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.
The Legend intake manifold has 3 air intake paths: long, short, and very short. Increased engine torque over a broad range is achieved by switching paths. High torque at less than 3100 RPM is achieved by using long intake path. High torque at higher RPM is achieved by using short intake path. Maximum horsepower is achieved by using very short intake path. DO NOT adjust throttle stop screw (preset at factory).
BY-PASS LOW & HIGH CONTROL SOLENOID VALVES
When engine RPM is less than 3100 RPM, signal from ECU activates by-pass low and high control solenoids valves, and intake air goes through long intake path. When engine speed is 3200-3800 RPM, by-pass low solenoid valve is shut off, and intake air goes through short intake path. At engine speeds greater than 3900 RPM, ECU switches off both solenoid valves, and intake air goes through very short intake path.
ELECTRONIC CONTROL UNIT (ECU)
ECU contains memories for basic injection timing and duration, electronic idle control, ignition timing control, ECU back-up functions, and other control functions. The ECU is located under the carpet, in passenger's footwell.
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.
A/C SWITCH SIGNAL
Switch signals ECU when A/C is turned on. ECU uses this input to change engine RPM to compensate for the extra engine load.
A/T SHIFT POSITION SIGNAL
Informs ECU when automatic transmission is in Neutral, Park, D3 or D4.
ALTERNATOR FR SIGNAL
Sends signals to inform ECU that alternator is charging.
ATMOSPHERIC PRESSURE (PA) SENSOR
The PA sensor converts atmospheric pressure into electrical signals. PA sensor is inside ECU.
BATTERY VOLTAGE (IGN 1)
When ignition switch is on, a battery voltage signal (ignition circuit) is sent to ECU.
BRAKE SWITCH SIGNAL
Switch signals ECU that brake pedal has been depressed. ECU uses this input signal to control idle speed.
COOLANT TEMPERATURE (TW) SENSOR
Coolant temperature sensor is a thermistor. The resistance of thermistor decreases as coolant temperature increases. Input is used by ECU to determine air/fuel mixture, timing and idle speed.
CLUTCH SWITCH
Signals ECU that clutch is engaged.
CRANK SENSOR
See IGNITION TIMING CONTROL under IGNITION SYSTEM.
CYLINDER (CYL) SENSOR
See IGNITION TIMING CONTROL under IGNITION SYSTEM.
EGR LIFT SENSOR
See EGR LIFT SENSOR under EMISSION SYSTEMS.
INTAKE AIR TEMPERATURE (TA) SENSOR
TA sensor is a thermistor. The resistance of thermistor decreases as intake air temperature increases. ECU uses input signals from sensor to determine air/fuel mixture.
IGNITION OUTPUT
Detects ignition signal. ECU will set a trouble code if ECU does not receive an ignition signal.
KNOCK SENSORS (LEGEND)
If knock sensor(s) detects detonation, ignition timing is retarded.
MAP SENSOR
MAP sensor converts manifold absolute pressure into electrical signals and sends this information to the ECU. Input is used by ECU to determine air/fuel mixture.
OXYGEN (O2) SENSOR
O2 sensor detects oxygen content in the exhaust gas and sends this information to the ECU. ECU uses input signals from sensor to vary the duration of fuel injection. Legend is equipped with 2 O2 sensors.
POWER STEERING PRESSURE SIGNAL
Provides signals to inform ECU of high power steering load. ECU increases idle speed through Electric Air Control Valve (EACV).
STARTER SIGNAL
Signals ECU of engine start-up (cranking). ECU uses this to control EACV to promote easy starting.
TDC SENSOR
See IGNITION TIMING CONTROL under IGNITION SYSTEM.
THROTTLE ANGLE SENSOR
Throttle angle sensor is a potentiometer connected to throttle valve shaft. As throttle angle changes, sensor varies voltage signal to ECU. Signal is used by ECU to determine fuel injection duration.
VEHICLE SPEED SENSOR (VSS)
Sensor monitors vehicle's speed and generates signal to ECU. Signal produces 4 pulses (switch closures to ground) per revolution of speedometer cable. ECU uses input signals from sensor to determine timing and fuel injection.
Not all of the following components are used on all models. See appropriate wiring diagram in L-WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section to determine components usage on specific models.
For theory and operation of specific output component, refer system indicated after component. ECU processes information from the input sensors, and sends appropriate voltage control signals to the following engine-controlled output components:
A/C COMPRESSOR CLUTCH RELAY SIGNAL
See FUEL SYSTEM - IDLE SPEED .
AIR SUCTION CONTROL SOLENOID VALVE
See EMISSION SYSTEMS .
CHECK ENGINE LIGHT
See SELF-DIAGNOSTIC SYSTEM .
EGR CONTROL SOLENOID
See EMISSION SYSTEMS .
ELECTRONIC AIR CONTROL VALVE
See FUEL SYSTEM - IDLE SPEED .
FUEL INJECTORS
See FUEL SYSTEM - CONTROL .
FUEL PRESSURE REGULATOR CUT-OFF SOLENOID VALVE
See FUEL SYSTEM - DELIVERY .
FAST IDLE CONTROL SOLENOID
See FUEL SYSTEM - IDLE SPEED .
FUEL PUMP MAIN RELAY
See FUEL SYSTEM - DELIVERY .
IGNITOR UNIT
See DIRECT IGNITION SYSTEM .
EVAP PURGE CUT-OFF SOLENOID VALVE
See EMISSION SYSTEMS .
FUEL PUMP
Fuel pump is located inside fuel tank. When engine is started, main relay operates fuel pump. The pump has a relief valve to prevent too much pressure. If discharge side is blocked, relief valve opens to by-pass fuel from discharge side to inlet side.
When engine is turned off, the pump stops. A check valve in the fuel pump maintains fuel pressure in fuel line.
The main relay is located at left side of firewall. Main relay contains 2 individual relays. The first relay operates with the ignition on, and supplies battery voltage to ECU, injectors and second relay (fuel pump power). The second relay is controlled by ECU for 2 seconds after ignition is turned on and whenever engine is running.
FUEL PRESSURE REGULATOR
Fuel pressure regulator maintains constant fuel pressure to injectors. When the difference between fuel pressure and manifold pressure exceeds 36 psi (2.6 kg/cm 2 ), the diaphragm is pushed upward, and excess fuel is fed back into fuel tank. A vacuum-operated diaphragm inside the regulator maintains fuel pressure at a specific range. This allows for changes in engine load.
At idle, intake manifold vacuum is high, causing diaphragm to be pulled up, thus reducing fuel pressure. When throttle is depressed, intake manifold vacuum decreases, and regulator spring overcomes manifold vacuum. This causes fuel pressure to increase.
When coolant temperature exceeds 221°F (105°C), or intake air temperature exceeds 176°F (80°C), pressure regulator cut-off solenoid valve cuts vacuum to pressure regulator. This ensures high fuel pressure to injectors and prevents vapor lock.
INJECTOR RESISTOR
Resistor lowers current supplied to injectors to prevent damage to injector coils.
When current is applied to the solenoid coil, valve lifts up, and pressurized fuel is injected close to intake valve. The ECU controls injector timing and length of time injector is opened. The injector is sealed by an "O" ring and a seal ring at each injector end (top and bottom).
FUEL-CUT
Fuel-cut operates to save fuel and reduce emissions. Fuel delivery is cut at speeds greater than 6800 RPM on Integra, or 6500 RPM on Legend. Fuel is also cut during closed throttle deceleration, when engine speed is greater than 1500 RPM on Integra, or 1050 RPM on Legend.
THROTTLE BODY
The throttle body is a single-barrel side-draft type. The lower portion of the throttle valve is heated by engine coolant. The idle adjusting screw (preset at factory) increases or decreases by-pass air. Canister/purge port is located on top of throttle body.
Idle speed is controlled by Electronic Air Control Valve (EACV) and fast idle control valve. EACV varies amount of air by-passing into intake manifold according to input signals received from ECU. When EACV is activated, valve operates to maintain proper idle speed.
On Integra, a dashpot is used to slow down the closing of throttle valve during gear shifting (M/T) or deceleration.
A/C COMPRESSOR CLUTCH RELAY
Relay signals ECU when A/C circuit is operating. ECU then sends signal to EACV to maintain proper idle speed.
AIR BOOST VALVE
Additional air enters the intake manifold during engine cranking. This helps engine start easier.
ELECTRONIC AIR CONTROL VALVE (EACV)
To maintain proper idle speed, EACV varies amount of air by-passing throttle body according to signals received from ECU.
FAST IDLE CONTROL VALVE
Valve is controlled by a thermowax plunger. When engine is cold, the thermowax contracts the plunger, allowing additional air to be by-passed into the intake manifold and increase engine speed. When engine reaches operating temperature, the valve closes.
Note. For other input components in idle speed circuit, see CEC INPUT DEVICES .
POWER STEERING PRESSURE SWITCH
Power steering pressure switch signals ECU to increase engine idle speed when power steering load is high.
Battery voltage is supplied through ignition switch, to ignition coil and ignitor unit. The PGM-FI control unit uses input signals from TDC/crank (or crank/CYL) sensor, throttle angle sensor, coolant sensor and MAP sensor to directly activate the ignitor, which in turn triggers ignition coil(s). On Integra, induced voltage from ignition coil is then distributed to each spark plug by the distributor.
DIRECT IGNITION SYSTEM (DIS)
Legend does not use a distributor. Each spark plug is fired directly by one of 6 coils. Each coil is controlled directly by the ignitor.
IGNITION TIMING CONTROL
A microcomputer in the ECU controls the ignition timing according to engine speed and vacuum pressure in the intake manifold. Engine speed and vacuum pressure are transmitted to ECU by signals from crank sensor, cylinder (CYL) sensor, TDC sensor (Integra), throttle angle sensor, coolant temperature sensor, knock sensors (Legend), and MAP sensor.
The ECU contains memories for basic ignition timing at various engine speeds, manifold pressure and coolant temperature. When these conditions have been processed, ECU sends signals to ignitor unit to control ignition timing.
Determines timing for fuel injection and ignition of each cylinder. It also detects engine RPM.
Detects position of No. 1 cylinder for sequential fuel injection to each cylinder.
If knock sensor(s) detects detonation, ignition timing is retarded.
TDC SENSOR (INTEGRA ONLY)
Determines ignition timing at start-up (cranking), and when crank angle signal is abnormal.
Note. For other inputs see CEC INPUT DEVICES .
AIR INJECTION SYSTEM (LEGEND)
System uses an air suction valve. When the air suction valve is activated, manifold vacuum lifts the diaphragm off the air suction valve. The pulsations of the exhaust gases draw fresh air from air cleaner into exhaust manifold through the reed valve of air suction valve.
AIR SUCTION CONTROL SOLENOID VALVE (LEGEND)
The Air Suction Control Solenoid Valve (ASCSV) supplies vacuum to the suction valve when engine speed is greater than 1500 RPM, and vehicle speed is greater than 9 MPH. Activation of ASCSV is delayed by ECU until engine is warm.
EGR SYSTEM
The EGR system is designed to reduce oxides of nitrogen (NOx) emissions. This is done by recirculating exhaust gas through the EGR valve and intake manifold, into the combustion chambers. System is composed of EGR valve, Constant Vacuum Control (CVC) valve and EGR control solenoid valve.
ECU contains memories for ideal EGR valve lifts for varying operating conditions. The EGR lift sensor detects amount of EGR valve lift and sends the information to ECU. ECU compares this information (actual EGR valve lift) with ideal EGR valve lift. If there is a difference between the 2 signals, ECU closes EGR control solenoid valve to reduce vacuum applied to EGR valve.
POSITIVE CRANKCASE VENTILATION (PCV)
PCV system prevents crankcase blow-by gas from escaping into the atmosphere. The plunger in the PCV valve is lifted in proportion to the intake manifold vacuum, drawing the blow-by gas into the intake manifold.
EVAP
This system prevents fuel tank vapors from escaping into the atmosphere.
When coolant temperature is less than 140-158°F (60-70°C), ECU supplies voltage to the purge cut-off solenoid valve to cut vacuum to purge control valve.
CHECK ENGINE LIGHT & LED INDICATOR
ECU supplies ground for the CHECK ENGINE light for about 2 seconds when ignition is turned on. When a defect is detected in signal from a sensor, ECU activates the CHECK ENGINE warning light and stores the failure code in the erasable memory.
On Integra, a Red LED display on ECU will flash code(s) every time ignition is turned on. On Legend, the CHECK ENGINE light will flash codes when diagnostic connector is jumped.
A/T CONTROL SOLENOID VALVE
Various engine sensor inputs to the ECU are used to control the A/T control solenoid valve. This provides precise timing for the gear shifts and torque converter lock-up system.
Note. For other input components, see CEC INPUT DEVICES .