Look for the Start/Ignition fusible link. It's blue and located at the left rear of the engine compartment.
It could be the ignition control module. I have attached 3 images showing the location of the module for the 3 possible engine/ignition system combinations. My reference manual doesn't have a schematic for the ignition circuit, so I can't tell you which wire does what.
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.
ELECTRONIC CONTROL ASSEMBLY (ECA) OPERATION
During system operation, ECA transmits electrical reference signals to engine sensors and analyzes
return signals to determine engine operating conditions. 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 control and resume control based on sensor signals.
Fig. 1: EEC-IV System Component Diagram (Typical)
Courtesy of FORD MOTOR CO.
INPUT DEVICES OPERATION
NOTE: Each vehicle uses various input devices. To determine input usage
on a specific model, see appropriate wiring diagram in WIRING
A/C COMPRESSOR CLUTCH SIGNAL
When battery voltage is supplied to A/C compressor clutch, a signal is sent to ECA. The ECA uses
this signal to increase engine idle speed to compensate for A/C compressor load.
AIR CHARGE TEMPERATURE (ACT) SENSOR
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. This sensor is similar in construction to Engine Coolant Temperature (ECT)
sensor, except it is packaged to improve sensor response time. The sensor is threaded into a
cylinder runner of intake manifold.
BAROMETRIC PRESSURE (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
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 1 of 19
advance, EGR flow and air/fuel mixture adjustments by ECA. Sensor input is updated during keyon
and every wide open throttle application. The sensor looks identical to Manifold Absolute
Pressure (MAP) sensor, except tubing nipple on BP sensor is open to atmosphere, while 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 (2.3L DIS)
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 PIP signal, utilized by DIS ignition module, and gives RPM signal to ECA. The second switch
produces 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 PRESSURE FEEDBACK ELECTRONIC (PFE) TRANSDUCER
This transducer converts a varying exhaust pressure signal into a proportional analog voltage,
digitized by EEC-IV processor. The EEC-IV processor uses PFE transducer input signal to
determine optimum EGR flow.
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.
ELECTRICAL LOAD CONTROL UNIT (ELCU)
The ELCU monitors electrical load placed on alternator and electrical system. It is mounted
beneath ECA on floor panel, behind front of center console. The ELCU signal affects idle speed.
ENGINE COOLANT TEMPERATURE (ECT) SENSOR
Threaded into an engine coolant passage near thermostat housing, this sensor inputs coolant
temperature to ECA. The signal from ECT affects following:
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 2 of 19
Boost Pressure (Turbo Engine)
Ignition Timing Output Signal From ECA
ENGINE COOLANT TEMPERATURE (ECT) SWITCH
Threaded into bottom of radiator, ECT monitors radiator coolant temperature and signals ECA.
The ECT signal affects air/fuel ratio, EGR flow and ignition timing output signal from ECA.
EXHAUST GAS OXYGEN (EGO) SENSOR
The O2 sensor is mounted in exhaust manifold where it monitors oxygen content of exhaust gases
when at operating temperature. The O2 sensor produces low voltage (as low as.1 volt) to indicate
a lean mixture (high amount of oxygen) and a high voltage (as high as.9 volt) to indicate a rich
mixture (low amount of oxygen). This voltage signal is transmitted to ECA where it is interpreted
as a rich or lean mixture signal.
On some models, a heating circuit is used to warm O2 sensor to operating temperature, enabling
faster conversion of feedback system to closed loop operation. With exception of 3.0L PFI engine,
all V6 and V8 engines are equipped with sensors in each exhaust bank.
The idle switch is threaded into throttle body and contacts throttle linkage. It signals ECA when
throttle valve is fully closed. This switch signal affects air/fuel ratio, injector timing, idle speed,
EGR flow and ignition timing.
IDLE AIR BYPASS VALVE
This valve is operated by ECA to control amount of air passing throttle plates at engine idle. The
valve affects cold engine fast idle, no-touch start, dashpot, overtemperature idle boost and engine
idle load correction.
MANIFOLD ABSOLUTE PRESSURE (MAP)
The speed density method is used to compute airflow rate on models equipped with a MAP sensor.
Manifold pressure and temperature are used by ECA to calculate airflow rate to ECA. The MAP
sensor responds to manifold vacuum changes due to engine load and speed changes.
The MAP sensor also uses frequency to measure manifold vacuum. The MAP sensor is used as a
barometric sensor for altitude compensation, updating ECA during Key On, Engine Off (KOEO)
and Wide Open Throttle (WOT). By monitoring MAP sensor output voltage, ECA determines
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 3 of 19
correct spark advance, EGR flow and air/fuel ratio. If MAP sensor fails, ECA will supply a fixed
MAP value and use Throttle Position Sensor (TPS) to control fuel distribution.
MASS AIRFLOW (MAF) SENSOR
This sensor directly measures mass of air flowing into engine. Sensor output is a DC analog signal
ranging from about.05 volts to 5.0 volts. The ECA uses this signal to calculate injector pulse
width. The sensing element is a thin platinum wire wound on a ceramic bobbin, coated with glass.
The ECA maintains hot wire at 392 F (200 C) above ambient temperature, as measured by a
constant cold wire. The current necessary to maintain the hot wire temperature is a direct reflection
of incoming air temperature and mass airflow.
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 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
PRESSURE FEEDBACK ELECTRONIC (PFE) EGR VALVE
The PFE exhaust gas recirculation valve is a conventional ported EGR valve with an integral
backpressure sensing element. It is used with backpressure transducer to inform ECA of EGR
valve position. The PFE transducer converts varying exhaust pressure signals into a proportional
analog voltage used by ECA to regulate EGR flow.
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 in this
article. The PIP signal is relayed to ECA by ignition module as an RPM reference. It is used 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 producing an AC
voltage signal that increases with RPM. The EDIS ignition module monitors this signal, passes this
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 4 of 19
information on to ECA, and modifies coil triggering signal based upon a Spark Angle Word
(SAW) signal sent back from ECA.
On distributor-type ignition systems, PIP signal is provided by a 12-volt reference by 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 blade enters window, 12-volt reference returns and PIP signal is
sent to ECA by TFI ignition module. The PIP signal is used by ECA as an RPM reference, and is
used in timing adjustment calculations. 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 Quick Test diagnostic procedure. The STI
is a single-pin connector, 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
VANE AIRFLOW METER (VAF)
Mounted in air inlet between air cleaner and throttle body, VAF measures air volume flowing into
engine. The VAF is essentially a door attached to a potentiometer. The door opening angle is in
direct proportion to quantity of air acting on it. The potentiometer electrically reflects door
position, sending this signal to ECA.
The VAF also contains a temperature sensor, permitting ECA to compute air temperature.
Analyzing both airflow and air temperature permits fuel flow adjustment for optimum air/fuel
mixture. In addition, VAF contains a fuel pump switching circuit to cut fuel pump operation when
engine is off.
VANE AIR TEMPERATURE (VAT) SENSOR
The Vane Air Temperature (VAT) sensor is mounted in VAF. It senses incoming air temperature
and sends a signal to ECA. This signal affects air/fuel ratio, idle speed and fuel pressure. Other
airflow systems use an Air Charge Temperature (ACT) sensor.
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 sensing
movement of a 35-tooth wheel with a gap for triggering where the 36th tooth would be. It is
located behind crankshaft pulley. The missing tooth is at 60 BTDC.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 5 of 19
The AC voltage signal, generated by VRS, increases with engine RPM and provides 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, used by ignition module along with VRS signal
to compute basic spark timing and determine which coil pack to trigger.
VEHICLE SPEED SENSOR (VSS)
The transmission-mounted VSS sends a pulsing signal to ECA when vehicle is in motion.
OUTPUT SIGNALS OPERATION
NOTE: Each vehicle uses various output devices. To determine output
usage on a specific model, see appropriate wiring diagram in
CANISTER PURGE SOLENOID VALVE
See EVAPORATIVE EMISSION CONTROL under EMISSION SYSTEMS in this article.
EGR CONTROL SOLENOID VALVE
See EGR SYSTEM under EMISSION SYSTEMS in this article.
EGR VACUUM REGULATOR (EVR)
Controlled by a signal from ECA, EVR regulates vacuum supplied to EGR valve. The EVR
replaces EGR solenoid vacuum vent valve assembly. During EVR operation, an electric current in
coil induces a magnetic field in armature. The magnetic field pulls disk closed, closing vent and
increasing vacuum level.
IDLE SPEED CONTROL (ISC) BY-PASS AIR VALVE
The ISC by-pass air valve, mounted on throttle body, is controlled by ECA to adjust idle speed by
regulating throttle plate by-pass air. Valve is a simple "on" or "off" solenoid which increases idle
speed for cold engine fast idle, no-touch starting, hot engine idle boost and engine idle load
MALFUNCTION INDICATOR LIGHT (MIL)
The MIL provides a visual signal when an ECA malfunctions. The MIL lens, located on
instrument panel, is labeled "CHECK ENGINE." The MIL flashes fault codes when Self-Test
Input (STI) circuit is grounded.
PRESSURE REGULATOR CONTROL (PRC) SOLENOID VALVE
The PRC solenoid valve controls vacuum to fuel pressure regulator. When solenoid is deenergized,
vacuum is supplied to pressure regulator, reducing fuel pressure. When energized,
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 6 of 19
vacuum is vented to atmosphere and spring pressure within regulator moves diaphragm increasing
fuel pressure. The solenoid, controlled by ECA, is mounted on firewall, next to canister purge
regulator solenoid valve.
SPARK OUTPUT (SPOUT) & SPARK ANGLE WORD (SAW)
When ECA receives a PIP signal from ignition module, it combines this information with input
collected from other input signals, performs calculations, and returns a timing control signal (PIP
or SAW) to ignition module.
WIDE OPEN THROTTLE A/C (WAC) CUT-OFF
During wide open throttle, (WAC) circuit interrupts power to A/C compressor clutch. The A/C
remains off for about 3 seconds after returning from WOT.
FUEL DELIVERY SYSTEM
Fuel delivery systems differ in their design, depending upon vehicle. Systems are classified by
type. Five types are available:
Type 1: Single tank with single pump.
Type 2: Single tank with dual pump.
Type 3: Dual tank with electric selector valve.
Type 4: Dual tank with mechanical selector valve/reservoir.
Type 5: Dual tank with pump sender in-tank reservoir.
LOW PRESSURE FUEL PUMP
All vehicles are equipped with a high pressure pump, but some are 2 pump systems having a
primary, or low pressure in-tank pump for supplying fuel to reservoir (types 2, 3, and 4 systems).
The low pressure pump rests in a sump, or 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.
HIGH PRESSURE FUEL PUMP
The type 5 system uses a high pressure fuel pump positioned inside fuel tank. This pump is similar
to type 1 pump. Type 5 systems have a reservoir built onto pump and sender assembly instead of
as a part of tank.
In a 2-tank system, sender assembly handles switching of high pressure fuel through internal
valves. In a type 5 system with 2 tanks, should one tank overfill during use (return line returns fuel
to wrong tank), it will be necessary to change pump and sender unit in tank that overfills.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 7 of 19
The fuel pump is capable of pumping in excess of 33 gallons (125 liters) of fuel per hour at a
working pressure of 39.2 psi (2.75 kg/cm2 ). These pumps also have internal pressure relief and
discharge check valves.
RESERVOIRS & FILTERS
Fuel reservoirs are used to prevent fuel flow interruptions during extreme vehicle maneuvers with
low tank fill levels. In-line reservoirs are used on type 2, 3 and 4 systems and are frame mounted
between low and high pressure pumps. When high pressure pump is located in-tank (type 1 and 5
systems), reservoir is either molded or welded into tank or into fuel pump and sender plastic
There are 2 types of in-line reservoirs: single function and dual function. Both contain a fine mesh
in-filter and the dual function contains a mechanical selector valve.
A driver-operated selector switch controls selector valve for switching fuel supply from one tank
to other. Two types of valves are used. These are electrical type (type 3) or mechanical type (type
The electric type, when energized by selector switch, shuts off fuel supply and return lines from
one tank and opens lines to other tank. Simultaneously, in-tank pump and fuel level sender are
turned off for one tank and energized for other.
The mechanical selector valve is contained within 6-port reservoir assembly and is found on type 4
high pressure fuel pump system only. 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 under 2 psi of fuel pressure are acting on upper
side of front tank and lower side for 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.
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
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,
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 8 of 19
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 differences in fuel flow among injectors.
FUEL PUMP SHUT-OFF (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 and fuel supply to electric fuel pump is shut off. Fuel
supply is interrupted even when engine is running.
A reset button is located on switch assembly. If electrical circuit trips, it is not possible to re-start
vehicle until switch is reset. Anytime switch opens, fuel system should be inspected before
CAUTION: DO NOT reset fuel pump shut-off (inertia) switch after an accident
until complete fuel system is inspected for leaks.
FUEL CONTROL SYSTEMS
Precise fuel metering is accomplished with EEC-IV system. The Electronic Control Assembly
(ECA), the heart of the EEC-IV system, continually monitors engine operating conditions based on
information received from various sensors and switches. In response to received information, ECA
calculates optimum air/fuel mixture in relation to present engine operating conditions and effects
required metering adjustments through output actuators control.
PORT FUEL INJECTION (PFI)
The electronic fuel injection system is a pulse time 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 mixture ratio throughout entire engine operational range.
FUEL PUMP CONTROL
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. If ignition switch is not turned to START position, timer in ECA will open ground
circuit, after approximately one second. 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.
Opening ground circuit de-energizes fuel pump relay (contacts opened) and de-energizes fuel
pump. This allows fuel system to depressurize. When ignition switch is turned to START position,
ECA operates fuel pump relay to provide fuel for starting engine while cranking.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 9 of 19
Each cylinder has a solenoid-operated injector which sprays fuel toward back of each intake valve.
ECA controls pulse width or time each injector is energized.
Fuel injectors meter and atomize fuel delivered to engine. Each injector receives battery voltage
through ignition switch circuit. The ECA controlled ground circuit completes circuit and energize
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 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.
IDLE SPEED CONTROL OPERATION
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 idle position signal to ECA.
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 throttle plates to control cold engine fast idle, no-touch start, dashpot, overtemperature
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 by-pass channel.
THROTTLE POSITION SENSOR (TPS)
The non-adjustable TPS, mounted to throttle shaft, supplies a voltage output signal according to
throttle position. The ECA uses TPS signal to determine engine operating conditions in relation to
DISTRIBUTORLESS IGNITION SYSTEM (DIS)
The DIS system, used on 4-cylinder engine, is a dual plug system consisting of a crankshaftmounted
dual Hall Effect sensor, two 4-tower DIS coil packs and a DIS ignition module.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 10 of 19
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 responding to 2 rotating metallic shutters mounted on
crankshaft. One output from Hall Effect sensor, Profile Ignition Pickup (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 ) and low (zero volts) for other half
The ECA determines spark angle (SPOUT) 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 are firing. 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 guess. Several attempts may be
needed until proper coil is selected, allowing vehicle to start and be driven until repairs can be
The Failure Mode Effects Management (FMEM) system will keep vehicle drivable in event of an
EEC-IV system or ignition failure that 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 6-cylinder 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 Pickup (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 back to EDIS module. The EDIS ignition module then processes
VRS and SAW signals and decides which coils to fire. In addition, EDIS ignition module
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 11 of 19
generates an ignition Diagnostic Monitor (DM) signal and sends it to ECA which uses it during
failure mode to provide a tach output signal.
The VRS, which transmits crankshaft position, is a passive electromagnetic device senses
movement of a 35-tooth wheel with a gap where the 36th tooth would be, located behind
crankshaft pulley (missing tooth is at 60 BTDC). An AC voltage signal, generated by VRS,
increases with engine RPM and provides basic spark timing information to EDIS ignition module.
The EDIS ignition module is a microprocessor that makes decisions about spark timing and coil
firing. The EDIS ignition module turns coils on and off at correct time and in proper sequence,
based on information from VRS and SAW signal generated from ECA. The EDIS ignition module,
upon receiving VRS and SAW signals, produces PIP and IDM output signals which are sent to
The ECA receives IGN GND and PIP signals from EDIS ignition module then generates a SAW
output signal, based upon fuel, air and other sensor information. An IDM signal is received 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
once. One plug is for cylinder which is to be fired (On compression stroke) and other goes to
mating cylinder which is on exhaust stroke. The next time coil is fired situation is reversed. The
next pair of spark plugs is fired according to engine firing order.
THICK FILM IGNITION (TFI)
All engines, except distributorless ignition, use TFI ignition system. 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 Pickup (PIP) signal to ECA. The ECA uses PIP input signal to produce an output
signal called SPOUT that is sent to TFI ignition module to be used to trigger coil secondary
The TFI ignition module, used on manual transaxle vehicles, features a push start mode. This
feature allows vehicle to be push started if necessary. An "E" core ignition coil is used.
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 -
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.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 12 of 19
VACUUM CONTROL VALVES (VCV)
These temperature operated vacuum switches have 2 or more ports. They utilize a wax pellet or
bimetallic material to open or close vacuum ports at normal engine operating temperatures.
These vacuum valves are usually mounted in some part of cooling system with their base
immersed in coolant. They 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 gradual application or vacuum release to engine or emission
control devices. One-way or two-way valves may be used depending on function and system.
The vacuum reservoir stores vacuum and provides an amplified vacuum signal, preventing rapid
fluctuations or sudden drops in a vacuum signal during conditions such as acceleration.
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 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 with ports pointing downward.
AIR INJECTION SYSTEMS
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.
Individual systems may vary in number and type of components, depending upon engine size and
PULSE AIR VALVE SYSTEM
The pulse air valve replaces air pump on some thermactor systems. The pulse air valve uses natural
pulses present in exhaust system to pull air into exhaust manifold. 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
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.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 13 of 19
In Managed Thermactor Air (MTA) system, air can be by-passed to atmosphere by a thermactor
air by-pass valve and/or directed near exhaust manifold or 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 air by-pass valve, diverts some thermactor air to
intake manifold during sudden drop in 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 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 valves with a vacuum vent provide a timed air dump during deceleration and also
dump when a vacuum pressure difference is maintained between signal port and vent port.
AIR CHECK VALVE & PULSE AIR VALVE
Both units function by allowing thermactor air to enter exhaust system while preventing exhaust
gases from passing in opposite direction. Although similar in appearance, air check valve is not
interchangeable with pulse air valve.
The air pump is a belt-driven, positive displacement, vane type pump that 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. 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.
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.
The air silencer, mounted in engine compartment, is a combination silencer and filter. It is
connected to system by a flexible hose and 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
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 14 of 19
Operated by vacuum, this valve directs air pump output to exhaust manifold or downstream to
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
A combined air by-pass and air control valve is used on vehicles with a GVWR over 8500 pounds
and a 4.9L engine. Both valves are normally closed and come in either a bleed type or a non-bleed
type. Bleed type valves will have bleed percentage molded into plastic case.
By-pass valve routes thermactor air to either the exhaust system or to atmosphere. When air is
routed to exhaust system, control valve routes air either upstream to exhaust manifolds or
downstream to catalytic converter.
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
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 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 listed components are used on any one system. Component
usage depends on calibration of complete vehicle.
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.
Some engines incorporate a modified, ported EGR valve and a remote backpressure transducer.
Vacuum applied to EGR valve is modulated by exhaust backpressure transducer which bleeds off
some vacuum when backpressure is low. This provides EGR flow proportional to engine load.
Vacuum supplied to EGR transducer is controlled by an EGR control solenoid valve.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 15 of 19
EGR CONTROL SOLENOID VALVE
The solenoid supplies vacuum to EGR valve when de-energized and vents vacuum through its air
filter when energized. It also receives a signal from ECA according to EGR requirements. EGR
solenoid valve is mounted on firewall.
EGR BACKPRESSURE VARIABLE TRANSDUCER
A vacuum bleed hole, located inside transducer, vents EGR vacuum to atmosphere until there is
sufficient exhaust backpressure to close bleed hole. When bleed is closed, vacuum is routed to
EGR and normal operation begins.
The EGR backpressure variable transducer modulates EGR vacuum so exhaust gas flow is
proportionate to throttle opening. This is accomplished by sensing exhaust backpressure and
bleeding off some of 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 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 EGR when energized. The second valve is a vent valve which vents
EGR valve to atmosphere when de-energized. Both solenoid valves receive variable duty cycle
signals from ECA according to EGR requirements.
EGR VACUUM CONTROL VALVE FILTER
The EGR vacuum control valve filter is used to vent various emission control components to
EGR VACUUM REGULATOR (EVR)
The vacuum regulator is an electromagnetic device controlling vacuum 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 means increased vacuum to EGR.
EGR VALVE POSITION (EVP) SENSOR
This sensor is attached to EGR valve assembly and indicates EGR valve position to EEC system.
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
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 16 of 19
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 tank in a
carbon canister when engine is not running. During engine operation, fumes are drawn into engine
for burning during combustion process, purging canister.
Four basic components are used in evaporative emission system:
Activated carbon canister.
Vacuum-operated canister control valve.
Tank pressure control valve.
For specific component application and vacuum-hose routing, see VACUUM DIAGRAMS article
in this section.
Carbon canister storage is used for evaporative fuel control on all vehicles.
CANISTER PURGE VALVE
Vacuum operated purge valve controls flow of fuel vapors from carbon canister to engine.
CANISTER PURGE SOLENOID VALVE
This normally closed solenoid valve controls fuel vapor flow from canister to intake manifold. It is
opened or closed by a signal from electronic control assembly during various engine operating
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, permitting
room for thermal expansion.
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 (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/cm2 ). Under normal conditions, fill cap allows air to enter fuel tank as fuel is used
without allowing fuel vapors to escape.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 17 of 19
VACUUM CHECK VALVE
A vacuum check valve blocks airflow in one direction. It allows free airflow in other direction.
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 that allows only vapor and 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
POSITIVE CRANKCASE VENTILATION (PCV)
The PCV system relays on intake manifold vacuum to eliminate blow-by gases from crankcase. In
this system, manifold vacuum draws gases from crankcase, through a connective hose and into
combustion chamber. The PCV valve is positioned in connective hose through which blow-by
gasses flow on their way to combustion chamber.
The PCV valve opens and closes in direct relation to engine vacuum acting against spring pressure,
thus metering blow-by gas flow entering combustion chamber. During periods of high manifold
vacuum, such as at idle and deceleration, valve is almost completely closed, limiting flow of
gasses. During cruise speeds valve permits greatest flow of gasses.
Under conditions where excessively high amounts of blow-by gases are produced (such as worn
cylinders or rings), system is designed to allow excess gases to flow back through crankcase vent
hose and into air inlet to be consumed during normal combustion.
THERMOSTATIC AIR CLEANER
The thermostatic air cleaner controls air inlet temperature to improve combustion. System consists
of an air cleaner housing, fresh air intake, hot air stove pipe connected to exhaust manifold,
vacuum motor and intake air temperature sensor. Some systems incorporate a vacuum delay valve
to vacuum motor. Vacuum Motor
The vacuum motor opens and closes air delivery mode door located in air intake duct. When
vehicle is cold, vacuum is allowed to act upon motor, closing off door to cold (underhood) air. As
underhood temperature increases (as sensed by air cleaner temperature sensor), vacuum is bled off
and mode door closes. When engine reaches operating temperature, air mode door should be in
cold air delivery position, completely blocking off hot air delivery.
INTAKE AIR TEMPERATURE SENSOR
Sensor is located in air cleaner housing, directly in path of intake air. When intake air is cool, an
internal bleed built into sensor is closed. This allows full vacuum to pass through sensor and act
upon vacuum motor, pulling air mode door into full hot air delivery position.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 18 of 19
As underhood temperature increases, internal bleed of air temperature sensor begins to open,
causing a gradual bleeding off of manifold vacuum to vacuum motor. This causes vacuum motor
to slowly close air delivery mode door, blocking off hot air and allowing cooler underhood air to
pass into air cleaner.
A delay valve may be installed in vacuum line to vacuum motor. Valve allows immediate
application of vacuum to vacuum motor, while causing vacuum to gradually bleed off if manifold
vacuum source is removed.
When air temperature sensor bleed is closed, full manifold vacuum is delivered to vacuum motor.
If a need for wide open throttle arises, manifold vacuum becomes virtually nonexistent in intake
manifold. Delay valve prevents loss of vacuum to vacuum motor in event of wide open throttle
operation before vehicle is entirely warmed up.
NOTE: All systems have self-diagnostic capabilities. For information on
procedures for entering self-test modes and reading service codes,
see TESTS W/CODES article in this section.
ENGINE CONTROLS - THEORY & OPERATION -1990 Ford Bronco Page 19 of 19
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Saturday, December 10th, 2011 AT 10:54 PM