Ford Mustang 1989-1993 Repair Guide

Description and Operation


See Figures 1 through 5

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Fig. Fig. 1: General layout of the fuel injection system on the 2.3L engine

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Fig. Fig. 2: Most of the fuel related parts are accesible from the top of the engine as shown on this 5.0L engine

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Fig. Fig. 3: Fuel supply components-5.0L engine

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Fig. Fig. 4: The inertia switch is reset by pushing the buttom mounted on top. The switch is located behind the rear panel in the truck where the lights are

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Fig. Fig. 5: When mounting the inertia switch, the button must point up

The Multi-point (EFI) and Sequential (SEFI) fuel injection sub-systems include a high pressure, in-line electric fuel pump, a low pressure, tank-mounted fuel pump, fuel charging manifold, pressure regulator, fuel filter and both solid and flexible fuel lines. The fuel charging manifold includes 4 or 8 electronically-controlled fuel injectors, each mounted directly above an intake port in the lower intake manifold. On the 4-cylinder EFI system, all injectors are energized simultaneously and spray once every crankshaft revolution, delivering a predetermined quantity of fuel into the intake air stream. On the V8 EFI engines, the injectors are energized in 2 banks of four, once each crankshaft revolution. On the SEFI system, each injector fires once every crankshaft revolution, in sequence with the engine firing order.

The fuel pressure regulator maintains a constant pressure drop across the injector nozzles. The regulator is referenced to intake manifold vacuum and is connected in parallel to the fuel injectors; it is positioned on the far end of the fuel rail. Any excess fuel supplied by the fuel pump passes through the regulator and is returned to the fuel tank via a return line.

The pressure regulator reduces fuel pressure to 39-40 psi under normal operating conditions. At idle or high manifold vacuum condition, fuel pressure is further reduced to approximately 30 psi.

The fuel pressure regulator is a diaphragm-operated relief valve, in which the inside of the diaphragm senses fuel pressure and the other side senses manifold vacuum. Normal fuel pressure is established by a spring preload applied to the diaphragm. control of the fuel system is maintained through the EEC-IV control unit, although electrical power is routed through the fuel pump relay and an inertia switch. The fuel pump relay is normally located on a bracket somewhere above the Electronic Control Assembly (ECA) and the inertia switch is located in the trunk. The in-line fuel pump is usually mounted on a bracket at the fuel tank, or on a frame rail. Tank-mounted pumps can be either high- or low-pressure, depending on the model.

The inertia switch opens the power circuit to the fuel pump in the event of a collision. Once tripped, the switch must be reset manually by pushing the reset button on the assembly.

Check that the inertia switch is reset before diagnosing power supply problems to the fuel pump.


Fuel Injectors

See Figures 6, 7 and 8

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Fig. Fig. 6: The cross section of a fuel injector shows the precision construction that allows the injector to meter such fine quantities of fuel

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Fig. Fig. 7: Fuel supply manifold-2.3L engine

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Fig. Fig. 8: Fuel supply manifold-5.0L engine

The fuel injectors used with the EFI and SEFI system are electro-mechanical (solenoid) type, designed to meter and atomize fuel delivered to the intake ports of the engine. The injectors are mounted in the lower intake manifold and positioned so that their spray nozzles direct the fuel charge in front of the intake valves. The injector body consists of a solenoid-actuated pintle and needle-valve assembly.The control unit sends an electrical impulse that activates the solenoid, causing the pintle to move inward off the seat and allow the fuel to flow. The amount of fuel delivered is controlled by the length of time the injector is energized (pulse width), since the fuel flow orifice is fixed and the fuel pressure drop across the injector tip is constant. Correct atomization is achieved by contouring the pintle at the point where the fuel enters the pintle chamber.

Exercise care when handling fuel injectors during service. Be careful not to lose the pintle cap and always replace O-rings to assure a tight seal. Never apply direct battery voltage to test a fuel injector.

The injectors receive high-pressure fuel from the fuel manifold (fuel rail) assembly. The complete assembly includes a single, pre-formed tube with four or eight connectors, the mounting flange for the pressure regulator, mounting attachments to locate the manifold and provide the fuel injector retainers and a Schrader® quick-disconnect fitting used to perform fuel pressure tests.

The fuel manifold is normally removed with the fuel injectors and pressure regulator attached. Fuel injector electrical connectors are plastic and have locking tabs that must be released when disconnecting the wiring harness.

Throttle Air Bypass Valve

See Figure 9

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Fig. Fig. 9: The air bypass valve allows a measured amount of air past the throttle plate to control the idle speed

The throttle air bypass valve is an electro-mechanical (solenoid) device whose operation is controlled by the EEC-IV control unit. A variable air metering valve controls both cold and warm idle air flow in response to commands from the control unit. The valve operates by passing a regulated amount of air around the throttle plate; the higher the voltage signal from the control unit, the more air is bypassed through the valve. In this manner, additional air can be added to the fuel mixture without moving the throttle plate. At curb idle, the valve provides smooth idle for various engine coolant temperatures, compensates for air conditioning load and compensates for transaxle load and no-load conditions. The valve also provides fast idle for start-up, replacing the fast idle cam, throttle kicker and anti-dieseling solenoid common to previous models.

There are no curb idle or fast idle adjustments. As in curb idle operation, the fast idle speed is proportional to engine coolant temperature. Fast idle kick-down will occur when the throttle is kicked. A time-out feature in the ECA will also automatically kick down fast idle to curb idle after approximately 15-25 seconds after coolant has reached approximately 160° F (71° C). The signal duty cycle from the ECA to the valve will be at 100% (maximum current) during the crank to provide maximum air flow to allow no-touch starting at any time (engine hot or cold).

Electronic Engine Control

See Figures 10 through 14

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Fig. Fig. 10: Remove the lower trim mounting screw

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Fig. Fig. 11: Pry out the plastic fasteners on the kick panel

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Fig. Fig. 12: Remove the kick panel

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Fig. Fig. 13: Remove the insulation

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Fig. Fig. 14: The computer is hidden in the passenger side kick panel

The electronic engine control sub-system consists of the Electronic Control Assembly (ECA) and various sensors and actuators. The ECA, also known as an Electronic Control Module (ECM), reads inputs from engine sensors, then outputs a voltage signal to various components (actuators) to control engine functions. The period of time that the injectors are energized (ON time or pulse width) determines the amount of fuel delivered to each cylinder. The longer the pulse width, the richer the fuel mixture.

The operating reference voltage (VREF) between the ECA and its sensors and actuators is 5 volts. This allows the components to work during engine cranking, when battery voltage drops.

In order for the ECA to properly control engine operation, it must first receive current status reports on various operating conditions. The control unit constantly monitors crankshaft position, throttle plate position, engine coolant temperature, exhaust gas oxygen level, air intake volume and temperature, air conditioning function (ON/OFF), spark knock and barometric pressure.

Universal Distributor

The primary function of the TFI-IV ignition system universal distributor is to direct the high secondary voltage to the spark plugs. In addition, it supplies crankshaft position and frequency information to the ECA using a Profile Ignition Pickup (PIP) sensor in place of the magnetic pickup or the crankshaft position sensor used on other models. This distributor does not have any mechanical or vacuum advance. The universal distributor assembly is adjustable for resetting base timing, if required, by disconnecting the SPOUT connector.

The PIP replaces the crankshaft position sensor found on other EEC-IV models.

The PIP sensor has an armature with four windows and four metal tabs that rotate past the stator assembly (Hall-effect switch). When a metal tab enters the stator assembly, a positive signal is sent to the ECA , indicating the 10°BTDC crankshaft position. The ECA calculates the precise time to energize the spark output signal to the TFI module. When the TFI module receives the spark output signal, it shuts off the coil primary current and the collapsing field energizes the secondary output.

Mis-adjustment of the base timing affects the spark advance in the same manner as a conventional, solid-state ignition system.

Thick Film Ignition (TFI-IV) Module

The TFI-IV module has six connector pins at the engine wiring harness that supply the following signals:

Ignition switch in RUN .
Engine cranking
PIP (crankshaft position to ECA)
Spark advance (from ECA)
Internal ground from the ECA to the distributor.

The TFI-IV module supplies the spark to the distributor through the ignition coil and calculates the duration. It receives its control signal from the ECA (spark output).

Distributorless Ignition System (DIS)

The DIS systems eliminate the need for a distributor by using multiple coils, which fire two spark plugs at the same time. This system uses either a dual-function crank sensor or a crank sensor and cam sensor. The cam sensor provides the cylinder identification (CID) signal, used to choose which coil to fire. The crank sensor provides a PIP signal for spark timing. The dual-function crank sensor provides both PIP and CID signals to the DIS module.

Distributorless Ignition System Module

The DIS module controls coil firing from the ECA commands similar to the way the TFI-IV module does. The DIS module functions include:

Selection of coils
Driving of coils
Driving of tachometer
PIP (crankshaft position to ECA)
CID (cylinder identification)
LOS (limited operating system base timing)

Throttle Position (TP) Sensor

See Figure 15

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Fig. Fig. 15: The throttle position sensor mounts on the end of the throttle shaft on all engines

The TP sensor is mounted on the throttle body. This sensor provides the ECA with a signal that indicates the angle of the throttle plate. The sensor output signal uses the 5-volt reference voltage previously described. From this input, the ECA controls:

  1. Operating modes, which are Wide-Open Throttle (WOT), Part-Throttle (PT) and Closed-Throttle (CT).
  3. Fuel enrichment at WOT.
  5. Additional spark advance at WOT.
  7. EGR cut-off during WOT, deceleration and idle.
  9. Air conditioning cut-off at WOT (30 second maximum)
  11. Cold start kick-down
  13. Fuel cut off during deceleration.
  15. WOT de-choke during cranking or starting.

On the EEC-IV system, the TP sensor signal to the ECA only changes the spark timing during the WOT mode. As the throttle plate rotates, the TP sensor varies its voltage output. As the throttle plate moves from a closed position to a WOT position, the voltage output of the TP sensor will change from a low voltage (approximately 1.0 volt) to a high voltage (approximately 4.75 volts) If the TP sensor used is not adjustable it must be replaced if it is out of specification. The EEC-IV programming compensates for differences between sensors.

Engine Coolant Temperature (ECT) Sensor

See Figure 16

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Fig. Fig. 16: The coolant temperature sensor will be placed in a coolant passage such as it is on this 2.3L engine. On the 5.0L engine it is in the heater water supply tube on the intake manifold

The ECT sensor is located either in the heater supply tube, or in the lower intake manifold. The ECT sensor is a thermistor which changes resistance as a function of temperature. The sensor detects the temperature of the engine coolant and provides a corresponding signal to the ECA. From this signal, the ECA will modify the air/fuel ratio (mixture), idle speed, spark advance, EGR and canister purge control. When the engine coolant is cold, the ECT signal causes the ECA to provide enrichment to the air/fuel ratio for good cold-engine performance.

Exhaust Gas Oxygen (EGO) Sensor

See Figure 17

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Fig. Fig. 17: The exhaust gas oxygen sensor reacts to the level of oxygen in the exhaust stream allowing the fuel injection to adjust the mixture

The EGO or HEGO sensor on the EEC-IV system is mounted in its own mounting boss, located between the two downstream tubes in the header near the exhaust system. The EGO sensor works between 0 and 1 volt output, depending on the presence (lean) or absence (rich) of oxygen in the exhaust gas. A voltage reading greater than 0.6 volts indicates a rich air/fuel ratio, while a reading of less than 0.4 volts indicates a lean mixture.

Never apply voltage to the EGO sensor; doing so could destroy the sensor's calibration. This includes the use of an ohmmeter. Before connecting and using a voltmeter, make sure it has a high input resistance (at least 10 megohms) and that it is set on the proper resistance range. Any attempt to use a powered voltmeter to measure the EGO voltage output directly will damage or destroy the sensor.

Operation of the sensor is the same previous models. A difference that should be noted is that the rubber protective cap used on to of the sensor on earlier models has been replaced with a metal cap. In addition, later model sensors incorporate a heating element (HEGO) to bring the sensor up to operating temperature more quickly and keep it there during extended idle periods to prevent the sensor from cooling off and placing the system into open-loop operation.

Vane Meter

The vane meter is actually two sensors in one assembly-a Vane Air Flow (VAF) sensor and a Fane Air Temperature (VAT) sensor. This meter measures air flow to the engine and the temperature of the air stream. The vane meter is located either behind or under the air cleaner.

Air flow through the body moves a vane mounted on a pivot pin. The more air flowing through the meter, the further the vane rotates about the pivot pin. The air vane pivot pin is connected to a variable resistor (potentiometer) on top of the assembly. The vane meter uses the 5-volt reference signal. The output of the potentiometer to the ECA varies between 0 volts and VREF (5 volts), depending on the volume of air flowing through the sensor. A higher volume of air will produce a higher voltage output.

The volume of air measured through the meter has to be converted into an air-mass value. The mass of the specific volume of air varies with pressure and temperature. To compensate for these variables, a temperature sensor in front of the vane measures incoming air temperature. The ECA uses the air temperature and a programmed pressure value to convert the VAF signal into a mass airflow value. This value is used to calculate the fuel flow necessary for the optimum air/fuel ratio. The VAT also affects spark timing as a function of air temperature.

Air Conditioning Clutch Compressor (ACC) Signal

Any time battery voltage is applied to the A/C clutch, the same signal is also applied to the ECA. The ECA then maintains the engine idle speed with the throttle air bypass valve control solenoid to compensate for the added load created by the A/C clutch operation. Shutting down the air conditioning will have a reverse effect. The ECA will maintain the engine idle speed at 850-950 rpm.

Knock Sensor

The knock sensor is used to detect detonation. In situations of excessive knock, the ECA receives a signal from this sensor and retards the spark accordingly. It is mounted in the lower intake manifold at the rear of the engine.

Barometric (BAP) Sensor

See Figure 18

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Fig. Fig. 18: Barometric and manifold pressure sensor look similar

The barometric pressure sensor is used to compensate for altitude variations. From this signal, the ECA modifies the air/fuel ratio, spark timing, idle speed and EGR flow. The barometric sensor is a design that produces a frequency based on atmospheric pressure (altitude). The sensor is mounted on the right fender apron.

Manifold Absolute Pressure (MAP) Sensor

The MAP sensor measures manifold vacuum and outputs a variable frequency. This gives the ECA information on engine load. It replaces the BAP sensor by providing the ECA updated barometric pressure readings during Key ON/Engine OFF and wide-open throttle. The MAP sensor output is used by the ECA to control spark advance, EGR flow and air/fuel ratio.

EGR Shut-off Solenoid

The electrical signal to the EGR shut-off solenoid is controlled by the ECA. The signal is either ON or OFF. It is OFF during cold start, closed throttle or WOT. It is ON at all other times.

The canister purge valve is controlled by vacuum from the EGR solenoid. The purge valve is a standard valve and operates the same as in previous systems.

The solenoid is the same as the EGR control solenoid used on previous EEC systems. It is usually mounted on the left side of the firewall, in the engine compartment. The solenoid is normally closed, and the control vacuum from the solenoid is applied to the EGR valve.