Engine Controls - Theory & Operation: Other Nissan Pathfinder II

VALVE TIMING CONTROL SYSTEM (SENTRA 1.6L)

Valve timing control system consists of intake camshaft pulley and advance/retard mechanism, valve timing control solenoid and oil control valve. (Scheme 1) Valve timing control is used to increase engine performance by advancing or retarding intake camshaft.

Intake valve opening and closing points are controlled by PCM according to engine operating conditions. This affects overall torque curve by allowing more favorable torque applications at low-to-medium speeds. Inputs from coolant temperature, throttle position and mass airflow sensors, as well as engine RPM and gear position, are used by PCM to determine operation.

Valve Timing Control Solenoid

Oil pressure, applied through valve timing control solenoid, is used to adjust intake camshaft timing. At idle or high speed, valve timing control solenoid is off, valve timing is retarded and valve overlap is decreased. At low-to-medium speed, valve timing control solenoid is on, valve timing is advanced and valve overlap is increased. This results in increased torque over a broad engine RPM range.

Scheme 1

COMPUTERIZED ENGINE CONTROLS

The Electronic Concentrated Control System (ECCS) is a computerized emission, ignition, and fuel control system. A single Engine Control Module (PCM) uses input voltage signals received from various input components to control output.

PCM compares each input signal to appropriate parameter preprogrammed in PCM and adjusts output voltage signals accordingly. This allows optimum vehicle performance under various conditions. On most models voltage to PCM is supplied by ECCS relay. ECCS relay has a self shut-off feature that cuts power to PCM a few seconds after ignition is turned off.

ENGINE CONTROL MODULE (PCM)

PCM consists of a microcomputer, diagnostic mode selector, connectors and wiring for voltage signal input, voltage signal output, and power supply. Unit is not serviceable and should not be opened. PCM contains memory and logic circuits, enabling it to interpret sensor inputs and control various engine systems. To locate PCM, see PCM LOCATIONS table.

PCM LOCATIONS

INPUT DEVICES

Note. Components are grouped into 2 categories. First category is INPUT DEVICES, which are components that control or produce voltage signals monitored by the PCM. Second category is OUTPUT SIGNALS, which are components controlled by the PCM.

Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. Input signals include

A/C Switch

Informs PCM when A/C system is on. PCM responds by increasing idle speed to improve idling and reduce emissions. During heavy engine load, PCM will also open A/C clutch relay to disengage A/C clutch.

Battery Voltage Compensation

PCM monitors battery voltage to control ignition timing, injector pulse width and idle speed.

Camshaft Position Sensor (Except Maxima)

CMP sensor monitors engine speed and piston position. CMP sensor, which is built into distributor, has a rotor plate and a wave-forming circuit. Rotor plate has 360 small outer slits (one degree apart) to determine engine speed and 4 (4-cylinder) or 6 (V6) larger inner signal slits, 90 (4-cylinder) or 60 (V6) degrees apart, to determine camshaft position. (Scheme 2) The signal slit for cylinder No. 1 is the largest inner signal slit to allow PCM to determine TDC for cylinder No. 1.

When signal rotor plate passes space between Light Emitting Diode (LED) and photo diode, slits in signal rotor plate alternately cuts light from LED to photo diode. This causes a pulsating voltage, which is converted into an on-off pulse by wave-forming circuit and sent to PCM. PCM uses this signal to control fuel injection, ignition timing and other functions.

Scheme 2

Camshaft Position Sensor (Maxima)

CMP sensor detects cylinder number signal. CMP sensor, which is mounted to front cover of engine, facing camshaft, consists of a permanent magnet, core and coil. When engine is running, gap between sensor and camshaft will periodically change. Voltage signal generated in coil is sent to PCM, which detects specific cylinder.

Crankshaft Position (CKP) Sensor

All models are equipped with one CKP sensor (POS) located at rear of engine, facing flywheel. CKP sensor monitors engine speed and piston position. Maxima models are equipped with a second CKP sensor (REF) that is mounted at front of engine near oil filter. This CMP sensor detects engine compression TDC signal.

CKP sensors consists of a permanent magnet, core and coil. When engine is running, gap between sensor and flywheel teeth (POS) or crankshaft pulley (REF) will periodically change. Voltage signal generated in coil is sent to PCM, which detects crankshaft position or compression TDC signal.

Engine Coolant Temperature (ECT) Sensor

ECT sensor is installed in coolant inlet housing or intake manifold coolant passage. Sensor senses changes in temperature by monitoring resistance of a thermistor. As temperature increases, thermistor resistance decreases.

Sensor sends temperature information to PCM for air/fuel mixture, timing and idle speed control. During warm-up from cold start, PCM increases fuel enrichment to maintain engine performance. As engine temperature increases, PCM gradually decreases fuel enrichment until engine reaches normal operating temperature.

EGR Temperature Sensor

EGR temperature sensor, located near EGR valve, detects temperature of exhaust gases passing through EGR valve. Sensor has a thermistor which changes its resistance value in response to EGR flow. As temperature increases, resistance of sensor decreases. If absence of EGR flow is detected when PCM calls for EGR operation, a low flow fault is set in PCM memory. If EGR flow is detected during no flow conditions, a high flow fault is set in PCM memory.

Ignition Switch

PCM detects when ignition switch is in ON or START position. When PCM receives voltage signal for START position, it will actuate injectors and initiate ignition timing sequence and other functions, including EGR control override during start-up.

Intake Air Temperature Sensor

Intake air temperature sensor is mounted on air duct housing. Sensor monitors temperature of incoming air. Sensor is a thermistor and increases (cold) or decreases (hot) its resistance in response to temperature changes. This input is only used for on-board diagnosis and is not used to control engine operation.

Park/Neutral Position Switch (A/T Models)

Park/neutral position switch is located on transmission/transaxle. Switch notifies PCM when transmission/transaxle is in Park or Neutral and signals PCM of increased engine load when vehicle is in gear. This signal is used to calculate required changes in idle speed, ignition timing and injector operation.

Knock Sensor (KS)

Basic ignition timing is preprogrammed. Knock sensor detects engine knocking, converts knocking vibration into voltage signal and transmits signal to PCM. Information is used by PCM to adjust ignition timing accordingly to eliminate detonation.

Mass Airflow Sensor

Mass airflow (MAP) sensor is located in main air intake duct. MAF sensor uses a hot-wire type sensing element. Incoming air passing through MAF airflow sensor causes hot wire to cool. As a result, PCM must apply additional current to maintain hot wire at precalibrated temperature. PCM measures airflow by monitoring amount of additional current required to maintain hot wire at precalibrated temperature.

If MAF sensor output current is outside normal operational range, a MAF sensor fault is indicated. PCM will use Throttle Position (TP) sensor as primary information on driving condition. During this period, PCM will limit engine speed to less than 2000 or 3000 RPM, depending on model. This will inform driver that vehicle is driving under fail-safe conditions and needs attention.

Neutral Switch (M/T Models)

Neutral switch is located on transmission/transaxle. Switch notifies PCM when transmission/transaxle is in Neutral and signals PCM of increased load when vehicle is in gear. Information is used to calculate required changes in idle speed, ignition timing and injector operation.

Heated Oxygen Sensor (HO2S)

HO2S sensor monitors amount of oxygen in exhaust gas. Sensor provides a voltage signal which is used to adjust air/fuel mixture to obtain optimum combustion. HO2S heater quickly brings sensor to operating temperature, reducing time engine is in open loop mode.

A rich exhaust gas mixture causes high sensor voltage (0.6-1.0 volt). A lean exhaust gas mixture causes low sensor voltage (0.1-0.3 volt). PCM monitors signal from sensor and adjusts air/fuel mixture accordingly.

Power Steering Oil Pressure Switch

Power steering oil pressure switch is attached to power steering high pressure line. Switch monitors power steering load and sends signal to PCM. When oil pressure exceeds a predetermined amount, PCM sends a voltage signal to idle speed control valve to increase idle speed.

Tank Fuel Temperature Sensor

Tank fuel temperature sensor is located in tank, next to fuel pump. Tank fuel temperature sensor is used to control EVAP system pressure. This input is only used for on-board diagnosis and is not used to control engine operation.

Throttle Position (TP) Sensor & Idle Switch

TP sensor is attached to throttle body housing and is actuated by movement of accelerator pedal. TP sensor has a potentiometer which varies output voltage in response to changes in throttle position. This information is relayed to PCM in form of an input voltage signal on 3-terminal TP sensor harness. TP sensor also has ability to inform PCM of rate of changes taking place in throttle plate movement. Wide open and closed throttle position switch, built into TP sensor, is not used for engine control.

Vehicle Speed Sensor (VSS)

VSS is a pulse generator, located in transaxle, which provides a vehicle speed signal to speedometer. Speedometer then sends a signal to PCM.

OUTPUT SIGNALS

Vehicles are equipped with different combinations of PCM-controlled components. Not all components listed below are used on every vehicle. For theory and operation on each output component, refer to indicated system.

A/C Clutch

See AC CLUTCH under MISCELLANEOUS CONTROLS.

Transmission Control Module (TCM)

See TRANSMISSION/TRANSAXLE CONTROLS under MISCELLANEOUS CONTROLS.

Cooling Fan Motor

See COOLING FAN under MISCELLANEOUS CONTROLS.

Cooling Fan Relay

See COOLING FAN under MISCELLANEOUS CONTROLS.

Evaporative Emission Control System

See EVAPORATIVE EMISSION CONTROL (EVAP) SYSTEM under EMISSION SYSTEMS.

Exhaust Gas Recirculation (EGR) Control

See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSION SYSTEMS.

Fuel Injector

See FUEL CONTROL under FUEL SYSTEM.

Fuel Pump Control Module (Sentra & 200SX 2.0L)

See FUEL DELIVERY under FUEL SYSTEM.

Fuel Pump Relay

See FUEL DELIVERY under FUEL SYSTEM.

Idle Air Control (IAC) Valve

See IDLE SPEED under FUEL SYSTEM.

IACV-Air Regulator

See IDLE SPEED under FUEL SYSTEM.

IACV-FICD

See IDLE SPEED under FUEL SYSTEM.

Idle Speed Control

See IDLE SPEED under FUEL SYSTEM.

Ignition Timing Control

See IGNITION TIMING CONTROL SYSTEM under IGNITION SYSTEM.

See MASS AIRFLOW SENSOR under INPUT DEVICES.

Power Transistor & Ignition Coil(s)

See DISTRIBUTOR IGNITION (DI) SYSTEM under IGNITION SYSTEM.

Pressure Regulator Control Solenoid

See FUEL DELIVERY under FUEL SYSTEM.

Valve Timing Control

See VALVE TIMING CONTROL SYSTEM (SENTRA 1.6L) under AIR INDUCTION SYSTEM.

Fuel Pump

An electric in-tank fuel pump is controlled by PCM. Fuel pump is activated when PCM supplies ground signal to fuel pump relay. Fuel pump relay will be energized by PCM for up to 5 seconds when ignition is first turned on and during cranking. When PCM receives 180-degree signal from Camshaft Position (CMP) sensor, fuel pump relay will remain energized until 180-degree signal is no longer received.

Fuel Pump Control Module (Sentra 2.0L)

When PCM supplies ground signal to fuel pump relay, battery voltage is supplied to Fuel Pump Control Module (FPCM) and fuel pump. FPCM regulates fuel flow by controlling fuel pump ground circuit. Fuel pump is grounded through either dropping resistor, which drops fuel pump voltage to about 7 volts, or through FPCM to ground. Battery voltage is supplied to fuel pump under the following conditions

1. During engine cranking.
2. Engine coolant temperature is less than 50°F (10°C).
3. Engine is running under heavy-load and high-speed conditions.

Under conditions other than those listed, fuel pump control module limits fuel pump voltage to about 7 volts.

Fuel Pump Relay (Except Sentra 2.0L)

Fuel pump relay receives voltage from ignition switch when switch is in ON position. Relay is energized when ground is supplied by PCM. Battery voltage is then supplied to fuel pump. On Sentra (2.0L), battery voltage is also supplied to fuel pump control module.

Fuel Pressure Regulator

Fuel is delivered to injectors from in-tank fuel pump. Fuel pressure at injectors is regulated by fuel pressure regulator, located in fuel return line between injectors and fuel tank. Pressure regulator is a sealed unit divided into 2 chambers (fuel and spring) by a diaphragm. Fuel chamber receives fuel through inlet side from injector fuel rail. Spring chamber is connected to intake manifold vacuum.

A vacuum-operated diaphragm inside regulator maintains fuel pressure at a specific range, taking into consideration changes in engine load. At idle, intake manifold vacuum is high, causing diaphragm to be pulled down, allowing excessive fuel to be returned to fuel tank. As throttle is depressed, intake manifold vacuum decreases, allowing diaphragm regulator spring to overcome manifold vacuum, causing diaphragm to block fuel returning to fuel tank. This causes an increase in fuel pressure.

Feedback System

PCM calculates base injection pulse width by processing signals from CMP sensor and mass airflow sensor. After receiving signals from sensors detecting engine conditions, PCM adds fuel enrichment (preprogrammed into control unit) to base injection pulse width to improve engine performance under the following conditions

1. During warm-up.
2. Starting
3. During acceleration.
4. Hot engine operation.
5. When transmission is shifted from Neutral to Drive (A/T models).
6. Heavy-load, high-speed operation.

PCM decreases injection pulse width under the following conditions

1. During deceleration.
2. During high-speed engine operation.

Fuel injection system incorporates mixture ratio feedback. It is designed to maintain a precise mixture ratio. Through input signals from heated oxygen sensor, PCM can adjust air/fuel ratio to optimally control exhaust emissions and engine performance. This function takes place during closed loop operation. PCM continuously monitors itself to stay within an acceptable emissions output range. However, this feedback system can be overridden and will operate in open loop when one or more of the following conditions exist

1. Starting.
2. Engine and/or oxygen sensor cold.
3. Driving at high speeds and under heavy load.
4. Malfunction of front heated oxygen sensor or circuit exists.
5. Vehicle is decelerating.
6. High engine coolant temperature.

The fuel injector is a small elaborate solenoid. PCM sends a duration signal to injector, which in turn opens to high pressure fuel supplied by fuel pump.

Sequential Fuel Injection (SFI)

Fuel injection system can operate in one of 2 injection modes: simultaneous or sequential. In simultaneous injection mode, fuel is injected into all cylinders at same time. In sequential injection mode, injectors are triggered in spark plug firing order. Fuel injection operates in sequential mode under most conditions. Fuel injection will shift from sequential to simultaneous mode during starting and when PCM is operating in fail-safe mode. For more information on fail-safe modes, see SELF-DIAGNOSTICS - INTRODUCTION article.

PCM controls engine idle speed according to engine operating conditions and component/model application. PCM will send a signal to adjust and compensate for idle speed under the following conditions: from starting to 20 seconds after start, low battery voltage, engine overheating, electrical load, A/C on, power steering load, during deceleration and when vehicle is moving at idle.

Idle speed control is achieved using various components. Models are equipped as follows

1. Except Sentra 2.0L IAC Valve, IACV-FICD, Fast Idle Cam
2. Sentra 2.0L IAC Valve, IACV-Air Regulator, IACV-FICD

Idle Air Control (IAC) Valve-Auxiliary Air Control (AAC) Valve

PCM processes signals received from sensors to determine optimum idle speed under varying engine conditions. PCM senses engine condition and determines best idle speed with regard to coolant temperature and transmission/transaxle gear position. PCM will then send an electrical on-off signal to adjust idle speed. IAC valve then regulates amount of air bypassing throttle valve. (Scheme 3) PCM controls IAC solenoid by varying pulse width (on time).

Scheme 3

IACV-Air Regulator (Sentra 2.0L)

IACV-Air Regulator provides an air by-pass when engine is cold for fast idle during warm-up.

IACV-Fast Idle Control Device (IACV-FICD)

IACV-FICD increases idle speed for a few seconds when vehicle is first started and when A/C compressor is engaged.

IGNITION SYSTEM

Note. All models except Maxima use a Light Emitting Diode (LED) Camshaft Position (CMP) sensor, located in distributor. For additional information on operation, see INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

ELECTRONIC IGNITION (EI) SYSTEM - MAXIMA

Electronic (direct) ignition system uses one coil per cylinder. Individual coils are plugged directly onto spark plugs. PCM receives inputs from sensors including Camshaft Position (CMP) sensor, Crankshaft Position (CKP) sensor, Mass Airflow (MAF) sensor, engine coolant temperature sensor and throttle position sensor to determine ignition timing. PCM supplies voltage to power transistor, which fires ignition coil.

DISTRIBUTOR IGNITION (DI) SYSTEM - EXCEPT MAXIMA

A camshaft position sensor, mounted inside distributor, monitors engine speed and piston location. (Scheme 2) Signals created by camshaft position sensor are sent to PCM.

Power transistor uses ignition signals received from PCM to trigger ignition coil. Power for ignition coil(s) is supplied through ignition switch.

IGNITION TIMING CONTROL SYSTEM

Ignition timing is controlled by PCM according to engine operating conditions, based on inputs from various sensors, including camshaft position sensor, mass airflow sensor and engine coolant temperature. Optimum ignition timing for various driving conditions is preprogrammed and stored in PCM.

Ignition Timing Retard

See KNOCK SENSOR under INPUT DEVICES under COMPUTERIZED ENGINE CONTROLS.

EVAPORATIVE EMISSION CONTROL (EVAP) SYSTEM

Fuel evaporation system is used to reduce emissions of hydrocarbons (HC) into atmosphere. System consists of a sealed fuel tank, vacuum relief filler cap, charcoal canister, canister purge volume control valve, fuel check valve, canister purge lines, vacuum signal lines, and vapor vent lines. Fuel vapor from sealed fuel tank is stored in charcoal canister when engine is not running. Fuel vapor is retained in canister until purged by canister purge volume control valve.

Evaporative Canister

Filled with activated charcoal, canister stores fuel vapors to be burned or returned to fuel tank.

Canister Purge Control Solenoid & Canister Purge Control Valve (Sentra Only)

PCM controls canister purge control solenoid ground circuit. When canister purge control solenoid is energized, intake manifold vacuum flows through canister purge control solenoid to canister purge control valve. This vacuum signal opens canister purge control valve, allowing fuel vapor to flow. Canister purge control valve remains closed under the following conditions

1. Throttle closed.
2. Low engine coolant temperature.
3. During deceleration.
4. Engine stopped.
5. Low vehicle speed (M/T models).
6. For 60 seconds after starting engine (when engine is at normal operating temperature).

Canister Purge Volume Control Valve

Canister purge volume control valve controls flow rate of fuel vapor from canister. On Sentra, canister purge volume control valve consists of a step motor with 4 windings. PCM controls ground circuits for windings, which opens or closes valve. On all models except Sentra, canister purge volume control valve is a duty cycle solenoid valve. PCM controls fuel vapor flow rate by changing duty cycle.

Canister Vent Control Valve

Canister vent control valve is located on canister, and is used to seal canister vent during on-board diagnosis of EVAP system. Canister vent control valve, controlled by PCM, normally remains open and is not used for normal EVAP system operation.

Vacuum Cut & Vacuum Cut By-Pass Valves

Vacuum cut valve prevents manifold vacuum from reaching fuel tank. Vacuum cut by-pass valve opens only during on-board diagnostic leak and flow checks.

EGR Control

To reduce tailpipe emissions of Oxides Of Nitrogen (NOx) and reduce engine knock, an Exhaust Gas Recirculation (EGR) system is used. A portion of exhaust gases from exhaust manifold is returned to intake manifold and then combustion chamber to dilute air/fuel mixture, therefore lowering cylinder temperature. On Maxima, EGR flow is controlled by EGR volume control valve and PCM. On all models except Maxima, EGR flow is controlled by EGR valve, PCM, EGR control solenoid and backpressure transducer. EGR system does not operate under certain conditions including the following

1. Engine starting.
2. Engine idling.
3. Low engine temperature.
4. High-speed engine operation.

EGR temperature sensor, located near EGR valve, is used to determine when EGR is actually operating. If sensor indicates a low exhaust gas temperature when EGR action is commanded, PCM can determine if an EGR fault is present.

EGR Backpressure Transducer (Except Maxima)

EGR-BPT regulates vacuum signal to EGR valve by monitoring exhaust backpressure. If excessive backpressure is detected, EGR-BPT allows EGR to function in turn lessening engine knock. If little or no exhaust backpressure is detected EGR function is reduced or cancelled. If excessive roughness is detected during off-idle operation EGR-BPT may be faulty.

EGR Control Solenoid (Except Maxima)

During prescribed operating conditions, PCM grounds EGR control solenoid to cut vacuum to EGR valve. When EGR control solenoid is not activated, vacuum signal is allowed to EGR valve.

EGR Volume Control Valve (Maxima)

EGR volume control valve consists of a step motor with 4 windings. Winding ground circuits are controlled by PCM to regulate EGR flow.

MALFUNCTION INDICATOR LIGHT (MIL)

All vehicles are equipped with a MIL on instrument panel. Light comes on as a bulb check when ignition switch is turned to ON position. Light also comes on when systems related to emission controls are malfunctioning during normal vehicle operation. For additional information, see SELF-DIAGNOSTICS - INTRODUCTION article.

MISCELLANEOUS CONTROLS

Note. Although some of the controlled devices listed here are not technically engine performance components, they can affect driveability if they malfunction.

If A/C is turned on while engine is at idle, PCM will signal Fast Idle Control Device (FICD) to increase idle speed. During hard acceleration, PCM cuts off A/C power relay circuit to disengage A/C clutch for several seconds to aid acceleration.

Cooling fan motor(s) is controlled by PCM through cooling fan relay(s). PCM will compensate idle speed when cooling fan is on.

On models with multiple relays, one relay operates under normal or low speed operation and other operates under high speed or extra load conditions (A/C on). A third relay is sometimes used for operating 2-speed cooling fan motor at high speed.

TRANSMISSION/TRANSAXLE CONTROLS

Some A/T models are equipped with an electronic Transmission Control Module (TCM). TCM receives input signals from ignition coil (RPM signal), idle switch (on-off signal), throttle position sensor (variable signal) and full throttle switch (on-off signal). Based on these values, TCM calculates optimum timing and duration to energize shift solenoids, overrun clutch solenoid and lock-up solenoid. TCM also indicates when Overdrive (OD) is engaged by illuminating OD indicator light.