for cold starting. ECU supplies voltage to injector, and a cold start time switch controls the ground circuit of the cold start injector.
Injectors are ECU activated solenoids which deliver fuel to individual cylinders.
Fuel Pressure Control
When starting engine at high temperature, the ECU switches on a Vacuum Switching Valve (VSV) to draw atmospheric pressure into the diaphragm chamber of the fuel pressure regulator. This causes fuel pressure to increase preventing fuel vapor lock and helping engine start-up.
Fuel Pump Speed Control (LS400)
This control system increases fuel pump output by switching the fuel pump speed to high if a large amount of fuel is required by the engine. During normal operations, when engine speed is low, the fuel pump rotates at low speed to reduce unnecessary consumption of electric power and to maintain fuel pump durability.
A stepper motor type ISC valve is used. ISC valve controls the idle RPM at a target speed based on signals from the ECU by adjusting the volume of air by-passing the throttle valve. When the engine is cold, the ISC valve is opened wider in relation to coolant temperature and engine speed, resulting in faster idle.
The ESA system replaces conventional mechanical and vacuum advance. The ECU controls the ignition spark advance curve for every driving condition. Spark advance is based on the following inputs: coolant temperature sensor, O2 sensors, engine RPM, vehicle speed sensor, A/C switch, airflow meter and cranking (starter) signal. A remote ignition coil design is used.
Crankshaft position and engine RPM are monitored by the ECU using permanent magnet pick-up coils in the distributor (ES250) or mounted on distributor and near crankshaft pulley (LS400). Crankshaft position is read by ECU at terminals G1 and G2, and engine RPM is read at terminal NE. (Scheme 5)and (Scheme 6).
Scheme 5
Scheme 6
The ECU uses the NE and G pick-up coil inputs to switch the primary ignition circuit on and off. Primary circuit is turned off when the ECU sends a signal to the ignitor on the IGT wire. At the same time, the ignitor sends an IGF signal to the ECU. The ECU feeds voltage to the IGF circuit. The ground for this voltage is momentarily cut when the primary circuit is turned off. The ECU monitors the IGF signal and detects if the primary was switched on and off. After sending a command to turn off the primary circuit on the IGT wire, the ECU monitors the IGF circuit to ensure primary switching occurred.
CRANKCASE VENTILATION
The Positive Crankcase Ventilation (PCV) system is designed to prevent contaminating hydrocarbons (HC) in the crankcase from escaping into the atmosphere.
Crankcase vapors are routed from the crankcase through a vacuum controlled PCV valve into the intake manifold. When vapors reach the intake manifold, they are mixed with air/fuel and burnt in the combustion chamber. (Scheme 7)
Scheme 7
The PCV system provides primary control by metering the flow of blow-by vapors, according to manifold vacuum. When manifold vacuum is high (at idle) the PCV restricts the flow to maintain a smooth idle.
DASHPOT SYSTEM
This system controls exhaust emissions during deceleration by holding the throttle plate at an above-idle position during deceleration. This aids in complete burning of the air/fuel mixture.
DECELERATION FUEL-CUT SYSTEM
During deceleration from high engine speed with throttle valve completely closed, the ECU stops fuel injection to improve fuel economy and lower exhaust emission.
When engine speed falls below a predetermined RPM or throttle valve is opened, fuel injection is resumed. Fuel cut-off and fuel injection resumption speeds are high when coolant temperature is low.
EGR SYSTEM
The Exhaust Gas Recirculation (EGR) system is used to reduce NOx emissions by lowering combustion temperatures. Recycling metered amounts of exhaust gas back into the intake system lowers combustion temperatures. (Scheme 8)
Scheme 8
When engine is below operating temperature, no exhaust gas recirculation is obtained. Increase in engine temperature allows control valves to regulate vacuum to EGR valve for gas recirculation. Vacuum modulator is used to regulate exhaust backpressure and balance atmospheric pressure and vacuum, to allow EGR operation at heavy throttle.
Based on inputs from the coolant temperature sensor, engine RPM and throttle position sensor, the ECU helps control EGR operation. The ECU controls vacuum through a Vacuum Switching Valve (VSV).
EVAPORATIVE EMISSION SYSTEM
The purpose of the Fuel Evaporative Emission Control (EVAP) system is to prevent the escape of fuel vapors (hydrocarbons) from the fuel tank into the atmosphere. In order to reduce hydrocarbons (HC) emissions, evaporated fuel from the fuel tank is routed through the charcoal canister then into the intake manifold for combustion in the cylinders. (Scheme 9) A coolant temperature sensing Bimetallic Vacuum Switching Valve (BVSV) controls the operation of canister purge.
Scheme 9
OXYGEN SENSOR HEATER CONTROL
The ECU controls the operation of the oxygen sensor heater according to intake air volume and engine speed. If engine load is light and exhaust gas temperature is low, the heater is turned on to maintain sensor efficiency. When engine load becomes high and exhaust temperature is high, the heater is turned off to prevent deterioration of the sensor.
SELF-DIAGNOSTIC SYSTEM
The ECU is equipped with a self-diagnostic system which detects system failures or abnormalities. When malfunction occurs, the CHECK ENGINE light on the instrument panel is turned on.
By analyzing various input signals, the ECU detects system malfunctions related to various operating parameter sensors. The ECU stores trouble codes associated with the detected failure until the diagnostic system is cleared.
All vehicles are equipped with a CHECK ENGINE light on the instrument panel. The light will illuminate when the ignition switch is turned to the ON position (bulb check) and when engine electrical systems are malfunctioning during normal operation (Mode 1) with the engine running. For additional information, see appropriate G - TEST W/ CODES article in ENGINE PERFORMANCE section.
MISCELLANEOUS CONTROLS
Note. Although not considered true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
When the ECU detects a signal from the air conditioner ECU that the air conditioner switch is turned on, the ECU outputs a signal to the magnetic clutch relay, turning it on.
The magnetic clutch operation is delayed about 0.5 second. During this time, the ECU opens the ISC valve to offset the drop in engine RPM due to the operation of the air conditioner compressor.
This system also helps maintain driveability by switching off magnetic compressor clutch when the vehicle is accelerated suddenly, or when engine is running under a heavy load condition.
Electronically Controlled Transaxle (ES250)
This system electronically controls gear shift timing, clutch and brake hydraulic pressure and engine torque during shifting to achieve optimum shift feeling. A shift lock system is also incorporated to prevent the possibility of incorrect operation of the transmission. The transmission ECU is located together with the engine ECU inside a single unit.
Electronically Controlled Transmission with Intelligent
Control System (LS400)
This system electronically controls gear shift timing, clutch and brake hydraulic pressure and engine torque during shifting to achieve optimum shift feeling. A shift lock system is also incorporated to prevent the possibility of incorrect operation of the transmission. The transmission ECU is located together with the engine ECU inside a single unit.
See also:
• MISCELLANEOUS CONTROLS
• SELF-DIAGNOSTIC SYSTEM
• IGNITION SYSTEM