Engine Controls - Theory & Operation Ford Probe I

INTRODUCTION

This article covers basic description and operation of engine performance-related systems and components. Before diagnosing vehicles or systems with which you are not completely familiar, read this article.

AIR INDUCTION SYSTEM

Note. The components listed here are components which modify the normal air flow to the engine. For inlet air sensors and meters, see ECA INPUT SENSORS.

SUPERCHARGER OPERATION

Supercharger is an option on the 3.8L Thunderbird. The supercharger uses a pair of 3-lobed rotors connected to drive gears housed in the nose of the supercharger assembly. The supercharger drive gears are belt driven from the crankshaft. When rotated, the rotors create a positive displacement pump to compress air.

The compressed air creates a surplus volume of intake air that increases air pressure and density in the intake manifold. Air passes through the supercharger and intercooler to the intake manifold. The throttle body controls the volume of air supplied to the supercharger through the intake plenum.

The supercharger is serviced as an assembly and overhaul is not recommended. The supercharger system includes the boost control solenoid, intercooler, knock sensor, knock control unit, overboost warning device, and vane air temperature sensor. A rev limiter begins closing off fuel to the injectors at about 6000 RPM.

INTERCOOLER

As air is compressed, its temperature increases. This heated, pressurized air is routed through the core of the intercooler. Outside air passing through the intercooler lowers the temperature of the compressed air inside of the intercooler.

The intercooler lowers the temperature of the compressed air from the supercharger and increases inlet air density. This allows a denser air/fuel charge to enter the combustion chamber and improve efficiency.

A vacuum controlled by-pass valve, installed at the supercharger outlet, allows a regulated amount of airflow to return into the supercharger at low RPM. As the throttle opens, the by-pass closes and directs all air from the supercharger to the intake manifold.

TURBOCHARGER OPERATION

Turbocharger is an option on the 2.2L equipped Probe. On acceleration, turbocharger boost pressure is controlled by a wastegate/actuator assembly. Operation of the wastegate is controlled by a solenoid valve. If a fault develops in the system, boost pressure will not be controlled and overboost will occur. If overboost occurs, the knock control system will retard spark advance and ECA will activate overboost warning chime.

The turbocharger assembly includes the turbine, compressor, wastegate, actuator, and intercooler. The turbocharger system includes the boost control solenoid, intercooler, knock sensor, knock control unit, overboost warning device, and vane air temperature sensor.

EXHAUST TURBINE & COMPRESSOR ASSEMBLY

Turbocharger uses exhaust gas to power the turbine assembly. Exhaust gas moving across the turbine blades causes blades to spin. The compressor, rotating on the same shaft as the turbine, draws air from the air cleaner and pumps it through the intercooler and into the intake plenum. An increase in exhaust gas volume will increase turbine and compressor speed. Faster rotation of the turbine increases compressor output. This process increases horsepower by pressurizing the air/fuel charge prior to entering combustion chamber.

The turbine and compressor wheels can reach speeds up to 180,000 RPM. A sufficient supply of clean engine oil is necessary for continued operation. Engine oil is fed directly to center housing rotating assembly. Any contamination or interruption of oil flow will result in severe turbocharger damage.

WASTEGATE & ACTUATOR

When intake manifold pressure reaches a specified level, the wastegate opens and allows a portion of the exhaust gas to by-pass the turbine blades to limit turbine speed and output.

The actuator responds to boost pressure and is controlled by the ECA. When the pressure differential across the compressor reaches a specified level, the diaphragm will partially open the wastegate.

An intercooler is used on turbocharged engines to lower inlet air temperature and increase inlet air density, allowing a denser air/f1e l charge to enter the combustion chamber.

INTAKE AIR CONTROL (IAC) SYSTEM OPERATION (TAURUS 3.8L SHO)

Each cylinder features 2 intake runners fed from an air plenum chamber that is connected to the throttle housing. The primary runner feeds air to the cylinders whenever the engine is running. The secondary runner is opened by an IAC valve when engine speed exceeds about 4000 RPM. This system improves low and mid range torque and improves fuel efficiency.

Scheme 1

Scheme 2

Scheme 3

DISTRIBUTORLESS IGNITION SYSTEM (DIS) OPERATION

The DIS consists of a crankshaft mounted Hall Effect Profile Ignition Pickup (PIP) sensor, a camshaft driven Hall Effect Cylinder Identification (CID) sensor, a 6 tower DIS coil, and a DIS ignition module. The DIS ignition system eliminates the distributor by using multiple coils.

Each coil fires 2 spark plugs at the same time. Spark plugs are paired so as one fires during compression stroke, the other fires during exhaust stroke. There are 3 coils mounted together in a "coil pack". Each coil pack has 3 tach wires, one for each coil.

The crank sensor is a digital output hall device that sends out a signal called Profile Ignition Pickup (PIP). The PIP is produced by a rotating metallic vane mounted on crankshaft damper assembly. The crankshaft sensor has 3 teeth and generates 3 PIP signals for every one revolution of the crankshaft.

The Cylinder Identification (CID) signal is generated by a single toothed vane cup hall device driven by camshaft that produces one signal every camshaft revolution, or once every 2 crankshaft revolutions. The 3.0L SHO CID sensor is mounted at end of the rear camshaft. The 3.8L SC CID sensor is mounted in normal distributor location.

The PIP output is a 50% duty cycle (50% on and 50% off) signal that provides base spark timing information. The CID signal output is also a 50% duty cycle signal and is used so DIS module knows which coil to fire. CID signal is high (10-12 volts) for half of the cam revolution (180 degrees) and low the other half.

The ECA determines spark angle (SPOUT) using the PIP signal to establish base timing. The SPOUT signal is produced and sent by ECA to DIS module and serves 2 purposes. The leading edge of the signal fires the coil and trailing edge of the signal controls dwell timing. This feature is called Computer Controlled Dwell (CCD).

The Ignition Diagnostic Monitor (IDM) signal is an output from DIS module that provides diagnostic information concerning the ignition system to EEC-IV module for self-test.

If the CID circuit fails and an attempt to start the engine is made, the DIS module will randomly select one of the 3 coils to fire. If hard starting results, turning ignition off and cranking engine again will result in another random selection. Several attempts may be needed until proper coil is selected allowing the vehicle to be started and driven until repairs can be made.

The Failure Mode Effects Management (FMEM) system attempts to keep the vehicle drivable in spite of certain EEC system failures that prevent the EEC module from providing spark angle or dwell commands. The ECA opens SPOUT signal line and the DIS module fires coils directly from PIP input. This results in a fixed spark angle of 10 degrees and fixed dwell.

THICK FILM IGNITION (TFI) SYSTEM OPERATION

All EEC-IV controlled engines, except distributorless ignition, use the TFI ignition system. The TFI distributor is a gear driven, die cast unit. A Hall Effect stator assembly is used to trigger the ignition coil. This distributor does not use conventional centrifugal/vacuum advance mechanisms. The TFI ignition module may be mounted in the base of the distributor bowl or on the cowl behind the engine. Vehicles which have a remotely mounted module are often referred to as Closed Bowl Distributor (CBD) TFI systems.

The TFI distributor uses a Hall Effect switch mechanism to switch primary voltage and send a Profile Ignition Pickup (PIP) signal to the ECA. The ECA uses PIP input signal to produce an output signal called SPOUT that is sent to the TFI ignition module to be used to trigger discharge of coil secondary voltage.

The TFI ignition module used on manual transaxle equipped vehicles features a push start mode. This feature allows vehicle to be push started if necessary. An "E" core ignition coil is used.

DURASPARK II IGNITION SYSTEM OPERATION

All 5.8L carbureted engines use Duraspark II ignition system. All 4-Cylinder Probe vehicles use an ignition system that is similar to Duraspark II. Each system consists of a breakerless distributor, ignition module, ignition coil, ignition ballast resistor, ignition switch, and battery.

The rotating reluctor causes fluctuations in a magnetic field produced by stator/pick-up coil assembly. These fluctuations induce a voltage in the stator/pick-up coil which sends a signal to the ignition module.

The occurrence of this signal to ignition module, in relation to initial spark timing, is controlled by centrifugal and vacuum advance mechanisms. Centrifugal advance controls spark timing in response to engine RPM. Vacuum advance controls spark timing in response to engine load.

EMISSION SYSTEMS

Several systems and components are used to control emissions. Operation and method of actuation is provided for most devices. For complete testing procedures, refer to the specific system or component test in I - SYS/COMP TESTS.

AIR CLEANER TEMPERATURE SWITCH

This bimetallic sensor is installed in the lower air cleaner tray and is subject to temperature changes within the air cleaner. At a given temperature, the sensor bleeds off vacuum to the air cleaner air control door, permitting the vacuum motor to open the duct door and allow fresh air in while shutting off full heat.

EXHAUST HEAT CONTROL VALVE

Used to divert hot gases from the exhaust manifold to the intake manifold riser pad to heat the incoming air/fuel charge. Two types are used, a bimetallic spring type and a vacuum actuated type.

FEEDBACK CARBURETOR ACTUATOR MOTOR

The actuator motor is threaded into carburetor body. Its actuator shaft moves a fuel metering pintle valve to adjust air/fuel mixture. Actuator shaft moves in response to signals from the MCU system.

SOLENOID VACUUM VALVE (SVV)

The normally close solenoid valve consists of 2 vacuum ports with an atmospheric vent. The valve assembly can be with or without control bleed. When de-energized, outlet port of valve is opened to atmospheric vent and closed to inlet port. When energized, outlet port is opened to inlet port and closed to atmospheric vent. The control bleed is provided to prevent contamination entering solenoid valve from intake manifold. This solenoid valve is used on throttle kicker and EGR shutoff.

TEMPERATURE VACUUM SWITCH (TVS)

The TVS incorporates a bi-metallic disc to open or close vacuum ports and may be used in conjunction with distributor, canister purge or EGR systems.

VACUUM CONTROL VALVES (VCV)

Temperature operated vacuum switches have 2 or more ports. They utilize wax pellet or bimetallic material to either open or close the vacuum ports when normal engine operating temperature is reached.

Valves are normally mounted in some part of cooling system so that the base is immersed in coolant. May be normally open or normally closed. One version includes an electrical vacuum switch on its top.

VACUUM DELAY VALVES

Inserted in vacuum lines to provide for gradual application or release of vacuum to engine or emission control devices. May be one-way or two-way valves depending on function and part of system affected.

VACUUM REGULATORS

A 2-port vacuum regulator provides a constant output signal when input signal is greater than a preset level. At a lower input vacuum, output equals input. A 3-port or 4-port regulator may be used to control vacuum advance to the distributor.

VACUUM RESERVOIR

The vacuum reservoir stores vacuum and provides an amplified vacuum signal. It prevents rapid fluctuations or sudden drops in a vacuum signal such as seen during acceleration.

VACUUM RESTRICTOR

This orifice-type flow restrictor is used in several emission calibrations to control the flow rate and/or actuation timing of components and systems.

VACUUM VENT VALVES

Controls induction of fresh air into system to prevent accumulation of fuel vapors which could cause decay of vacuum diaphragms. May be vent valve only or combined vent and delay valve. Valves should always be mounted so ports point downward.

AIR INJECTION SYSTEM OPERATION

The air injection system reduces carbon monoxide (CO) and hydrocarbon (HC) content of exhaust gases. It injects fresh air into the exhaust gas stream which continues combustion of unburned gases. Individual systems may vary in number and type of components, depending upon engine size and application.

PULSE AIR VALVE SYSTEM

The pulse air valve replaces the air pump on some thermactor systems. It uses a pulse air valve instead of an air pump. It permits air to be drawn into exhaust system by vacuum created by exiting exhaust pulse.

Natural pulses present in the exhaust system are used to pull air into exhaust manifold through a pulse air valve. The pulse air valve is connected to exhaust manifold by a tube, and to air cleaner by a hose. This allows fresh air to complete oxidation of exhaust gases and blocks backflow of high pressure exhaust pulses.

THERMACTOR AIR SYSTEM

Individual systems may vary in number and type of components, depending upon engine size and application but all systems use the same basic components. A typical system consists of an air supply pump, air by-pass valve, centrifugal filter, check valve(s), air control valve, air manifold and air hoses.

In the Managed Thermactor Air (MTA) system, air can be by-passed to the atmosphere by a thermactor air by-pass valve and/or directed near exhaust manifold, or underbody catalytic converter. Some models may use a combined air by-pass/air control valve.

ANTI-BACKFIRE (GULP) VALVE

The anti-backfire valve, located downstream from the air by-pass valve, diverts a portion of thermactor air to intake manifold during periods of sudden decrease of intake manifold pressure.

AIR BY-PASS VALVE

Valves direct airflow from thermactor air pump to exhaust system or atmosphere as required. They may be mounted on air pump or in-line (remote). Air By-Pass Valves are vacuum operated and may be normally open or closed.

Normally closed valves supply air to the exhaust system with medium and high applied vacuum signals during normal modes, short idles and some acceleration. With low or no vacuum applied, the pump air is dumped through the silencer ports of the valve.

Normally open valves with a vacuum vent provide a timed air dump during deceleration and also dump when a vacuum pressure difference is maintained between the signal port and the vent port.

AIR CHECK VALVE & PULSE AIR VALVE

Both units function as a valve that allow thermactor air to enter exhaust system while preventing exhaust gases from passing in the opposite direction. Air check valve is not interchangeable with pulse air valve.

AIR PUMP

The air pump supplies air under pressure to exhaust port near exhaust valve by either an external air manifold or through an internal drilled passage in cylinder head or exhaust manifold. This pressurized air, combined with hot exhaust gases, creates a secondary combustion stage which produces carbon monoxide and water.

The air pump is a belt-driven, positive displacement, vane type pump that provides air for the air injection system. Air is received from a remote silencer/filter attached to air inlet nipple of pump or through a centrifugal fan on front of the pump. The by-pass valve performs pressure relief. Air pumps are available in 11 cubic inch and 19 cubic inch sizes. Various drive belt pulley ratios permit a wider range of vehicle applications.

AIR SILENCER/FILTER

The air silencer, mounted in engine compartment, is a combination silencer and filter. It is connected to system by means of a flexible hose. This component is used on pulse air injection system, or for a system not using an air supply pump with impeller type centrifugal air filter fan.

AIR SUPPLY CONTROL VALVES

Operated by vacuum to direct air pump output to exhaust manifold or downstream to catalytic converter, depending on system requirements, depending upon engine mode and control system.

CHECK VALVES

Check valves are used on all thermactor systems in various locations. These valves allow airflow in one direction only.

COMBINATION AIR BY-PASS & AIR CONTROL VALVE

The combination air by-pass and air control valve combines the functions of the by-pass valve and the air control valve into a single unit. There are 2 types of normally closed valves, the non-bleed type and the bleed type, both of which look alike. One distinguishing feature will be that the bleed type will have the percent of bleed molded into the plastic case.

DUAL THERMACTOR AIR CONTROL SOLENOID VALVE

The dual thermactor air control solenoid valve assembly consists of 2 normally closed solenoid valves with vents. One valve controls thermactor air by-pass valve and the other controls thermactor diverter valve. Both valves pass air when deactivated and do not pass air when activated.

THERMACTOR IDLE VACUUM (TIV) VALVE

The Thermactor Idle Vacuum (TIV) valve vents the vacuum signal to atmosphere when preset manifold vacuum or pressure is exceeded. During periods of extended idle conditions, this valve is used to divert thermactor airflow to limit exhaust temperature to prevent excessive underbody temperature. TIV Valve also cuts EGR in a heavy boost mode for turbocharged applications.

EGR SYSTEM COMPONENTS OPERATION

Note. Not all listed components are used on any one system. Component usage depends on calibration of complete vehicle.

EGR VALVE

The Exhaust Gas Recirculation (EGR) system distributes exhaust gas into the intake mixture. This lowers combustion temperatures due to lower concentrations of oxygen. Lowering of combustion temperatures reduces amount of NOx emissions.

Turbocharged engine uses an electronic EGR valve where EGR flow is controlled according to ECA demands by means of an EGR valve position sensor attached to the valve. The EGR valve is operated by a vacuum signal from EGR control solenoid valve.

Non-turbocharged engine incorporates a modified ported EGR valve and a remote backpressure transducer where the EGR vacuum applied to EGR valve is modulated by sensing exhaust backpressure and bleeding off some vacuum when the backpressure is low. This provides EGR flow proportional to engine load. Vacuum supplied to EGR transducer is controlled by an EGR control solenoid valve.

EGR CONTROL SOLENOID VALVE

Non-turbocharged engines uses a single EGR control solenoid. The solenoid supplies vacuum to the EGR valve when de-energized and vents vacuum through its air filter when energized. It also receives a signal from the ECA according to EGR requirements. Both turbocharged and non-turbocharged EGR solenoid valves are mounted on the firewall.

The turbocharged engine uses a dual type EGR control solenoid. One is a vacuum valve which supplies vacuum to the EGR valve when energized, and the second is a vent valve which vents the EGR valve vacuum to atmosphere when de-energized. Both solenoid valves receive variable duty cycle signals from the ECA according to EGR requirements.

EGR BACKPRESSURE VARIABLE TRANSDUCER

A vacuum bleed hole, located inside the transducer, vents EGR vacuum to the atmosphere until sufficient exhaust backpressure is applied, closing the bleed hole. When this happens, vacuum is then routed to the EGR and normal operation begins.

The EGR backpressure variable transducer modulates EGR vacuum so amount of gas flow is in proportion to throttle opening. It does this by sensing exhaust backpressure and bleeding off some of the vacuum when backpressure is low. Since exhaust pressure depends on engine load, it is equivalent to throttle opening signal. The EGR backpressure variable transducer is mounted just above the EGR valve.

EGR SOLENOID VACUUM VALVE ASSEMBLY

The dual EGR solenoid valve assembly consists of 2 solenoid valves. One is a vacuum vent valve which supplies vacuum to the EGR when energized. The second valve is a vent valve which vents the EGR valve to the atmosphere when de-energized. Both solenoid valves receive variable duty cycle signals from the ECU according to EGR requirements.

EGR VACUUM CONTROL VALVE FILTER

The EGR vacuum control valve filter is used to vent various emission control components to the atmosphere.

EGR VACUUM REGULATOR (EVR)

The vacuum regulator is an electromagnetic device which controls vacuum output to EGR valve. The EVR is used in place of EGR Solenoid Vacuum Vent Valve Assembly. Regulator operation is measured as a duty cycle, increased duty is increased vacuum to EGR.

EGR VALVE POSITION (EVP) SENSOR

This sensor is attached to the EGR valve assembly and indicates position of EGR valve to the EEC system. It is located on top of the electronic EGR type valve.

ELECTRONIC EGR VALVE

The Electronic EGR (EEGR) valve is required in EEC systems where EGR flow is controlled according to computer demands of EGR valve position sensor. The EGR valve is operated by a vacuum signal from dual EGR solenoid valves or electronic vacuum regulator.

EVAPORATIVE EMISSION CONTROL SYSTEMS OPERATION

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

CARBURETED ENGINES (5.8L V8 MCU)

Carbureted engines are equipped with a fuel bowl solenoid vent valve, purge control valve, canister purge regulator valve, vacuum/thermostatic bowl vent valve, thermal vent valve and vacuum bowl vent valve. Carbon canister is purged by drawing vapors into intake manifold when engine is started.

The time at which vapors are drawn into engine will depend on operating mode of engine. Fuel vapors which might otherwise collect in carburetor bowl and pass directly into atmosphere are vented to carbon canister when engine is stopped. Flow of vapors is controlled by a fuel bowl vent valve or a fuel bowl thermal vent valve.

FUEL INJECTED ENGINES

Fuel injected engines are equipped with an in-line purge solenoid or a purge valve. Carbon canister is purged by drawing vapors into air cleaner.

CARBON CANISTER

Carbon canister storage is used for evaporative fuel control on all vehicles. The function of evaporative emission control system is to store gasoline fumes from fuel tank and carburetor float bowl (if equipped) in a carbon canister until fumes can be drawn into engine for burning during combustion process.

The 4 basic components used in evaporative emission system are as follows

1. Activated carbon canister.
2. Vacuum operated canister control valve.
3. Computer controlled solenoid.
4. Tank pressure control valve.

For specific component application and vacuum hose routing, see appropriate VACUUM DIAGRAMS article.

CANISTER PURGE VALVE

Vacuum operated purge valve controls flow of fuel vapors from carbon canister to engine.

CANISTER PURGE SOLENOID VALVE

Normally closed solenoid valve controls the flow of fuel vapors from the canister to the intake manifold. Opened or closed by a signal from the electronic control assembly during various engine operating modes.

CARBURETOR FUEL BOWL THERMAL VENT VALVE

Inserted in carburetor-to-canister vent line, valve is closed when engine compartment is cold. This prevents fuel tank vapors (generated when fuel tank heats up before engine compartment) from being vented through carburetor fuel bowl.

FILL CONTROL/VENT SYSTEM

Fill limiting is accomplished through configuration of fill neck and/or internal vent lines within fill neck and tank. Vent system is designed to permit air space in 10-12 percent of tank when tank is filled to capacity. Air space provides for thermal expansion of fuel as well as being an aid to in-tank vapor vent system.

FUEL BOWL THERMAL VENT VALVE

Thermal vent valve, located in carburetor-to-canister vent line, is closed when engine compartment temperature is cold. This prevents fuel tank vapors, generated when fuel tank heats up before engine compartment does, from being vented through carburetor fuel bowl.

FUEL VAPOR RETURN SYSTEM

This system consists of a vapor return line from fuel pump to fuel return outlet of fuel sender, reducing amount of fuel vapor entering carburetor.

PURGE CONTROL VALVE

Purge control valve, located in line with carbon canister, controls flow of fuel vapors from carbon canister to intake manifold. Air should not flow through PCV purge line port, unless more than 16 in. Hg vacuum is applied to carburetor vacuum port.

PURGE CONTROL SOLENOID VALVE

Purge control solenoid valve controls vapor flow from canister to intake manifold. Being normally closed, it is opened by a signal from Electronic Control Assembly (ECA).

PRESSURE/VACUUM RELIEF FUEL CAP

This system consists of a sealed filler cap with an integral pressure/vacuum relief valve. Fuel system vacuum relief is provided after 1.0 in. Hg of vacuum, and pressure relief after 1.8 psi. (.13 kg/cm 2 )

Under normal conditions, fill cap allows air to enter fuel tank as fuel is used while preventing vapors from escaping.

VACUUM BOWL VENT & VACUUM/THERMOSTATIC BOWL VENT VALVES

The vacuum bowl vent valve is a vacuum/temperature actuated on/off valves. and vacuum/thermostatic bowl vent valve are vacuum and vacuum/temperature actuated on/off valves. Vacuum bowl vent valve and vacuum/thermostatic bowl vent valve are similar in appearance. The valves are used in evaporative emission system to control vapor flow from the carburetor bowl to carbon canister.

With either valve, the flow path from the bowl to the canister is closed by manifold vacuum when the engine is running. The thermostatic valve also closes the bowl-to-canister flow path when the temperature of the valve is 90°F (32°C) or less (even without manifold vacuum). When the temperature of the valve is 120°F (49°C) or more, the valve is open (unless closed by manifold vacuum).

VACUUM CHECK VALVE

A vacuum check valve blocks airflow in one direction. It allows free airflow in the other direction.

VAPOR VENT SYSTEM

System provides a vapor space above gasoline surface in fuel tank. This area is sufficient to permit adequate breathing space for tank vapor valve assembly. All vapor valves are mounted on fuel tank and use a small orifice that allows only vapor and not liquid fuel to pass into line running to canister.

Fuel vapors trapped in sealed fuel tank are vented though vapor valve assembly on top of fuel tank. Vapors are routed through a single vapor line to carbon canister in engine compartment.

Vapors are stored in carbon canister until they are purged into engine while engine is operating. On vehicles equipped with fuel/vapor return lines, vapor generated in fuel supply line is continuously vented back to fuel tank. Venting prevents engine surging from fuel enrichment and assists in hydrocarbon emission control.

POSITIVE CRANKCASE VENTILATION (PCV)

The PCV system uses intake manifold vacuum to eliminate blow-by vapors from the crankcase. The mixture is then passed into the combustion chamber and burned. The PCV valve provides primary control by metering the flow of the blow-by vapors, according to manifold vacuum.

Under conditions where abnormal amounts of blow-by gases are produced (such as worn cylinders or rings), the system is designed to allow the excess gases to flow back through crankcase vent hose into the air inlet and be consumed during normal combustion.

SELF-DIAGNOSTIC SYSTEMS

Note. All systems have self-diagnostic capabilities. For information on procedures for entering self-test modes and reading service codes, see appropriate SELF-DIAGNOSTICS article.

CARBURETED (5.8L MCU VEHICLES ONLY)

The computer used in carbureted models is called the Microprocessor Control Unit (MCU). The MCU monitors engine operating conditions by receiving inputs from a series of engine switches and sensors. Control of output actuators determines fuel mixture and idle speed.

The MCU is a solid-state micro-computer located on the left fender panel. It receives input signals and sends control signals through a 24-pin connector. The MCU is capable of operating in 3 modes: Initialization, Open loop and Closed loop.

Initialization mode occurs when the engine is started. In this mode the MCU richens the fuel mixture for easy starting.

Open loop operation is controlled by MCU programming. Air/fuel ratio is fixed at a pre-determined level and allows good driveability at idle, moderate-to-heavy acceleration, and deceleration.

Closed loop operation occurs when the engine is warm and vehicle is operated at light load conditions. In closed loop, the MCU controls air/fuel mixture in response to signals from the oxygen sensor.

FUEL INJECTED

The computerized control system used on fuel injected models is either EEC or EEC-IV. The computers of the EEC and EEC-IV systems is a microprocessor called the Electronic Control Assembly (ECA). The computer monitors engine operating conditions by input received from engine switches and sensors. Control of output actuators determines fuel mixture and idle speed. On EEC models equipped with turbochargers (Probe Turbo), the ECA also controls the ignition output signal.

The engine control system consists of the ECA, sensors, switches, and actuators. The ECA sends out electrical reference signals to engine sensors and then analyzes the return signals. The engine sensors supply the ECA with specific information, in the form of electrical signals, to determine engine operating conditions. In the event of a sensor or actuator failure, the ECA initiates an alternative strategy to allow the vehicle to maintain driveability. This strategy is called Failure Mode Effects Management (FMEM).

The "Check Engine" light will remain on whenever FMEM is in operation. FMEM substitutes a fixed sensor signal and continues to monitor the failed sensor. If the signals from a faulty sensor return to within operating limits, the ECA will resume using the sensor's signal.

ECA INPUT SENSORS OPERATION

Note. Each vehicle may be equipped with different combinations of input sensors. 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.

A/C COMPRESSOR CLUTCH SIGNAL

When battery voltage is supplied to A/C compressor clutch, a signal is sent to the ECA. The ECA uses this signal to increase engine idle speed to compensate for load added by A/C compressor.

AIR CHARGE TEMPERATURE (ACT) SENSOR

The ACT sensor is located in the intake tract, air cleaner or airflow sensor. It provides ECA with intake air temperature information. The ECA uses this information to adjust fuel mixture, primarily during cold start enrichment periods.

BAROMETRIC PRESSURE (BP) SENSOR

Measures barometric pressure of atmosphere. Variations in atmospheric pressure are converted to electrical signals and sent to ECA. The ECA uses this signal to adjust fuel mixture due to changes in altitude. The BP sensor affects the air/fuel ratio and idle speed.

The sensor looks identical to the Manifold Absolute Pressure (MAP) sensor, except the tubing nipple on the BP sensor is open to the atmosphere and the MAP sensor is connected to the intake manifold.

BRAKE ON-OFF (BOO) SWITCH

The BOO switch is mounted on the brake pedal. It signals deceleration for air/fuel ratio adjustment.

CLUTCH ENGAGE SWITCH

This switch is mounted at the clutch pedal. It signals the ECA when the transmission is in gear. Clutch engage switch signal to the ECA affects air/fuel ratio, idle speed, and ignition timing (turbo engine).

COOLANT TEMPERATURE SENSOR (CARBURETED)

There are 2 sensors used to signal temperature changes to the MCU. The sensors are mounted in coolant passages ahead of the carburetor. One sensor is open when the engine is warm; the other is open when the engine is cold and/or when it has overheated.

COOLANT TEMPERATURE SENSOR (FUEL INJECTED)

See ENGINE COOLANT TEMPERATURE (ECT) SENSOR.

CRANKSHAFT POSITION SENSOR (CPS)

This sensor, located in the base of the distributor, is used on Probe Turbo engines only. It detects crankshaft position and signals the ECA, which uses the signal to determine engine RPM. This signal affects, air/fuel ratio, injector timing, idle speed, EGR flow, canister purge flow, fuel pressure, turbo boost pressure and ignition timing.

Note. Vehicles equipped with EEC-IV systems use the PIP ignition signal to determine crankshaft position.

CYLINDER IDENTIFICATION (CID) SENSORS

CID sensors, are used on Probe Turbo engines, detect TDC position for cylinders No. 1 and 4. This signal affects injector timing and ignition timing.

EXHAUST GAS OXYGEN (EGO) SENSOR

The voltage signal produced by the EGO sensor indicates the oxygen content of engine exhaust gases. The EGO sensor is threaded into the exhaust manifold.

The EVP sensor detects EGR valve position. this information is transmitted to the microprocessor. The sensor is mounted on the EGR valve. The EVP signal affects EGR flow and ignition timing.

ELECTRICAL LOAD CONTROL UNIT (ELCU)

The ELCU monitors the electrical load placed on the alternator and electrical system. It is mounted beneath the ECA on the floor panel, behind the front of the center console. The ELCU signal affects idle speed.

ENGINE COOLANT TEMPERATURE (ECT) SENSOR

This sensor inputs the coolant temperature to the ECA. It is threaded into an engine coolant passage near the thermostat housing. The signal from the ECT affects the following

1. Air/Fuel Ratio
2. Idle Speed
3. EGR Flow
4. Purge Flow
5. Fuel Pressure
6. Boost Pressure (Turbo Engine)
7. Ignition Timing Output Signal From ECA

ENGINE COOLANT TEMPERATURE (ECT) SWITCH

ECT monitors radiator coolant temperature and signals the ECA. The switch is threaded into the lower part of the radiator. The ECT signal affects the air/fuel ratio, EGR flow and ignition timing output signal from ECA.

HEATED EXHAUST GAS OXYGEN (HEGO) SENSOR

An electrically heated O2 sensor is mounted in the exhaust manifold. This sensor monitors oxygen content of exhaust gases. When it is at operating temperature, a voltage signal is produced which varies according to oxygen content of exhaust gases. The signal is transmitted to the ECA and is translated into a rich or lean mixture signal.

This sensor uses a built-in heating circuit. The heating circuit is used to bring the HEGO sensor up to operating temperature, enabling faster conversion to closed-loop operation. With the exception of the 3.0L PFI engine, all V6 and V8 engines are equipped with 2 sensors. One for each exhaust bank.

IDLE SWITCH

The idle switch is threaded into the throttle body and contacts the throttle linkage. It signals the ECA when the throttle valve is fully closed. The switch signal affects air/fuel ratio, injector timing, idle speed, EGR flow and ignition timing.

KNOCK CONTROL UNIT

This control unit is used on Probe Turbo engines. It monitors the signal sent to the ECA to determine if knock sensor signal is due to pre-ignition or vibration. The ECA uses the signal to retard ignition timing. It is mounted on the firewall next to the BP sensor.

KNOCK SENSOR (KS)

The KS measures vibrations and converts them into an electrical signal. The signal is sent to the Knock Control Unit (KCU) where it is filtered and sent to the ECA. The KS is threaded into the engine block near the oil pressure switch.

MANIFOLD ABSOLUTE PRESSURE (MAP)

The speed density method is used to compute the airflow rate on models equipped with a MAP sensor. Manifold pressure and temperature are used to calculate the airflow rate to the ECA. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes.

The MAP sensor uses frequency to measures manifold vacuum. The MAP sensor is used as a barometric sensor for altitude compensation, updating the ECA during Key On, Engine Off (KOEO) and at Wide Open Throttle (WOT). By monitoring MAP sensor output voltage, the ECA can determine the correct rate of spark advance, EGR flow and air/fuel ratio. If MAP sensor fails, the ECA will supply a fixed MAP value and use the Throttle Position Sensor (TPS) to control fuel distribution. See ECA INPUT SENSORS.

MASS AIR (MA) SENSOR

Measures airflow by measuring temperature differential. Incoming air is heated by a heated platinum wire that is wound on a ceramic bobbin and coated with glass. The "Hot Wire" is heated to approximately 450°F above a cold wire sensor located down stream of the "Hot Wire".

As air passes through the airflow sensor, the temperature of the air is measured as it passes over the cold wire sensor. The ECA uses the temperature differential to compute the quantity of air flowing across the sensor and into the engine.

MASS AIRFLOW (MAF) METER

Mass airflow sensor measures flow of air entering the engine in grams per second. This measurement of airflow is a reflection of engine load (throttle opening), similar to the relationship of engine load to MAP or vacuum sensor signal. MAF signal should remain relatively constant at cruise, gradually changing with throttle angle and rapidly changing on sudden acceleration. The ECA uses this information to control fuel delivery.

NEUTRAL GEAR SWITCH (NGS)

The NGS monitors in-gear conditions and signals the ECA. This signal affects, air/fuel ratio, idle speed, and ignition timing.

NEUTRAL SAFETY SWITCH (NSS)

The switch monitors gear position on automatic transmission vehicles. It sends a gear position to the 4EAT control unit. The signal affects air/fuel ratio, idle speed and EGR flow. The switch is mounted on side of transmission case.

PROFILE IGNITION PICK-UP (PIP)

The PIP signal is generated by the ignition system. It informs the ECA of crankshaft position and engine speed. The PIP assembly is integral with distributor. It has an armature with windows and metal tabs, that rotate past a stator assembly (Hall Effect switch). The distributor does not use mechanical or vacuum advance.

POWER STEERING (P/S) PRESSURE SWITCH

The P/S switch monitors power steering pressure. When the power steering is in operation, the switch signals the ECA. The switch is located in the high pressure line from the P/S pump to the steering rack assembly. The P/S signal affects the idle speed.

PRESSURE FEEDBACK ELECTRONIC (PFE) EGR VALVE

The PFE exhaust gas recirculation valve is a conventional ported EGR valve with an integral backpressure sensing element. The valve is used in conjunction with the backpressure transducer, to inform ECA of EGR valve position. The PFE transducer converts varying exhaust pressure signals into a proportional analog voltage, which is used by the ECA to regulate EGR flow.

SELF-TEST OUTPUT/SELF-TEST INPUT (STO/STI) CONNECTORS

The STO connector is a 6-pin connector that is used to perform the Quick Test diagnostic procedure. The STI is a single-pin connector located next to the STO. When the STI is grounded, it activates the fault code output function. Codes are retrieved through the STO connector.

THROTTLE POSITION SENSOR (TPS)

The TPS monitors throttle plate opening. Its signal to the ECA is proportional to opening angle. It is mounted on the throttle body at throttle plate rod. The TPS signal affects air/fuel ratio, injector timing, idle speed, EGR flow, fuel pressure, and ignition timing (turbo engine). If a malfunction in TPS system is detected, a code 12 will be set in the ECA memory.

VANE AIRFLOW METER (VAF)

The VAF is mounted in the air inlet between the air cleaner and the throttle body. It measures the volume of air flowing into the engine. A movable door, attached to a potentiometer, signals the ECA as to door position. The ECA translates door position to volume of air entering the engine.

The VAF also contains a temperature sensor. The temperature of air entering the engine is monitored so that the information can be transmitted to the ECA. The EEC-IV module is programmed to compute airflow and air temperature so that fuel flow can be adjusted to obtain optimum air/fuel mixture. The VAF also contains a fuel pump switching circuit to cut fuel pump operation when the engine is off.

VACUUM SWITCHES (CARBURETED)

Engine load is monitored by vacuum level. To measure vacuum level, 3 vacuum switches are used to signal the MCU of cruise, deceleration, and wide open throttle conditions.

VANE AIR TEMPERATURE (VAT) SENSOR

The Vane Air Temperature (VAT) sensor is mounted in the VAF. It senses the temperature of incoming air and inputs a signal to the ECA. This signal affects air/fuel ratio, idle speed, fuel pressure, and turbo boost pressure on turbo engine. Other airflow systems use an Air Charge Temperature (ACT) sensor.

VEHICLE SPEED SENSOR (VSS)

Sensor is transmission mounted, and sends a constant pulse signal to the ECA when the vehicle is in motion.

ECA OUTPUT ACTUATORS OPERATION

Note. Each vehicle may be equipped with different combinations of input sensors. 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.

AIR CLEANER COLD WEATHER MODULATOR

A cold weather modulator is sometimes used in addition to the air cleaner temperature control sensor to control the inlet air temperature. The cold weather modulator traps vacuum in the system, so air control door will not switch to cold air when vacuum drops during acceleration.

AIR CLEANER VACUUM MOTOR

Regulates position of air control door within air cleaner duct to allow warm or cold air in as signaled by the air cleaner temperature sensor and cold weather modulator.

A/C WIDE OPEN THROTTLE CUT-OFF

During Wide Open Throttle (WOT) operation, the WAC circuit interrupts power to the A/C compressor clutch. The A/C remains off for about 3 seconds after returning from WOT.

BY-PASS AIR CONTROL (BAC) VALVE

The BAC valve, mounted on the throttle body, controls idle smoothness by regulating throttle plate by-pass air. The BAC valve consists of the air by-pass valve, which functions during cold engine conditions below 122°F (50°C), and the idle speed control solenoid valve which works throughout the entire temperature range.

The air by-pass valve is controlled by the engine coolant temperature. The idle speed control solenoid is controlled by the ECA. The BAC controls, cold engine fast idle, no touch starting, dashpot, hot engine idle and engine idle load correction.

CANISTER PURGE REGULATOR (CPR) SOLENOID VALVE

The CPR controls the vacuum supply to the canister purge valve. When energized, the CPR solenoid supplies vacuum to the canister purge valve. When de-energized, the vacuum line to the purge valve is vented to the atmosphere. The CPR is mounted on the firewall next to the pressure regulator control solenoid valve and is regulated by a signal from the ECA.

EXHAUST GAS RECIRCULATION (EGR) CONTROL SOLENOID VALVE

Two types of solenoid valves are used. A single solenoid is used on non-turbo vehicles. The solenoid provides vacuum to EGR valve when de-energized. The line to the EGR valve is vented to the atmosphere when the solenoid valve is energized.

The turbo engine uses a dual solenoid valve. One solenoid supplies vacuum to the EGR valve when energized and the other vents vacuum to the atmosphere when de-energized. The ECA controls the variable duty cycle of both valves.

EGR SHUT-OFF (EGRSO) SOLENOID

The EGR shut-off solenoid is an electrically-operated vacuum valve located between manifold vacuum source and EGR valve. A controlled vacuum bleed is located between solenoid and EGR valve. Vacuum bleed is a backpressure variable transducer. These 2 devices operate EGR valve. Solenoid switched vacuum is also supplied to canister purge valve.

EGR VENT (EGRV) SOLENOID

Solenoid vents EGR control solenoid vacuum line. Valve is normally open. When energized, vacuum is supplied to EGR valve.

Controlled by a signal from the ECA. Regulates the amount of vacuum supplied to the EGR valve.

MALFUNCTION INDICATOR LIGHT (MIL)

The MIL provides a visual signal when an ECA malfunction has occurred. The lens of the MIL is labeled "CHECK ENGINE" and it is located on the driver's instrument panel. The MIL will flash fault codes when the Self-Test Input (STI) circuit is grounded.

PRESSURE REGULATOR CONTROL (PRC) SOLENOID VALVE

The PRC solenoid valve controls the vacuum to the fuel pressure regulator. When the solenoid is de-energized, vacuum is supplied to the pressure regulator. When energized, vacuum is vented to the atmosphere. The solenoid is controlled by the ECA. It is mounted on the firewall next to the canister purge regulator solenoid valve.

TURBO BOOST CONTROL SOLENOID VALVE

This actuator controls the vacuum supply to the wastegate actuator. When the solenoid is de-energized, vacuum is supplied to the actuator. When the solenoid is energized, wastegate actuator vacuum is vented to the atmosphere. The solenoid is controlled by a signal from the ECA and is mounted on the turbocharger.

FUEL PUMP

Fuel is supplied by chassis mounted or in-tank electric fuel pump. Some models use both in-tank low pressure pump and externally mounted high pressure pump. Pump delivers fuel from fuel tank through 20-micron fuel filter to fuel charging manifold assembly.

Fuel charging manifold assembly incorporates electrically actuated fuel injectors directly above each intake port. Injectors spray metered quantity of fuel into intake air. Constant fuel pressure is maintained to injector nozzles by pressure regulator.

FUEL PRESSURE REGULATOR

Fuel pressure regulator is connected in series with fuel injectors and is positioned downstream from them. Excess fuel supplied by pump, but not consumed by engine, passes through regulator and returns to fuel tank through fuel return line.

ECA determines required fuel flow rate necessary to maintain prescribed air/fuel ratio for given engine operation by measuring quantity of air entering engine. Computer determines needed injector pulse width (period of time that injectors are energized) and energizes injector to meter quantity of fuel.

VAF system measures intake air quantity with vane airflow meter and integral air charge temperature sensor. Speed/density control system uses Throttle Position Sensor (TPS), Manifold Absolute Pressure (MAP) sensor and Air Charge Temperature (ACT) sensor to determine intake air quantity.

FUEL SUPPLY MANIFOLD ASSEMBLY

Fuel supply manifold assembly (fuel rail) delivers high pressure fuel from fuel pump supply line to fuel injectors. Fuel rail consists of tubular rail or stamping with injector connectors. Fuel pressure regulator is mounted on flange attached to rail. Rail also has mounting attachments which locate and secure fuel injectors in intake manifold.

Fuel pressure regulator is attached to fuel supply manifold assembly downstream of fuel injectors. It regulates fuel pressure supplied to injectors. Regulator is diaphragm operated relief valve in which one side of diaphragm senses fuel pressure and other side is subjected to intake manifold pressure.

Fuel pressure is controlled by spring preload applied to diaphragm. Balancing one side of diaphragm with manifold pressure maintains constant fuel pressure at injectors. Excess fuel is by-passed through regulator and returned to fuel tank.

The regulator controls fuel line vapor formation, allows for rapid restarts, assists in engine idle stabilization and maintains fuel pressure when the engine is turned off. On 1.9L engines, fuel pressure is maintained at 14.5 psi (1.0 kg/cm 2 ). On 2.5L engines, fuel delivery pressure is maintained at approximately 39 psi (2.7 kg/cm 2 ). On all models, this value may vary from unit to unit. Pressure is adjusted at the factory to compensate for differences in fuel flow among injectors.

FUEL PUMP SHUT-OFF (INERTIA) SWITCH

Fuel injected models use an electrical interrupt switch in the fuel system. In the event of a collision or vehicle roll-over, electrical contacts within the inertia switch open and fuel supply to the electric fuel pump is shut off. Fuel supply will be interrupted even if the engine is still running.

A reset button is located on the switch assembly. If the electrical circuit trips, it is not possible to re-start the vehicle unless the switch is reset by depressing the reset button. Anytime the switch opens, the fuel system should be inspected for damage prior to resetting.

CARBURETED (5.8L V8 MCU)

The model 7200 VV (Variable Venturi) carburetor changes venturi opening size to allow for varying engine speed and load conditions. This is done by dual venturi valves which are controlled by engine vacuum and throttle position.

Airflow to engine is controlled by single butterfly valve mounted in a die cast aluminum throttle body. Butterfly valve is identical in configuration to throttle plate of carburetor. It is actuated by a similar linkage and pedal cable arrangement.

Depending upon engine demands, venturi valve positions change to determine amount of airflow through carburetor. Venturi valves are connected to 2 tapered main metering rods. As venturi valve position changes, metering rods adjust amount of fuel flow through main metering jets.

The carburetor changes air/fuel ratio in response to commands from MCU control module. The system uses a stepper motor to regulate amount of bleed air allowed into main metering fuel system. The greater amount of air entering the system, the leaner the air/fuel ratio.

THROTTLE BODY INJECTION (TBI)

The TBI system is single point, pulse time modulated injection system. Fuel is metered into air intake stream according to engine demands by single injector, which is mounted in throttle body on intake manifold.

Throttle body assembly controls airflow to engine through a butterfly valve. Throttle position is controlled by either linkage or cable/cam mechanism. Body is one-piece aluminum casting with single bore and air by-pass channel.

Fuel is supplied by a rail or fuel tank mounted electric fuel pump. Fuel is filtered and sent to injector and then to pressure regulator. A single injector nozzle is mounted vertically above throttle plate. Fuel pressure regulator is downstream from injector in fuel system. Fuel in excess of that needed for engine demand is returned to fuel tank by fuel return line.

PORT FUEL INJECTION (PFI)

The electronic fuel injection system is a pulse time, PFI system. The ECA controls fuel injectors to meter fuel quantity into intake ports. The ECA receives inputs from engine sensors to compute fuel flow necessary to maintain air/fuel ratio throughout entire engine operational range. Injector "on" time is the only controlled variable in fuel delivery system.

FUEL PUMP CONTROL (ELECTRIC)

When ignition switch is turned to "ON" position, EEC power relay is energized (contacts closed). Power is provided to fuel pump relay and to timer in ECA. Fuel pump receives power through fuel pump relay contacts. If ignition switch is not turned to "START" position, timer in ECA will open ground circuit after approximately one second. ECC senses engine speed and shuts off fuel pump by opening ground circuit to fuel pump relay when engine stops, or when engine speed drops to less than 120 RPM.

Opening ground circuit de-energizes fuel pump relay (contacts opened) and de-energizes fuel pump. This function allows pressurization of fuel system. When ignition switch is turned to "START" position, ECA operates fuel pump relay to provide fuel for starting engine while cranking.

FUEL PUMP CONTROL (MECHANICAL)

The fuel pump is bolted to the left side of the engine. Fuel pump arm extends into the block and is operated by an eccentric on the camshaft. As a safety feature, the pump has a built-in check valve to cut fuel supply in case of rollover.

FUEL PUMP RELAY

Fuel pump relay and ignition is activated by ECA when the ignition switch is in the "ON" or "START" positions. When ignition is turned on, the relay is activated to supply initial fuel line pressure to system. Some models use an Integrated Relay Controller Module (IRCM) which incorporates the fuel pump relay.

FUEL INJECTORS

On PFI engines, each cylinder has a solenoid-operated injector which sprays fuel toward back of each intake valve. On TBI engines, a single injector is mounted in the throttle body. On all fuel injected models, the ECA controls length of time each injector is on, or energized.

Fuel injector nozzles are solenoid operated valves which meter and atomize fuel delivered to engine. Each injector receives battery voltage through an ignition switch circuit. The ECA controlled ground circuit is used to complete the circuit and energize (on time) the injector. The on time governs the amount of fuel delivered.

Injector bodies consist of solenoid actuated pintle and needle valve assembly. Injector flow orifice is fixed and fuel pressure at injector tip is constant. Fuel flow to engine is regulated by how long solenoid is energized. This length of time is known as "pulse width". Atomization spray is obtained by shape of pintle.

FUEL INJECTOR NOZZLE

The fuel injector nozzle is an electromechanical device which meters and atomizes the fuel delivered to the engine. The injector valve body consists of a solenoid actuated ball and seat assembly.

An electrical control signal from the EEC-IV processor operates the solenoid, allowing the ball to move off its seat and fuel to flow. The injector flow orifices are fixed, and fuel supply pressure is constant. Amount of fuel flow to the engine is controlled by amount of time the solenoid is energized.

THROTTLE BODY ASSEMBLY

Throttle body assembly controls airflow to engine through a butterfly valve. Throttle position is controlled by either linkage or cable/cam mechanism. Body is one-piece aluminum casting with single bore and air by-pass channel.

Air by-pass channel carries idle airflow which is regulated by air by-pass valve. Air by-pass valve is controlled by ECA to adjust both cold and warm idle speeds. Air by-pass valve uses solenoid valve to vary volume of idle airflow allowed to enter by-pass channel.

IDLE SPEED CONTROL (ISC) MOTOR

This DC motor controls idle speed according to signals from ECA. Idle speed motor also controls high cam RPM, anti-diesel shutoff, dashpot, and pre-positioning for next vehicle start up. The ISC includes an integral Idle Tracking Switch (ITS) which sends a signal to the ECA that the throttle is at idle position.

IDLE SPEED CONTROL BY-PASS AIR (PFI)

The throttle air by-pass valve is a solenoid-operated valve controlled by ECA. The valve allows air to by-pass around throttle plates to control cold engine fast idle, no-touch start, dashpot, overtemperature idle boost, and engine load idle correction.

A non-adjustable Throttle Position Sensor (TPS) is mounted to the throttle shaft and is used to supply a voltage output signal proportional to the throttle position. The TPS signal is used by the computer to determine engine operating conditions in relation to throttle position.