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.
INTAKE MANIFOLD RUNNER CONTROL SYSTEM
Intake Manifold Runner Control (IMRC) system uses a series of valves inside intake manifold to control length of intake air path. Intake air path has 2 lengths; short and long. Increased engine torque or power over a broad operating range is achieved by using valves to switch between paths. This results in high torque at low engine speed and high power at high engine speed.
High torque at low speed is achieved when IMRC valve(s) are closed. High power at high engine speed is achieved when IMRC valve(s) are open.
INTAKE AIR BY-PASS CONTROL SYSTEM
During engine operation, Intake Air By-Pass (IAB) control valve sends intake air to fuel injectors. This increases fuel atomization at injectors during throttle tip-in. (Scheme 1), (Scheme 2) or (Scheme 3).
Scheme 1
Scheme 2
Scheme 3
MDX, RSX, 3.2CL & 3.2TL
Note. On all models except RSX with K20A2 engine, Variable Valve Timing & Lift Electronic Control System (VTEC) rocker arms are used only for intake valves. On RSX with K20A2 engine, both intake and exhaust camshafts are used.
VTEC uses 3 different camshaft lobes and rocker arms, synchronizer pistons, and a VTEC solenoid valve. (Scheme 4)- (Scheme 6). The 2 low lift cam lobes operate primary and secondary (outer) rocker arms. The high lift cam lobe operates the connecting (middle) rocker arm. The PCM controls VTEC solenoid valve which in turn operates synchronizer pistons.
At low speed, primary and secondary rocker arms control valve timing, lift and duration. The connecting (middle) rocker arm remains disengaged, and has no effect on engine operation. At high RPM with heavy engine load, PCM activates VTEC solenoid valve to apply oil pressure to synchronizer pistons located in primary and secondary rocker arms. This locks primary, connecting, and secondary rocker arms together so they are driven as a single unit by high lift cam lobe on which the connecting rocker rides.
RSX has Variable Valve Timing Control (VTC) mechanism on intake camshaft in addition to VTEC system. System improves fuel efficiency and reduces exhaust emissions at all levels of engine speed, vehicle speed, and engine load. VTC system makes continuous intake valve timing changes corresponding to operating conditions. It optimizes intake valve timing to allow engine to produce maximum power. Cam advance is reduced at idle, which stabilizes combustion and reduces engine speed.
Scheme 4
Scheme 5
Scheme 6
POWERTRAIN CONTROL MODULE
The PCM controls fuel injector timing and duration, idle speed, ignition and camshaft timing, PCM back-up functions, and other functions. The PCM is located in front passenger footwell area. On RSX and 3.5RL, ECM/PCM is located under right side of dash, behind carpet. On MDX, 3.2CL and 3.2TL, ECM/PCM is located below left center of dash, behind carpet.
Note. Components are grouped into 2 categories. The first category is INPUT DEVICES , which are components that control or produce signals monitored and processed by the PCM. The second category is OUTPUT SIGNALS , which are components controlled by the PCM.
INPUT DEVICES
These vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine input device usage on a specific model, see appropriate wiring diagram under ENGINE PERFORMANCE in WIRING DIAGRAMS.
Accelerator Pedal Position (APP) Sensor (MDX)
As the accelerator pedal position changes, the sensor varies the signal voltage to PCM. This signal is one of many the PCM uses to control current to the throttle actuator to maintain the correct idle speed.
A/C Pressure Switch
A/C pressure switch signals PCM when A/C is on. PCM uses this input to control Idle Air Control (IAC) valve and adjust engine idle to compensate for extra engine load.
Air Fuel Ratio (A/F) Sensor (MDX & RSX)
A/F sensor is located upstream of TWC and sends signals to the ECM/PCM which varies duration of fuel injection accordingly.
A/T Gear Position Signal
Switch informs the PCM when automatic transmission is in Neutral or Park.
Generator FR (ALT FR) Signal
ALT FR signal is used by PCM to control generator output in accordance with electrical load and driving conditions. This ensures optimum engine load and increased fuel economy.
Barometric Pressure (BARO) Sensor
Sensor converts atmospheric pressure into electrical signals. Barometric pressure sensor is located inside PCM.
Battery Voltage (IGN 1)
When ignition is on, a battery voltage signal (ignition circuit) is sent to the PCM.
Brake Switch Signal
Switch signals the PCM that the brake pedal has been pressed. The PCM uses this input signal to control idle speed.
Crankshaft Position (CKP) Sensor
Sensor detects speed and position of crankshaft. Used for fuel injection timing and ignition timing.
Crankshaft Speed Fluctuation (CKF) Sensor
Sensor produces a pulse signal with which PCM can detect minute changes in crankshaft speed. Signal is used to detect misfires.
Cylinder Position (CYP) Sensor
Sensor detects when piston in No. 1 cylinder is at TDC on compression stroke. Signal is used to maintain sequential injection.
EGR Valve Position Sensor
EGR valve position sensor signals PCM how much EGR valve lift there is during engine operation. See EGR VALVE POSITION SENSOR under EGR SYSTEM under EMISSION SYSTEMS.
Electrical Load Detection (ELD)
Electrical load signal is monitored by PCM. Signal is used for generator control. System reduces engine load to improve fuel economy.
Engine Coolant Temperature (ECT) Sensor
This sensor is a thermistor which sends a signal to PCM proportional to engine coolant temperature. PCM uses this input to control air/fuel mixture, timing and idle speed.
Intake Air Temperature (IAT) Sensor
IAT sensor is a thermistor which determines intake air temperature. IAT sensor resistance decreases as intake air temperature increases. PCM uses the signal from this sensor to control air/fuel mixture.
Knock Sensor (KS)
Piezo-electric knock sensor creates small current when detecting a detonation. System adjusts ignition timing to minimize engine knock.
Manifold Absolute Pressure (MAP) Sensor
MAP sensor converts absolute pressure (vacuum) into an electrical input signal. PCM uses this input signal to control air/fuel mixture.
Power Steering Pressure (PSP) Switch Signal
PSP switch signals PCM of increased power steering load. PCM then increases idle speed with the Idle Air Control (IAC) valve.
Primary Heated Oxygen Sensor (HO2S-1)
Sensor measures oxygen content in exhaust gas before catalytic converter. PCM uses input from this sensor to control duration of fuel injection. On 3.5RL equipped with V6 engine, 2 primary oxygen sensors are used. See OXYGEN SENSOR HEATER under EMISSION SYSTEMS.
Secondary Heated Oxygen Sensor (HO2S-2)
Sensor measures oxygen content in exhaust gas after catalyst. Determines efficiency of catalytic converter. See OXYGEN SENSOR HEATER under EMISSION SYSTEMS.
Starter Signal
Starter signals the PCM of engine start-up (cranking). PCM uses this signal to control IAC valve to assist starting.
Throttle Position (TP) Sensor
TP sensor is a potentiometer connected to throttle valve shaft. As throttle opening changes, TP sensor varies voltage signal to PCM. PCM uses this signal to calculate fuel injector pulse duration.
Top Dead Center (TDC) Sensor
TDC sensor determines ignition timing at engine start-up during cranking and when crank angle signal is abnormal. PCM uses this signal for sequential fuel injection to each cylinder.
Variable Valve Timing & Lift Electronic Control (VTEC) System Pressure Switch Signal
This switch sends a signal to PCM indicating VTEC system oil pressure. PCM uses information to control activation of VTEC synchronizer pistons. If failure occurs, PCM will set a Diagnostic Trouble Code (DTC).
Vehicle Speed Sensor (VSS)
This sensor monitors vehicle speed and generates a signal for the PCM. Sensor produces a number of pulses per minute increasing and decreasing with speed of vehicle. PCM uses input from this sensor to calculate timing and fuel injection.
OUTPUT SIGNALS
Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation of each output component, refer to indicated system.
By-Pass Control Solenoid Valve(s)
See INTAKE AIR BY-PASS (IAB) CONTROL SYSTEM under AIR INDUCTION SYSTEM.
EGR Control Solenoid (EGR-SOL)
See EMISSION SYSTEMS .
EVAP Purge Control Solenoid Valve
See EMISSION SYSTEMS .
Fuel Injectors
See FUEL INJECTORS under FUEL CONTROL under FUEL SYSTEM.
Fuel Pressure Regulator Control Solenoid Valve
See FUEL PRESSURE REGULATOR CONTROL SOLENOID VALVE (3.5RL) under FUEL DELIVERY under FUEL SYSTEM.
Heated Oxygen Sensor Heaters
See EMISSION SYSTEMS .
Idle Air By-Pass
See IDLE SPEED CONTROL under FUEL SYSTEM.
Idle Air Control (IAC) Valve
See IDLE AIR CONTROL (IAC) VALVE under IDLE SPEED CONTROL under FUEL SYSTEM.
Ignition Coil Output
See IGNITION SYSTEM .
Ignition Control Module (ICM)
See IGNITION SYSTEM .
Malfunction Indicator Light (MIL)
See MALFUNCTION INDICATOR LIGHT under SELF-DIAGNOSTIC SYSTEM.
PGM-FI Relay
See FUEL DELIVERY under FUEL SYSTEM.
Throttle Actuator (MDX)
When the engine is cold, the A/C compressor is on, transmission in gear, brake pedal depressed, power steering load is high, or generator charging, the PCM controls current to the throttle actuator to maintain correct idle speed.
Fuel Pump
Fuel pump is located within fuel tank. When engine is started, PGM-FI main relay supplies battery voltage to the fuel pump. When engine is not running, fuel delivery stops. A check valve in fuel pump maintains fuel pressure in fuel line when pump is off.
Fuel Pressure Regulator
Fuel pressure regulator is a vacuum-operated diaphragm which maintains fuel pressure within a specific range, depending upon changes in engine load. At idle, intake manifold vacuum is high, reducing fuel pressure. When throttle is opened, intake manifold vacuum decreases and causes fuel pressure to increase.
Under certain conditions, pressure regulator control (cut-off) solenoid valve shuts off vacuum to fuel pressure regulator to increase fuel pressure to injectors.
PGM-FI Main Relay
PGM-FI main relay consists of 2 relays. When ignition is on, first relay is energized and supplies battery voltage to PCM, fuel injectors and second relay. Second relay is energized for 2 seconds when ignition is turned on, (to pressurize system) and when engine is running, to supply battery voltage to the fuel pump, through the fuel pump relay on 3.5RL.
Fuel Pump Relay (3.5RL)
Fuel pump relay is located in forward area of trunk. Fuel pump relay control circuit is energized by PCM. When ignition is on, battery voltage is supplied to fuel pump relay via PGM-FI relay.
PCM stores calculations for basic fuel injector discharge durations at various engine speeds and manifold pressures. Based on stored information and current sensor information, PCM controls injector timing and duration.
IDLE SPEED CONTROL (EXCEPT MDX)
Note. For other input components in idle speed circuit, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.
Note. MDX uses a throttle actuator to control idle speed. See IDLE SPEED CONTROL (MDX) .
Idle speed is controlled by Idle Air Control (IAC) valve and fast idle thermovalve. IAC valve controls amount of air into the intake manifold according to input signals received from the PCM. The following PCM inputs have influence on IAC valve operation and idle speed
- A/C Clutch Signal
- Generator FR Signal
- A/T Gear Position
- Brake Switch
- Engine Coolant Temperature Sensor
- Power Steering Pressure Switch
- Starter Switch Signal
To maintain proper idle speed, the IAC valve controls the amount of air by-passing the throttle plate. The PCM determines appropriate idle speed according to various inputs and sends an appropriate voltage signal to control IAC valve.
Idle Air Control Thermovalve (A/T)
Valve is mounted on bottom of throttle body and has coolant hoses connected to it. A thermowax plunger inside valve allows additional air to by-pass throttle plate when engine coolant is cold, which increases engine speed. As engine warms, thermowax expands, closing off by-pass air, reducing engine speed. No adjustment is possible.
Throttle Body
Throttle body is a single-barrel, side-draft type. Lower portion of throttle valve is heated by engine coolant. Idle adjuster screw (located at top of throttle body) increases or decreases idle airflow when turned. Throttle position sensor is mounted on side of throttle body (not serviced separately).
IDLE SPEED CONTROL (MDX)
Note. For other input components in idle speed circuit, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.
Idle speed is controlled by a throttle actuator according to input signals received from the PCM. The following PCM inputs have influence on throttle actuator operation and idle speed
- Accelerator Pedal Position Signal
- A/C Clutch Signal
- Generator FR Signal
- A/T Gear Position
- Brake Switch
- Engine Coolant Temperature Sensor
- Power Steering Pressure Switch
Throttle body is a single-barrel, side-draft type. Lower portion of throttle valve is heated by engine coolant. The throttle is electronically controlled by the electronic throttle control system.
Throttle Actuator
- Idle Control - The PCM controls the throttle actuator to maintain the proper idle speed according to the engine loads.
- Acceleration Control - When the accelerator pedal is pressed, the PCM opens the throttle valve depending upon the Accelerator Pedal Position (AAP) sensor signal.
- Cruise Control - The PCM controls the throttle actuator to maintain set speed when cruise control is operating. The throttle actuator replaces the cruise control actuator.
DIRECT IGNITION SYSTEM
PCM sends sequential low current signals (one for each coil) to the Ignition Control Module (ICM). ICM in turn grounds each coil in proper sequence and time. Spark plugs are fired directly by coils mounted on each spark plug.
PCM uses input signals from Top Dead Center (TDC), Cylinder Position (CYP) and Crankshaft Position (CKP) sensors for basic ignition timing. In addition, ignition timing is influenced by throttle position sensor, coolant temperature sensor and MAP sensor.
IGNITION TIMING CONTROL
Note. For other inputs, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.
PCM controls ignition timing according to signals transmitted to PCM by Crankshaft Position (CKP) sensor, Cylinder Position (CYP) sensor, TDC sensor, Throttle Position (TP) sensor, Engine Coolant Temperature (ECT) sensor, Knock Sensors (KS), and Manifold Absolute Pressure (MAP) sensor.
PCM calculates ignition timing for each engine speed, manifold pressure, and coolant temperature. When these conditions have been processed, PCM sends signals to Ignition Control Module (ICM) to trigger coil(s).
Sensor detects the speed and position of crankshaft. Signal is used for fuel injection timing and ignition timing.
Sensor detects when piston in No. 1 cylinder is at TDC of compression stroke. Signal is used to maintain sequential ignition and fuel injection timing.
Piezo-electric sensor generates a small current when detecting spark detonation (knock). Sensor signal line is monitored by the PCM. When knock sensor responds to detonation, the PCM will retard ignition timing to minimize knock.
CMP (TDC) Sensor
This sensor determines ignition timing at start-up (cranking) and when crank position signal is abnormal.
EGR SYSTEM
The EGR system reduces oxides of nitrogen (NOx) emissions by recirculating exhaust gas into the intake manifold and combustion chambers. The system consists of the EGR valve, EGR valve vacuum control valve, EGR valve vacuum control solenoid valve, PCM and various sensors.
Based upon internal calibrated tables, the PCM calculates and adjusts for ideal EGR valve lifts for varying operating conditions. EGR valve position sensor detects EGR lift and sends the information (actual position) to the PCM. PCM compares this information with operating conditions to determine continued adjustment requirements.
EVAPORATIVE CONTROL SYSTEM
System consists of an activated charcoal filled canister to temporarily store hydrocarbon vapors, a purge solenoid valve and a 2-way valve to prevent excessive pressure or vacuum in fuel tank. This system minimizes fuel tank vapor release into the atmosphere. The PCM allows purge flow (using purge solenoid valve) under the most effective conditions.
HEATED OXYGEN SENSOR
Oxygen sensor heater cuts oxygen sensor warm-up time and stabilizes sensor operation, allowing more accurate determination of exhaust oxygen content. Heater allows fuel system to enter closed loop operation earlier and prevents sensor cool down during prolonged idle.
POSITIVE CRANKCASE VENTILATION
PCV system prevents crankcase blow-by gas from escaping into the atmosphere. Engine blow-by gases are drawn into intake manifold through the PCV valve for burning during the combustion process. PCV valve plunger position is regulated by intake manifold vacuum. As increased engine load produces increased blow-by gases, PCV valve plunger adjusts to accommodate the increase.
MALFUNCTION INDICATOR LIGHT
As a bulb check, PCM supplies a ground for Malfunction Indicator Light (MIL) for about 15-20 seconds when ignition is first turned on. When a defect is detected in a sensor signal, PCM activates MIL and stores appropriate fault code in memory.
MISCELLANEOUS CONTROLS
Note. Although not true engine performance-related systems, some controlled devices may affect driveability if they malfunction.
A/T Control Solenoid Valves
Various engine sensor inputs to the PCM are used to control the A/T shift control and torque converter lock-up control solenoid valves. This provides precise timing for the gear shifts and torque converter lock-up system.
Note. For other input components, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.