Engine Controls - Theory & Operation: Other Mazda MX-5 NB

VARIABLE INERTIA CHARGING SYSTEM (VICS - MIATA & PROTEGE 1.8L)

Shutter valves in intake manifold control length of intake air path, depending upon engine RPM. With engine speed at 1100-4800 RPM, the shutter valves close, improving torque output. At more than 4800 RPM, the shutter valves open, increasing airflow while improving high RPM performance.

VARIABLE RESONANCE INDUCTION SYSTEM (VRIS - MILLENIA 2.5L & 626 2.5L)

VRIS uses 2 shutter valves in the dynamic chamber to change intake runner effective length depending upon engine RPM. (Scheme 1) To improve torque output at low-to-medium engine RPM, intake runners are opened into a resonance chamber, providing improved airflow to cylinder. At high engine speed, shutter valve closes, shortening intake runner length and improving high RPM performance. Millenia and 626 models are equipped with 2 VRIS shutter valve actuators and 2 VRIS solenoid valves.

Scheme 1

VARIABLE TUMBLE CONTROL SYSTEM (VTCS - PROTEGE 1.6L)

Shutter valves in intake manifold runners close at low RPM to improve emissions and fuel economy on California models. When valve is closed, air follows upper wall of intake and tumbles in combustion chamber. Valve opens during high speed operation to allow full airflow and improve high RPM performance. System includes shutter valves, actuator, delay valve, solenoid, vacuum chamber and check valve.

COMPUTERIZED ENGINE CONTROLS

Computerized engine control system monitors various engine/vehicle functions to control engine operation and lower emissions, while maintaining fuel economy and driveability.

POWERTRAIN CONTROL MODULE (PCM)

Note. All models except Millenia 2.3L and 626 2.0L M/T use a Powertrain Control Module (PCM). Millenia 2.3L and 626 2.0L M/T use an Engine Control Module (ECM). For purpose of this article, all references to PCM include ECM unless otherwise specified.

The PCM, through various input sensors, monitors battery voltage, engine RPM, intake air volume, cranking signal, camshaft position, crankshaft position, intake air temperature, radiator and engine coolant temperatures, exhaust oxygen content, throttle position, EGR valve position, atmospheric pressure, gearshift lever position, clutch engagement, vehicle speed, braking, power steering operation, and A/C compressor operation.

PCM utilizes this input information to control fuel injection system and operation of other output devices. Spark timing is also controlled by PCM.

PCM has a built-in fail-safe mechanism. If a fault occurs while driving, PCM will substitute preprogrammed values. Driving performance will be affected, but vehicle may still be driven.

PCM has a self-diagnostic function, which allows module to store diagnostic trouble codes in its memory. A Malfunction Indicator Light (MIL), located on instrument cluster, informs driver of system problems.

ECM/PCM LOCATIONS

Note. Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by PCM. The second category covers OUTPUT SIGNALS, which are components controlled by PCM.

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 WIRING DIAGRAMS article. The available input signals include the following

A/C Amplifier

A/C amplifier controls A/C operation according to vehicle conditions and sends A/C operation condition signal to PCM.

A/C Pressure Switch

A/C pressure switch detects A/C operation and sends signal to PCM.

Brake Switch

Switch signals PCM of vehicle braking condition. May also be referred to as a Brake On/Off (BOO) switch.

Camshaft Position (CMP) Sensor

CMP sensor detects camshaft angle at 180-degree intervals (4-cylinder) or 120-degree intervals (V6) and sends signal to PCM. Sensor also detects cylinder No. 1 TDC (4-cylinder) or No. 1 and No. 4 TDC (V6) and sends signal to PCM. For CMP sensor location, see CAMSHAFT POSITION SENSOR LOCATIONS table.

CAMSHAFT POSITION SENSOR LOCATIONS

Closed Throttle Position Switch

Switch detects fully closed throttle plate position and sends signal to PCM. On Millenia and 626 2.5L, closed throttle position switch is part of TP sensor, which is adjusted automatically when TP sensor is adjusted.

Clutch Pedal Position Switch (M/T)

Clutch switch signals PCM of clutch operation.

Coolant Temperature Sensor

See ENGINE COOLANT TEMPERATURE (ECT) SENSOR .

Crankshaft Position (CKP) Sensor

CKP sensor detects crankshaft angle at 60-degree intervals, providing PCM with data to trigger ignition coils, control injection timing and meter fuel delivered by injectors.

Differential Pressure Feedback EGR (DPFE) Sensor See EXHAUST GAS RECIRCULATION (EGR) SYSTEM under EMISSIONS SYSTEMS.

EGR Boost Sensor

The EGR boost sensor detects intake manifold pressure and sends a voltage signal to PCM.

EGR Position Sensor

Detects EGR control valve position and sends signal to PCM.

Engine Coolant Temperature (ECT) Sensor

ECT sensor is a thermistor device which changes resistance proportionate to temperature changes. ECT sensor inputs engine coolant temperature to PCM. For ECT sensor location, see ECT SENSOR LOCATIONS table.

ECT SENSOR LOCATIONS

Heated Oxygen Sensor (HO2S)

At operating temperature, Heated Oxygen Sensor (HO2S) monitors oxygen content of exhaust gases. A heating element is used to warm HO2S to operating temperature, enabling faster conversion of feedback system to closed loop operation.

HO2S produces low voltage (less than .4 volt) to indicate lean mixture (high amount of oxygen) and a high voltage (more than .6 volt) to indicate rich mixture (low amount of oxygen). This voltage signal is transmitted to PCM. All models use more than one sensor.

Intake Air Temperature (IAT) Sensor

IAT sensor inputs air temperature to the PCM. The IAT sensor changes resistance in response to air temperature. The IAT sensor resistance decreases as air temperature increases. For IAT sensor location, see IAT SENSOR LOCATIONS table.

IAT SENSOR LOCATIONS

Knock Sensor (Millenia & 626)

Knock sensor transmits signal to PCM which retards ignition timing when knocking occurs.

Main Relay

Main relay provides battery voltage to PCM and injectors.

Mass Airflow (MAF) Sensor (Except 626 2.5L)

MAF sensor measures flow of air entering engine. This measurement of airflow is a reflection of engine load (throttle opening). Sensing element (hot wire) is a thin platinum wire wound on a ceramic bobbin and coated with glass.

Hot wire is maintained at 392°F (200°C) greater than cold wire (ambient) temperature. Cold wire is located downstream of hot wire. Air temperature is measured as air passes through airflow sensor over cold wire sensor. PCM uses this information to control fuel delivery.

Manifold Absolute Pressure (MAP) Sensor (Millenia 2.3L)

MAP sensor varies voltage signal to PCM in relation to intake manifold pressure.

Neutral Switch

Neutral switch signals PCM of transaxle gear selection.

Power Steering Pressure Switch

Detects power steering operation and sends signal to PCM.

Speedometer Sensor

Speedometer sensor is located in the instrument cluster. Vehicle Speed Sensor (VSS) inputs a pulse signal (sine wave) to speedometer sensor that coverts signal to square wave for input to PCM.

Throttle Position (TP) Sensor

TP sensor provides signal in response to throttle plate position.

Vehicle Speed Sensor (VSS)

VSS located on transmission, is a magnetic pickup type sensor that detects vehicle speed.

Volume Airflow Sensor (626 2.5L)

Volume airflow sensor uses a measuring core to determine air mass entering engine.

OUTPUT SIGNALS

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

The PCM processes information from input sensors and sends appropriate voltage control signals to the following engine controls

A/C Cut-Out System

See IDLE SPEED under FUEL SYSTEM.

By-Pass Air Control (BAC) Valve

See IDLE SPEED under FUEL SYSTEM.

Canister Purge Solenoid

See FUEL EVAPORATION SYSTEM under EMISSION SYSTEMS.

Malfunction Indicator Light (MIL)

See SELF-DIAGNOSTIC SYSTEM .

Fuel Injectors

See FUEL CONTROL under FUEL SYSTEM.

Fuel Pump

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.

Ignition Timing Advance Control

See IGNITION TIMING CONTROL SYSTEMS under IGNITION SYSTEMS.

Pressure Regulator Control (PRC) Solenoid

See FUEL DELIVERY under FUEL SYSTEM.

Fuel, pressurized by electric fuel pump, flows through fuel filter, injector fuel rail and fuel pressure regulator. Pump is located in fuel tank. PCM controls fuel pump relay ground circuit during run and start modes.

The PCM activates fuel pump relay based on inputs from the ignition switch and the camshaft position sensor signal. During cranking, ignition switch cranking circuit energizes fuel pump relay. After engine starts and key is released to RUN position (engine speed more than 50 RPM), PCM provides fuel pump relay ground.

Fuel Pressure Regulator

Pressure regulator is a sealed unit, divided into 2 chambers (fuel and spring chambers) by a diaphragm. The fuel chamber receives fuel through the inlet side of injector fuel rail. The spring chamber is connected to intake manifold vacuum.

At idle, intake manifold vacuum is high. The diaphragm is pulled back by intake manifold vacuum, and excess fuel is returned to fuel tank. As throttle opens, intake manifold vacuum decreases. The regulator spring overcomes manifold vacuum, increasing fuel pressure.

PRC system includes a solenoid connected to vacuum line of fuel pressure regulator. During hot engine restarts, PCM energizes PRC solenoid. This prevents intake manifold vacuum from going to fuel pressure regulator and increases fuel pressure about 8 psi (.6 kg/cm 2 ).

The PCM controls fuel injector "on" time (pulse width) to meter fuel quantity into intake ports. The PCM receives inputs from engine sensors to compute fuel flow necessary to maintain correct air/fuel operation throughout engine operating range.

Each cylinder has a solenoid-operated injector that sprays fuel toward each intake valve. Each injector receives battery voltage from main relay. The PCM-controlled ground circuit is used to complete the circuit and energize the injector. Fuel injectors "fire" sequentially (in engine firing order) once per complete cycle.

Injectors consist of solenoid-actuated pintle and needle valve assembly. Amount of fuel delivered to engine is regulated by length of time solenoid is energized (pulse width). Atomized spray pattern is obtained by shape of injector pintle.

During acceleration (60 percent throttle or more with transmission in gear and clutch pedal released) and about 5 seconds after starting the engine, the PCM opens A/C relay, cutting off power to A/C compressor clutch. This improves idle after start-up and during heavy acceleration. A/C is cut off for about 10 seconds.

The BAC valve contains an air valve and Idle Air Control (IAC) valve. Engine coolant is directed around the air valve, warming the thermowax element. When engine coolant temperature is less than 122°F (50°C), the wax is contracted and the engine idles fast. When coolant temperature is more than 122°F (50°C), the wax is fully expanded, closing valve.

IAC valve is an electromechanical device controlled by PCM. The IAC valve is mounted on throttle body and allows air to by-pass the throttle plate. The amount of air allowed to by-pass the throttle plate is determined by PCM and is controlled by a duty signal.

During cold engine operation, the IAC valve opens, raising fast idle speed to a predetermined RPM. The IAC valve also compensates for all engine loads during warm engine operation to maintain a preset idle RPM.

Millenia 2.3L

The distributorless ignition system uses a camshaft position sensor to control ignition timing. Six individual ignition coils are used, one located on each spark plug.

The camshaft position sensor, located at right front of engine, monitors TDC of cylinder No. 1 and crankshaft angle at 180-degree intervals. Signals created by camshaft position sensor are sent to PCM.

PCM delivers ignition signals to the ignitor to control ignition. Ignitor triggers appropriate ignition coil. Power for ignition coils is supplied from ignition switch.

Millenia 2.5L

Mitsubishi breakerless electronic ignition consists of an ignitor, ignition coil and distributor. The control module is mounted inside the distributor. When ignition is on, ignition coil primary circuit is energized. As distributor shaft rotates, the reluctor rotates inside the stator assembly.

As armature teeth pass pegs of pick-up coil, a signal is sent to the ignitor. The ignitor then breaks the primary circuit in coil, causing a high voltage surge in coil secondary circuit required to fire spark plugs.

All models use a PCM-controlled Electronic Spark Advance (ESA) system. The PCM determines ignition timing based on signals from input devices.

EMISSION SYSTEMS

Note. Not all listed components are used on every vehicle system. Component usage depends on calibration of vehicle. See VACUUM DIAGRAMS article.

DECELERATION CONTROL SYSTEM

Fuel injected engines include a system to reduce emissions during deceleration, closed throttle and high RPM conditions. At speeds greater than 2000 RPM, PCM shuts off fuel injection whenever TP sensor detects closed throttle position.

EXHAUST GAS RECIRCULATION (EGR) SYSTEM

EGR system allows measured amounts of exhaust gas into intake manifold to reduce oxides of nitrogen (NOx).

The EGR system on B2500, B3000 and B4000 consists of EGR control valve, Differential Pressure Feedback EGR (DPFE) sensor, EGR Vacuum Regulator (EVR) solenoid, connecting hoses and pipes.

EGR system on Protege consists of EGR control valve, EGR boost sensor solenoid valve, EGR boost sensor, connecting hoses and pipes.

EGR system on Millenia consists of EGR control valve, EGR solenoid valves (vacuum side and vent side), EGR position sensor, connecting hoses and pipes.

EGR system on 626 consists of EGR control valve, EGR boost sensor solenoid valve, EGR boost sensor, EGR solenoid valves (vacuum side and vent side), EGR position sensor, connecting hoses and pipes.

FUEL EVAPORATION SYSTEM

Fuel evaporation system prevents escape of raw fuel vapor to atmosphere. System components include fuel tank with integral vapor separator, check-and-cut valve, purge control solenoid valve, charcoal canister, fuel filler cap and connecting lines.

Additional components include a 2-way check valve and PCM.

Canister Purge Solenoid Valve

This normally-closed solenoid valve controls fuel vapor flow from canister to intake manifold. Solenoid is opened or closed by a signal from PCM during various engine operating modes.

Vapor Vent System

All vapor valves are mounted on fuel tank and use a small orifice which allows vapor (but not liquid) fuel to pass into line running to canister. Fuel vapors in fuel tank are vented though vapor valve assembly on top of fuel tank. Vapors are routed through a vapor line to carbon canister in engine compartment.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system uses intake manifold vacuum to eliminate crankcase pressure. Manifold vacuum draws gases from crankcase, through PCV hose and valve, into combustion chamber.

By opening and closing in direct relation to engine vacuum, the PCV valve meters crankcase gas flow to combustion chamber. During periods of high manifold vacuum, such as at idle and deceleration, valve is almost completely closed, limiting flow of gases. During cruise speeds, valve permits greatest flow of gases.

Under conditions in which excessively high amounts of crankcase pressure are produced (such as heavy load), system allows excess gases to flow back through crankcase vent hose and into intake manifold.

Also called CHECK ENGINE light, MIL illuminates when ignition is turned on. Light remains on for several seconds after engine has started. If an abnormal input signal occurs, light illuminates and Diagnostic Trouble Code (DTC) is stored in memory. If the abnormal input signal returns to normal, PCM turns light off but DTC remains stored in memory until cleared. If ignition is turned on again, light will not illuminate until PCM detects another malfunction during system operation.

Note. PCM diagnostic memory is retained by direct power supply from battery. Memory is not erased by turning off ignition, but is erased if battery or PCM 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 CYCLING PRESSURE SWITCH (CCPS)

On models with manual A/C system, the CCPS is mounted on top of the receiver-drier. Based on refrigerant system pressure, a signal is sent to the PCM. The PCM uses this signal to maintain system pressure within the programmed range.

WIDE OPEN THROTTLE A/C (WAC) CUT-OFF

During wide open throttle, WAC circuit interrupts power to A/C compressor clutch. The A/C remains off for about 3 seconds after returning from WOT.