INTRODUCTION
Note. Unless otherwise specified, references to Pickup include the F350 Super Duty commercial chassis.
This article covers basic description and operation of engine performance-related systems and components. Read this article before diagnosing vehicles or systems with which you are not completely familiar.
ELECTRONIC CONTROL ASSEMBLY (ECA)
During system operation, ECA transmits electrical reference signals to engine sensors and analyzes return signals to determine engine operating conditions. (Scheme 1) If a sensor or actuator fails, ECA initiates an alternative strategy, allowing vehicle to maintain driveability. This strategy is called Failure Mode Effects Management (FMEM).
The CHECK ENGINE light will illuminate and remain on whenever FMEM is in operation. The ECA, overriding failed component with a FMEM substitute operation value, continues to monitor failed sensor. Should signals from faulty sensor return to within operational limits, ECA will cancel FMEM and resume control based on sensor signals.
| Application | Location |
|---|---|
| Aerostar | Under Left Instrument Panel |
| Bronco & Pickup | Behind Left Kick Panel |
| Explorer & Ranger | Behind Right Kick Panel |
| Van | Under Right Instrument Panel |
ECA LOCATION
Scheme 1
Note. Components are grouped into 2 categories. The first category is INPUT DEVICES, covering components which control or produce voltage signals monitored by the ECA. The second category is OUTPUT SIGNALS, covering components controlled by the ECA.
INPUT DEVICES
Vehicles are equipped with different combinations of input devices. Not all devices are used on all models. To determine the input device usage on a specific model, see WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. The available input signals include
A/C Compressor Clutch Signal
When battery voltage is supplied to A/C compressor clutch, ECA receives a signal, which it uses to increase engine idle speed to compensate for A/C compressor load.
Air Charge Temperature (ACT) Sensor
Threaded into a cylinder runner of intake manifold, the ACT sensor provides electronic fuel injection system with mixture temperature information. The ACT sensor is used both as a density corrector for airflow calculation and to proportion cold enrichment fuel flow.
Barometric Pressure (BP) Sensor
BP sensor measures barometric pressure of atmosphere. Variations in atmospheric pressure (changes in altitude) modify an electrical signal monitored by ECA. The BP sensor input affects spark advance, EGR flow and air/fuel mixture adjustments by ECA. Sensor input is updated during key-on and wide open throttle applications. The sensor looks identical to Manifold Absolute Pressure (MAP) sensor, except tubing nipple on BP sensor is open to atmosphere, while tubing nipple on MAP sensor is connected to intake manifold.
Brake On-Off (BOO) Switch
The BOO switch is mounted on brake pedal. It signals deceleration for air/fuel ratio adjustment.
Clutch Engage Switch
This switch is mounted at clutch pedal and signals ECA when transmission is in gear. Clutch engage switch signal to ECA affects air/fuel ratio and idle speed.
Coolant Temperature Sensor
See ENGINE COOLANT TEMPERATURE (ECT) SENSOR.
Crankshaft Sensor
A dual Hall Effect crankshaft sensor, mounted at crankshaft pulley, is used on 2.3L dual-plug Distributorless Ignition System (DIS). Sensor signals are generated when 2 trigger wheels, mounted on pulley assembly, pass through air gap of 2 Hall Effect switches. One switch produces a Profile Ignition Pick-up (PIP) signal, monitored by DIS ignition module to provide RPM signal to ECA. The second switch produces Cylinder Identification (CID) signal, used by DIS ignition module to identify which coil pack to trigger. If CID signal is not present, vehicle will be hard to start.
EGR Valve Position (EVP) Sensor
Mounted on EGR valve, EVP sensor detects EGR valve position and transmits this information to ECA. The EVP signal affects EGR flow and ignition timing.
Engine Coolant Temperature (ECT) Sensor
Threaded into an engine coolant passage near thermostat housing, ECT sensor inputs coolant temperature to ECA. The signal from ECT affects air/fuel ratio, idle speed, EGR flow, purge flow, fuel pressure, boost pressure (turbo engine) and ignition timing output signal from ECA.
Fuel Injection Pump Lever (FIPL) Sensor
The FIPL sensor is used on 7.3L vehicles with E40D automatic transmission. FIPL is mounted to fuel injection pump and actuated by throttle lever. FIPL transmits a signal, proportional to level of fuel delivery, to the Transmission Electronic Control Assembly (TECA) for assisting in transmission shift and torque capacity.
If FIPL circuit malfunctions, TECA will recognize erroneous signals and provide high capacity operating mode to protect transmission from damage. High capacity operating mode includes singular shift schedule at maximum throttle valve pressure, resulting in harsh shifting.
Heated Exhaust Gas Oxygen (HEGO) Sensor
The O2 sensor is mounted in exhaust manifold. When at operating temperature, the O2 sensor monitors oxygen content of exhaust gases. A heating circuit is used to warm O2 sensor to operating temperature, enabling faster conversion of feedback system to closed loop operation.
The O2 sensor produces low voltage (less than .4 volt) to indicate a lean mixture (high amount of oxygen) and a high voltage (more than .6 volt) to indicate a rich mixture (low amount of oxygen). This voltage signal is transmitted to ECA.
Manifold Absolute Pressure (MAP) Sensor
Manifold pressure and temperature are used by the ECA to calculate the airflow rate. The MAP sensor responds to manifold vacuum changes due to engine load and speed changes.
The MAP sensor uses frequency to measure 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 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.
Mass Airflow (MAF) Sensor
MAF sensor measures flow of air entering the engine. This measurement of airflow is a reflection of engine load (throttle opening). The sensing element (hot wire) is a thin platinum wire wound on a ceramic bobbin and coated with glass. The hot wire is maintained at 392°F (200°C) above cold wire (ambient) temperature. Cold wire is located downstream of hot wire. As air passes through the airflow sensor, the air temperature is measured as air passes over the cold wire sensor. The ECA uses this information to control fuel delivery.
Neutral Gear Switch (NGS)
The NGS monitors in-gear conditions and signals ECA. This signal affects air/fuel ratio, idle speed and ignition timing.
Neutral Safety Switch (NSS)
Mounted on side of transmission, this switch monitors gear position on automatic transmission vehicles. The signal affects air/fuel ratio, idle speed and EGR flow.
Power Steering (P/S) Pressure Switch
The P/S pressure switch monitors power steering pressure, signaling ECA to adjust engine idle speed under load conditions. The switch is located in high pressure line from P/S pump to steering rack assembly.
Profile Ignition Pick-Up (PIP)
On 2.3L DIS ignition systems, modulated PIP and CID signals are produced by dual Hall Effect crankshaft sensor and monitored by DIS ignition module. See CRANKSHAFT SENSOR under INPUT DEVICES. Ignition module relays the PIP signal to ECA, which uses it as an RPM reference to determine timing adjustments.
ECA timing control returns to ignition module in form of a Spark Output (SPOUT) signal. The leading edge of SPOUT signal fires coil, and trailing edge controls dwell on-time.
On 4.0L EDIS ignition systems, PIP signal is generated by Variable Reluctance Sensor (VRS), located near crankshaft pulley. The VRS is a Permanent Magnet (PM) generator, which produces an AC voltage signal which increases with RPM. The EDIS ignition module monitors this signal, passes this information on to ECA and modifies coil triggering signal based upon a Spark Angle Word (SAW) signal sent from ECA.
On distributor-type ignition systems, PIP signal is provided by a 12-volt reference from ignition module to Hall Effect switch, inside distributor. This reference voltage is modulated by window/shutter Hall Effect switch. Monitored reference is pulled low when shutter blade is out of Hall Effect switch window.
When shutter blade enters window, 12-volt reference returns, and TFI ignition module sends PIP signal to ECA. ECA uses the PIP signal as an RPM reference to calculate timing adjustments. ECA timing control returns to ignition module in form of a Spark Output (SPOUT) signal.
Self-Test Output/Self-Test Input (STO/STI) Connectors
The STO connector is a 6-pin connector used to perform self-test diagnostic procedure. The STI is a single-pin connector, located next to STO. When STI is grounded, it activates fault code output function. Codes are retrieved through STO connector.
Throttle Position Sensor (TPS)
Mounted on throttle body at throttle plate rod, TPS monitors throttle plate opening. The TPS signal to ECA is proportional to throttle opening angle. The TPS signal affects air/fuel ratio, injector timing, idle speed, EGR flow and fuel pressure. If TPS system malfunctions, Code 12 will be set in ECA memory.
Variable Reluctance Sensor (VRS)
The VRS is used on 4.0L Electronic Distributorless Ignition System (EDIS). The VRS transmits crankshaft position and RPM information. It is a passive electromagnetic device, which senses movement of a 35-tooth wheel (with a gap at 60 degrees BTDC, where the 36th tooth would have been, for triggering). The VRS is located behind crankshaft pulley.
An AC voltage signal, generated by VRS, increases with engine RPM and provides basic spark timing information to EDIS ignition module. Ignition module uses this input to produce a PIP signal to ECA. The ECA responds with a Spark Angle Word (SAW) signal, which, along with VRS signal, is used by ignition module to compute basic spark timing and to determine which coil pack to trigger.
Vehicle Speed Sensor (VSS)
The transmission-mounted VSS sends a pulsing signal to ECA when vehicle is moving. The VSS generates pulses with axle shaft revolution. The ECA interprets speed sensor input along with TPS closed throttle input.
This input enables ECA to differentiate between closed throttle deceleration and closed throttle idle (vehicle stopped) conditions. During deceleration, ECA controls Idle Speed Control (ISC) Valve to maintain a desired manifold pressure. During idle, ECA controls ISC motor to maintain a desired idle speed.
OUTPUT SIGNALS
Note. Vehicles are equipped with different combinations of computer-controlled components. Not all components listed below are used on every vehicle. For theory and operation on components, refer to indicated system.
A/C Clutch Cycling Switch
Canister Purge Solenoid Valve
See EMISSION SYSTEMS .
CHECK ENGINE Light
See SELF-DIAGNOSTIC SYSTEM .
Cooling Fan
See MISCELLANEOUS CONTROLS .
EGR System
See EMISSION SYSTEMS .
Fuel Injectors
See FUEL SYSTEMS .
Fuel Pump
See FUEL SYSTEMS.
Fuel Pressure Regulator
See FUEL SYSTEMS
Idle Speed Control (ISC) Valve
See FUEL SYSTEMS .
Self-Diagnostic
See SELF-DIAGNOSTIC SYSTEM.
Self-Test Output/Self-Test Input Connectors - See SELF
Wide Open Throttle Cutoff
See MISCELLANEOUS CONTROLS .
System Types
Fuel delivery systems differ in design depending upon model. System designs use one of the following configurations
- Single tank with single pump.
- Single tank with dual pump.
- Dual tank with mechanical selector valve/reservoir.
Low Pressure Fuel Pump
All vehicles are equipped with a high pressure pump, but some vehicles use multiple pump systems. These systems have a primary, low pressure in-tank pump for supplying fuel to reservoir. The low pressure pump rests in a sump (depression) in fuel tank. A nylon screen protects low pressure pump inlet from contaminating particles, but allows passage of small amounts of water which may accumulate in fuel tank sump.
When dual tanks are used, each tank is equipped with a low pressure pump. Such a system has a total of 3 pumps: 2 low pressure and one high pressure.
High Pressure Fuel Pump
The high pressure fuel pump is positioned inside fuel tank. A reservoir is built onto pump and sender assembly, instead of as part of tank.
In a 2-tank system, sender assembly handles switching of high pressure fuel through internal valves. Should one tank overfill during use (return line returns fuel to wrong tank), pump and sender unit in overfilling tank need replacement.
The high pressure fuel pump is capable of pumping over 33 gallons (125 liters) of fuel per hour at a working pressure of 39.2 psi (2.75 kg/cm 2 ). This pump also has internal pressure relief and discharge check valves.
Mechanical Fuel Pump
The 7.3L diesel engine uses a mechanical fuel pump to pump fuel from fuel tank to a combination fuel filter/fuel heater/water separator.
Mechanical Selector Valve
A driver-operated selector switch controls selector valve for switching fuel supply from one tank to other. The mechanical selector valve is contained within 6-port reservoir assembly. This valve switches fuel supply and return lines from one tank to other in response to fuel pressure from in-tank pumps acting on its actuating diaphragm.
The diaphragm switches tank connection when less than 2 psi of fuel pressure is acting on upper side of front tank and lower side of rear tank. Valve functioning depends upon proper operation of in-tank low pressure pumps. In all dual tank vehicles, excess fuel not used by engine is returned to same tank from which it was pumped.
Reservoirs & Filters
Fuel reservoirs are used to prevent fuel flow interruptions during extreme vehicle maneuvers with low tank fill levels. Models using multiple pumps use in-line reservoirs, frame mounted between low and high pressure pumps. On models using one pump, reservoir is either molded or welded into either tank or fuel pump and sender plastic housing.
There are 2 types of in-line reservoirs: single function and dual function. Both contain a fine mesh in-filter; the dual function contains a mechanical selector valve.
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 secure fuel injectors in intake manifold.
The fuel pressure regulator is attached to fuel supply manifold assembly, downstream of fuel injectors. It regulates fuel pressure supplied to injectors. Regulator is a diaphragm-operated valve with one side responding to fuel pressure and the other side to intake manifold vacuum.
When intake manifold vacuum is low, an internal spring increases pressure on diaphragm, blocking off fuel return passage and increasing fuel pressure. When manifold pressure is high, spring pressure is overcome by vacuum, opening fuel return passage and lowering fuel pressure. Excess fuel is by-passed through regulator and returned to fuel tank.
The regulator also controls fuel line vapor formation, allowing for rapid restarts, assistance in engine idle stabilization and maintenance of fuel pressure when engine is turned off. Pressure is adjusted at factory to compensate for fuel flow differences between injectors.
Fuel Pump Actuation
When ignition switch is turned to START position, ECA operates fuel pump relay to provide fuel for starting engine. ECA senses engine speed and shuts off fuel pump by opening ground circuit to fuel pump relay when engine stops or speed drops to less than 120 RPM. When ignition switch is in ON position, EEC power relay is energized (contacts closed). Power is provided to fuel pump relay and timer in ECA. Fuel pump receives power through fuel pump relay contacts.
Fuel Pump Shutoff (Inertia) Switch - All models use an electrical interrupt switch in fuel system. During a collision or vehicle roll-over, electrical contacts within inertia switch open, shutting off fuel supply to electric fuel pump. Fuel supply is interrupted even when engine is running.
A reset button is located on switch assembly. If electrical circuit trips, vehicle will not re-start until switch is reset. Fuel system should be inspected before resetting switch.
On Aerostar, inertia switch is located in right kick panel. On Van, inertia switch is located on right cowl panel, near front of door. On Bronco and Pickup, inertia switch is located on left toe board, near parking brake assembly. On Ranger and Explorer, inertia switch is located on toe board, near right side of transmission hump.
Note. DO NOT reset fuel pump shutoff (inertia) switch after an accident until inspecting entire fuel system for leaks.
FUEL CONTROL (GASOLINE ENGINES)
Precise fuel metering is accomplished with EEC-IV system. The ECA continually monitors engine operating conditions based on information received from various sensors and switches. In response to information received, ECA calculates optimum air/fuel mixture in relation to present engine operating conditions and affects required metering adjustments through output actuators control.
Fuel Injection
The ECA controls fuel injectors to meter pulse width or time each injector is energized. Each injector receives battery voltage through ignition switch circuit. The ECA-controlled ground circuit completes circuit to energize injector. The ECA receives inputs from engine sensors to compute fuel flow necessary to maintain air/fuel mixture ratio throughout entire engine operational range.
Each cylinder has a solenoid-operated injector which sprays fuel toward back of each intake valve. Injector bodies consist of solenoid-actuated pintle and needle valve assembly. Injector flow orifice is fixed, and fuel pressure at injector tip is constant. Atomizing spray is obtained by shape of pintle.
Throttle Body Assembly
The throttle body assembly controls air-flow 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 CONTROL (7.3L DIESEL)
The fuel pump delivers fuel from fuel tank to the fuel filter. After being filtered, fuel enters the injection pump and is delivered under high pressure through injection nozzles into cylinders. Injectors are equipped with a fuel return outlet. Excess fuel collected at the injectors and injector pump are recirculated back to the fuel tank.
The ISC valve, mounted on the throttle body, controls idle smoothness by regulating throttle plate by-pass air. ISC valve consists of an 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 affected by the engine coolant temperature. The idle speed control solenoid is controlled by ECA.
Idle is controlled by idle speed control air by-pass valve. The throttle air by-pass valve is a solenoid-operated valve controlled by ECA. The valve allows air to by-pass throttle plates to control cold engine fast idle, no-touch start, dashpot, over-temperature idle boost and engine load idle correction.
Air by-pass channel carries idle airflow 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 idle airflow volume allowed to enter engine.
IDLE SPEED (7.3L DIESEL)
Idle is controlled by curb idle speed adjustment screw, located on injection pump. Fast (cold) idle is controlled by cold idle solenoid, located by curb idle speed adjustment screw. For additional information, see ADJUSTMENTS article in the ENGINE PERFORMANCE Section.
DISTRIBUTORLESS IGNITION SYSTEM (DIS)
The DIS system, used on 4-cylinder engines, is a dual plug system consisting of a crankshaft-mounted dual Hall Effect sensor, two 4-tower DIS coil packs and a DIS ignition module.
The DIS eliminates distributor by using multiple coils, each of which simultaneously fires 2 paired spark plugs. The first time a pair of spark plugs is fired, one fires during compression stroke and other during exhaust stroke. The next time same pair of spark plugs is fired, the roles are reversed. Although spark in exhaust stroke is wasted, little of coil's energy is lost.
Two ignition coils are mounted together in a coil pack. Since there are 2 plugs per cylinder, 2 coil packs are required. The right coil pack operates continuously, whereas left coil pack may be switched on or off by ECA.
The ECA computes spark angle and dwell for ignition system. The crankshaft-mounted dual Hall Effect sensor is a dual digital output device, which responds to 2 rotating metallic shutters mounted on crankshaft. One output from hall effect sensor, Profile Ignition Pick-Up (PIP), is a 50 percent duty cycle signal, providing base spark timing information. The other output signal, Cylinder Identification (CID) signal, is required for DIS module to know which coil to fire. CID is high (battery voltage) for half of crankshaft revolution (180 degrees) and low (zero volts) for other half of revolution.
The ECA determines Spark Output (SPOUT) by using 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 signal fires coil, and trailing edge of signal controls dwell on-time. This feature is called Computer Controlled Dwell (CCD).
Another feature of system is Ignition Diagnostic Monitor (IDM). This is an output from DIS module to ECA which provides diagnostic information about ignition system for self-test.
Dual Plug Inhibit (DPI), another system feature, allows ECA processor to switch ignition system from single-to-dual plug operation. During cranking, vehicle is in single plug mode: only plugs on right side of engine fire. When engine starts, ECA sends a command to DIS module to switch to dual plug operation.
If CID circuit fails, DIS module will randomly select one of 2 coils to fire. If hard starting results, turning key off and then cranking again will result in another random selection. Several attempts may be needed before DIS module selects proper coil, which will allow vehicle to start and be driven until repairs can be made.
The Failure Mode Effects Management (FMEM) system will keep vehicle drivable in event of an EEC-IV system or ignition failure, which would otherwise prevent spark angle or dwell commands. During FMEM, ECA opens SPOUT line and DIS module fires coils directly from PIP output. This results in a fixed spark angle of 10 degrees and a fixed dwell.
Electronic Distributorless Ignition System (EDIS)
System is used on 4.0L engines and consists of a Variable Reluctance Sensor (VRS), an EDIS ignition module, an ECA and one 6-tower coil pack.
During system operation, EDIS ignition module receives crankshaft position information from VRS. In turn, EDIS ignition module generates a Profile Ignition Pick-Up (PIP) signal and sends it to ECA. The ECA responds with a Spark Angle Word (SAW) signal containing advance or retard timing information, which it sends to EDIS module. The EDIS ignition module then processes VRS and SAW signals to decide which coils to fire. In addition, EDIS ignition module generates an Ignition Diagnostic Monitor (IDM) signal and sends it to ECA, which uses it during failure mode to provide a tach output signal.
The EDIS ignition module is a microprocessor and makes decisions about spark timing and coil firing. The EDIS ignition module turns coils on and off at correct times and in proper sequence, based on VRS and SAW signals. The EDIS ignition module, upon receiving VRS and SAW signals, produces PIP and IDM output signals and sends these signals to ECA.
The ECA receives IGN GND and PIP signals from EDIS ignition module, and then generates a SAW output signal, based upon fuel, air and other sensor information. ECA also receives an IDM signal from EDIS ignition module to determine if a failure mode should be recorded.
The coil pack receives active low signals from EDIS ignition module and fires 2 spark plugs at a time. One plug is for cylinder which is to be fired (on compression stroke) and other goes to mating cylinder (on exhaust stroke). The next time coil is fired, situation is reversed. Coils are fired according to engine firing order.
Thick Film Ignition (TFI)
All engines, except those with distributorless ignition systems, use TFI ignition systems. The TFI distributor is a gear-driven, die-cast unit. A Hall Effect stator assembly is used to trigger ignition coil. This distributor does not use conventional centrifugal/vacuum advance mechanisms. The TFI ignition module may be mounted in base of distributor bowl or on cowl behind 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 Pick-Up (PIP) signal to ECA. The ECA uses PIP input signal to produce a Spark Output signal (SPOUT), which is sent to TFI ignition module to trigger coil secondary voltage discharge.
On manual transaxle vehicles, the TFI ignition module features a push start mode, which allows vehicle to be push started if necessary. An "E" core ignition coil is used.
EMISSION SYSTEMS
Several systems and components are used to control emissions. Operation and actuation method is provided for most devices. For testing procedures, refer to specific system in I - SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.
Temperature Vacuum Switch (TVS)
The TVS incorporates a bimetallic disc which opens and closes vacuum ports. TVS may be used in conjunction with distributor, canister purge or EGR systems.
Vacuum Control Valve
This temperature-operated vacuum switch has 2 or more ports. Valve uses a wax pellet or bimetallic material to open or close vacuum ports at normal engine operating temperatures.
Vacuum control valve is usually mounted in some part of cooling system with its base immersed in coolant. Valve may be either normally open or closed.
Vacuum Delay Valve
Vacuum delay valve is inserted in vacuum lines to provide gradual application or vacuum release to engine or emission control devices. One-way or two-way valve may be used, depending on function and system.
Vacuum Reservoir
The vacuum reservoir stores vacuum and provides an amplified vacuum signal, preventing rapid fluctuations or sudden drops in a vacuum signal during such conditions as acceleration.
Vacuum Restrictor
This orifice-type flow restrictor is used as an emission calibration to control flow rate and/or actuation timing of components and systems.
Vacuum Vent Valve
Vacuum vent valve controls induction of fresh air into system, preventing accumulation of fuel vapors which could cause decay of vacuum diaphragms. Valve may be vent only or combined vent and delay valve. Valve should always be mounted with ports pointing downward.
AIR INJECTION SYSTEM
The air injection system reduces carbon monoxide (CO) and hydrocarbon (HC) content of exhaust gases. It injects fresh air into exhaust gas stream, which continues combustion of unburned gases. Air can be by-passed to atmosphere by a thermactor air by-pass valve and/or directed near exhaust manifold or catalytic converter.
Depending upon engine size and application, individual systems may vary in number and type of components. All systems use same basic components: air supply pump, air by-pass valve, filter, check valve(s), air control valve, air manifold and air hoses. Some models may use a combined air by-pass/air control valve.
Air By-Pass Valve
Valve directs airflow from thermactor air pump to exhaust system or atmosphere as required. Valve may be mounted on air pump or in-line (remote). Air by-pass valve is vacuum operated and may be either normally open or closed.
Normally closed valve supplies air to exhaust system with medium and high applied vacuum signals during normal modes, short idles and some acceleration. With low or no vacuum applied, pump air is dumped through silencer valve ports.
Normally open valve with a vacuum vent provides a timed air dump during deceleration. Valve also dumps when a vacuum pressure difference is maintained between signal and vent ports.
Air Pump
The air pump is a belt-driven, positive displacement, vane-type pump which provides air for 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 pump. The by-pass valve performs pressure relief.
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 dioxide and water.
Air Supply Control Valve
Operated by vacuum, this valve directs air pump output upstream to exhaust manifold or downstream to catalytic converter.
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
By-pass valve routes thermactor air to either the exhaust system or atmosphere. When air is routed to exhaust system, control valve routes air either upstream to exhaust manifolds or downstream to catalytic converter.
Both valves are normally closed and come in either a bleed or non-bleed type. Bleed-type valves will have bleed percentage molded into 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 other controls thermactor air diverter valve. Both valves pass air when deactivated and do not pass air when activated.
Thermactor Idle Vacuum (TIV) Valve
The TIV valve vents vacuum signal to atmosphere when preset manifold vacuum or pressure is exceeded. During periods of extended idle, this valve is used to divert thermactor airflow to limit exhaust temperature. This prevents excessive underbody temperature. TIV valve also cuts EGR in a heavy boost mode for turbocharged applications.
Note. Not all vehicles use EGR systems. EGR system usage depends on engine application.
The Exhaust Gas Recirculation (EGR) system distributes exhaust gas into intake mixture. This lowers combustion temperatures due to lower concentrations of oxygen. Lowering combustion temperatures reduces NOx emissions.
Electronic EGR Valve
The Electronic EGR valve is required in EEC systems where EGR flow is controlled according to computer demands of EGR Valve Position (EVP) sensor. The EGR valve is operated by a vacuum signal from EGR Vacuum Regulator (EVR).
EGR valve is mounted on top of an intake and exhaust gas port. The EGR pintle blocks exhaust gas from entering intake system. When vacuum is applied to EGR valve, the diaphragm is actuated. This lifts EGR pintle (attached to diaphragm) and allows exhaust gas to recirculate into intake system.
EGR Vacuum Regulator (EVR)
The vacuum regulator is an electromagnetic device controlling vacuum to EGR valve. Regulator operation is measured as a duty cycle: increased duty means increased vacuum to EGR.
This sensor is attached to EGR valve assembly and indicates EGR valve position to ECA.
EVAPORATIVE EMISSION CONTROL
Note. Not all listed components are used on every vehicle system. Component usage depends on calibration of vehicle.
The function of evaporative emission control system is to store gasoline fumes from fuel system in a carbon canister when engine is not running. During engine operation, fumes are drawn into engine for burning during combustion process, purging canister.
Three basic components are used in evaporative emission system
- Activated carbon canister.
- Computer-controlled solenoid.
- Tank pressure control cap.
For specific component application and vacuum hose routing, see VACUUM DIAGRAMS article in the ENGINE PERFORMANCE Section.
Carbon Canister
Carbon canister storage is used for evaporative fuel control on all vehicles.
This normally closed solenoid valve controls fuel vapor flow from canister to intake manifold. It is opened or closed by a signal from ECA during various engine operating modes.
Fill Control/Vent System
Fill limiting is accomplished through configuration of fill neck and/or internal vent lines. Vent system is designed to permit air space in 10-12 percent of tank to allow for thermal expansion.
Vapor generated in fuel supply line is continuously vented back to fuel tank. Venting prevents engine surging from fuel enrichment and assists in hydrocarbon (HC) emission control.
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. Pressure relief is provided after 1.8 psi (.13 kg/cm 2 ). Under normal conditions, fill cap allows air to enter fuel tank as fuel is used, without allowing fuel vapors to escape.
Vapor Vent System
System provides a vapor space above gasoline surface in fuel tank. This area is sufficient to permit adequate breathing room for tank vapor valve assembly.
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 blow-by gases from crankcase. Manifold vacuum draws gases from crankcase, through PCV hose, into combustion chamber. The PCV valve is positioned in hose through which blow-by gases flow on their way to combustion chamber.
By opening and closing in direct relation to engine vacuum, the PCV valve meters blow-by 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 blow-by gases are produced (such as worn cylinders or rings), system allows excess gases to flow back through crankcase vent hose and into intake manifold.
SELF-DIAGNOSTIC SYSTEM
Note. All systems have self-diagnostic capabilities. For information on procedures for entering self-test modes and reading service codes, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.
The CHECK ENGINE light (if equipped) will illuminate whenever ignition is turned to ON position (bulb check) or systems related to the EEC-IV system malfunction during normal engine operation. For additional information, see appropriate TESTS W/CODES article in the ENGINE PERFORMANCE Section.
The STO connector is a 6-pin connector used to perform self-test diagnostic procedure. The STI is a single-pin connector, located next to STO. When STI is grounded, it activates fault code output function. Codes are retrieved through STO connector.
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 ECA. The ECA uses this signal to maintain system pressure within the programmed range.
The ECA regulates operation of the electric cooling fan through an ECA-controlled relay, which controls the ground or power circuit of the cooling fan. This allows the ECA to operate the cooling fan based on engine temperature. A malfunction of the cooling fan will cause engine overheating and possible detonation.
WIDE OPEN THROTTLE A/C (WAC) CUTOFF
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.
WAIT TO START INDICATOR LIGHT
On 7.3L models, this light comes on when engine is cold and ignition is in the RUN position. The light will remain on for 5-10 seconds, depending on time required for glow plugs to warm.
WATER IN FUEL INDICATOR LIGHT
On 7.3L models, this light comes on when engine is in the START position. The light will remain on if fuel filter water separator has a water level that exceeds predetermined level.