Acura Coupes and Sedans 1986-1993 Repair Guide

General Information


Programmed Fuel Injection (PGM-FI) System is a fully electronic microprocessor based engine management system. The Electronic Control Unit (ECU) is given responsibility for control of injector timing and duration, intake air control, ignition timing, cold start enrichment, fuel pump control, fuel cut-off, A/C compressor operation, alternator control as well as EGR function and canister purge cycles.

The ECU receives electric signals from many sensors and sources on and around the engine. The signals are processed against pre-programmed values; correct output signals from the ECU are determined by these calculations. The ECU contains additional memories, back-up and fail-safe functions as well as self diagnostic capabilities.

Diagnosis is generally based on symptom diagnosis and stored fault codes, if any. Testing always requires the use of the diagnostic charts in conjunction with the Acura test harness (pin-out box), a voltmeter and an ohmmeter.


In order to get fuel into the cylinders at the correct instant and in correct amount, the control system must perform various separate functions. The ECU (Electronic Control Unit), the heart of the PGM-FI, uses a microcomputer and consists of a CPU (Central Processing Unit), memories, and I/O (Input/Output) ports. Basic data stored in the memories are compared to the signals sent from the various sensors to provide the correct air/fuel mixture for all engine needs.

The unit contains memories for the basic discharge duration at various engine speeds and manifold pressures. The basic discharge duration, after being read out from the memory, is further modified by signals sent from various sensors to obtain the final injector duration. Other functions also include those listed below.

Starting Control

The fuel system must vary the air/fuel ratio to suit different operating requirements. For example, the mixture must be rich for starting. The memories also contain the injector duration values to be triggered by signals from the starter switch, engine speed and coolant temperature sensors, thereby providing extra fuel needed for starting.

Electronic Air Control

The ECU controls the EACV to maintain correct idle speed based on engine and accessories demand.

Ignition Timing Control

The ECU controls the basic ignition timing based on engine load, engine rpm, vehicle speed and coolant temperature.

Fuel Pump Control

When the engine is not running but the ignition is ON , electric current to the fuel pump is cut off, preventing the injectors from discharging fuel. Additionally, the fuel pump is energized for 2 seconds when the ignition is first switched ON . Once the engine is running, the pump is constantly energized.

Fuel Cut-Off Control

During deceleration with the throttle valve nearly closed, electric current to the injectors is cut off at engine speeds of approximately 1000-1500 rpm, depending on the engine. This contributes to improved fuel economy. Fuel cut-off also takes place when engine speed exceeds the red line or safe operation rpm limit, regardless of throttle position.

VTEC Control

The Integra GS-R and the 1993 Legend Coupe employ Variable Valve Timing and Lift Electronic Control (VTEC). The ECU is responsible for the control of this system as well as control of the intake manifold chamber and fuel pump discharge pressure.

Fail Safe

If the ECU notes the loss or change out-of-range of a sensor signal, the computer will ignore the faulty signal and substitute a fixed value in its place. This value may not necessarily be correct for the immediate driving situation, but will allow the engine to operate. For example, many ECU decisions are based on the engine coolant temperature. Should the coolant temperature sensor (TW sensor) signal be lost just after a cold start, the ECU will substitute the default value. This substitution value is based on an engine at normal temperature so vehicle performance will be affected until fully warmed up.

Back-up System

The ECU also tests itself while operating. If an abnormality occurs within the ECU, the system switches to a back-up circuit independent of the computing system. This system substitutes fixed values for all inputs and controls the injectors accordingly. Vehicle performance is usually reduced to minimal driveability. The back up function serves as a lifeboat to prevent the vehicle from being stranded in the event of ECU failure.


When a abnormality occurs, the ECU lights the engine warning light and stores a failure code in its memory. The fault code can be retrieved during diagnosis.



The CRANK or crankshaft angle sensor signal is used to determine the timing for fuel injection and ignition timing. It is also used to generate the engine RPM signal. The TDC sensor signal, found only on Integra, Vigor and 1986-1990 Legend, is used to determine ignition timing during start-up or when the CRANK signal is abnormal. The CYL sensor is used to detect the position of No. 1 cylinder; the signal is used to trigger the sequential fuel injection.

On Integra, Vigor and 1986-1990 Legend, these sensors are contained within the distributor. The signals are generated by the rotation of toothed wheels passing through pick-up coils. In some cases, the CYL sensor is incorporated with the other two. The function of these components does not change, but if any single sensor is defective, the entire distributor must be replaced as an assembly. The 1986-1990 Legend uses only a TDC sensor within the distributor.

For all Legend and Legend Coupe, the CRANK and CYL sensors are located behind camshaft pulleys on the left or front bank of cylinders.

Manifold Air Pressure (MAP) Sensor

See Figures 1 through 6

The sensor converts manifold air pressure readings into electrical voltage signals and sends them to the ECU. This information is used along with signals from the crank angle sensor to compute the basic injector duration.

For Legend and Legend Coupe, the MAP sensor is located within the emission control box. On Integras, the sensor is locate on a bracket on the firewall. The Vigor sensor is located on the left fender inner apron, just behind the air cleaner housing.

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Fig. Fig. 1: PGM-FI system component locations-1986-90 Integra

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Fig. Fig. 2: PGM-FI system component locations-1991-93 Integra

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Fig. Fig. 3: Component location and vacuum hose routing-Integra with automatic transaxle

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Fig. Fig. 4: Emission control box components-1986-90 Legend

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Fig. Fig. 5: Component location and vacuum hose routing-Vigor

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Fig. Fig. 6: Emission control box components-1991-93 Legend

Coolant Temperature (TW) Sensor

The sensor uses a temperature-dependent diode (thermistor) to measure differences in the coolant temperature. The basic injector duration is partially based on the signals sent from this sensor through the ECU. The resistance of the thermistor decreases with a rise in coolant temperature.

Intake Air Temperature (TA) Sensor

This device is also a thermistor and is placed in the intake manifold. It acts much like the water temperature sensor but with a reduced thermal capacity for for quicker response. The injector duration determined by the ECU is altered for different operating conditions by the signals sent from this sensor.

Throttle Angle Sensor

This sensor is a potentiometer which translates the position of the throttle plate to an electrical signal. The signal is near zero at idle and increases to just under 5 volts at wide-open throttle. The sensor is mounted to the side of the throttle body.

Oxygen Sensor

The oxygen sensor detects the oxygen content in the exhaust gas and sends an electrical signal to the ECU. The ECU uses this signal to control the fuel injector duration, maintaining the stoichiometric air/fuel ratio of 14.7:1. The sensor is located in the exhaust manifold; 6 cylinder engines use two sensors, one in each manifold.

This sensor is a hollow shaft of zirconia with a closed end. The inner and outer surfaces are plated with platinum, thus forming a platinum electrode. The inner surface or chamber is open to the atmosphere; the outer surface is exposed to the exhaust gas flow through the manifold.

Voltage is induced at the platinum electrode when there is any difference in oxygen concentration on the outside and inside. Operation of the device is dependent upon the changes in the induced voltage as the exhaust stream content changes.

Oxygen sensors only become efficient when operating at the proper temperature. All models except Integra use a heated oxygen sensor. The heater stabilizes the sensor's output and allows the sensor to heat quicker after the engine as been started.

EGR Valve Lift Sensor

The EGR lift sensor is found on all Acura vehicles except Integra with manual transaxle. The ECU controls the function of the EGR valve through the EGR control solenoid. The lift sensor translates the movement of the EGR shaft into an electric signal which is sent back to the ECU. The control unit compares this signal to pre-programmed values for optimum function. The sensor is located on the EGR valve.

Atmospheric Pressure (PA) Sensor

Used only on Integra and the 1986-1990 Legend, this unit converts atmospheric pressures into voltage signals and sends them to the ECU. The signals then modify the basic injector duration to compensate for changes in the atmospheric pressure.

The PA sensor is located inside the passenger compartment of the vehicle. On 1986-1990 Legend, it is located under the left dash, immediately behind the main relay. The Integra sensor is built into the ECU and is not accessible.

Vehicle Speed Sensor (VSS)

The signal from the vehicle speed sensor is used by the ECU as well as the cruise control and/or automatic transmission control units. For 1986-1990 Legends and all Integra, the signal is generated by a pulser unit incorporated into the back of the speedometer/odometer unit.

Vigor and 1991-93 Legend use an external sensor mounted on top of the power steering speed sensor in the engine compartment. The power steering speed sensor is gear-driven by the engine and controls the steering boost in relation to the speed of the vehicle. An electric pulser unit is mounted on top of this unit and is coupled to it by a small shaft. The motion from the lower unit is transferred to the upper unit; the resultant electrical signal is sent to the appropriate control units.

Starter Signal

The START position on the ignition switch causes a signal to be sent to the ECU. During cranking, the ECU will increase the amount of fuel injected into the manifold according to the engine temperature. The amount of fuel injected is gradually reduced when the starter switch is released.

Alternator FR Signal

The ECU receives a signal from the alternator when the system is charging.

Air Conditioning Signal

The load on the engine increases when the compressor is engaged; the ECU must increase the idle as necessary. In some cases, the ECU will limit A/C operation, cutting it off through the A/C control relay in cases of wide-open throttle or preventing it from engaging in cold engine conditions. If the vehicle has a separate control unit for the air conditioning, the two controllers exchange information regarding system status.

Battery Voltage (IGN 1 )

A 12 volt signal controlled by the IGN 1 (ON or RUN) position of the ignition switch is sent to the ECU.

Automatic Transaxle Shift Position Signal Manual Transaxle Neutral and Clutch Signals

This signal is sent to the ECU through the neutral safety switch. The control unit will not allow the engine to start if the shift selector is not in N or PARK .

1991-93 Legend vehicles with manual transaxle employ switches at the clutch pedal and the shifter. The signals from the switches are used by the ECU to determine that the vehicle is in N and/or the clutch is disengaged.

Power Steering Fluid Pressure Switch

This switch is used on all vehicles. When the power steering fluid pressure exceeds a predetermined level, the ECU is signaled by a pressure operated switch. If the idle is below a predetermined rpm, as during a parking maneuver, the control unit will increase the idle speed of the engine to compensate for the additional load.

Electric Load Detector (ELD)

This sensor is used on Vigor vehicles; it is located in the underhood fuse and relay box. It detects the present of moderate to large electrical current in the system and sends a signal to the ECU.

The ECU will control the idle speed to compensate for the additional load of headlights, rear defroster, heater fan or similar heavy electrical loads.

The ELD unit is integral with the fuse box in which it is mounted; should the sensor fail, the fuse box assembly must be replaced.

Brake Switch Signal

This signal is sent to the ECU from the brake switch.

Knock Sensors

See Figures 7 and 8

These sensors are used on Legend and Vigor engines to detect detonation or engine knock. When this condition is detected, the ECU retards the ignition timing to compensate. The knock sensors are referred to as left and right on Legend and No.1 or No.2 on Vigor.

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Fig. Fig. 7: PGM-FI system component locations-1991-93 Legend

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Fig. Fig. 8: Knock sensor locations-Legend and Vigor

Ignition Timing Adjuster

This component allows the electronic ignition advance to be set to 15 degrees BTDC. The sensor provides signals to the ECU; it is a solid-state electrical component having no mechanical parts. On Legend the sensor is located within the emission control box; on Vigor, it is located on the left fender apron, behind the air filter housing. The adjuster is covered or concealed by a cover held by 2 rivets.

VTEC Oil Pressure Switch Signals

For each set of two valves-intake or exhaust-in each cylinder, there are three corresponding lobes on the camshaft. The 2 outboard lobes have a significantly different profile from the center lobe. The center or third lobe is not used during part-throttle or low load conditions.

During high speed or high load operation, the ECU signals a spool valve which delivers engine oil to small pistons in the rocker arms. The pressure causes the pistons to move outward, locking all 3 rockers together. Once locked, the rockers are forced to follow the center cam lobe, thus changing the performance of the engine. The change to high lift occurs in approximately 0.1 second and is virtually undetectable by the operator.

Once engine operation returns to part throttle or low load, the additional oil pressure is cut off; the pistons retract and the rockers are controlled by the outer lobes of the camshaft.

The ECU receives signals from both spool valves and both oil pressure switches indicating system status. The ECU will set one of 4 possible fault codes if any out-of-range signals occur.


The prime mission of the ECU is to maintain good driveability and proper emission levels under all operating conditions. The ECU interprets and compares the varied input signals from the sensors and uses the resulting calculations to control electrical outputs. In many cases, the circuit control provided by the ECU closes the ground rather than the power side of the circuit.

Fuel Injectors

All models use fuel injectors in the intake manifold (port injection) controlled individually by the ECU.

The ECU controls the system electrically, energizing the magnet within each injector according to engine speed, manifold pressure and other variables. The ECU can recognize an injector which is not functioning electrically and set a fault code. Physical problems, such as dirt lodged within the injector or polluted fuel cannot be detected because they are not electrical in nature. When tracing an apparently fuel-related driveability problem, the electrical and physical condition of the injector and condition of the fuel must be considered.

Main Relay

Although part of the main relay is energized by the ignition switch, the ECU controls ground to another part of the relay. When the ECU receives information that the the ignition switch is ON and a either a cranking or engine rpm signal is present, the ECU closes the relay circuit for the fuel pump system.


The dashboard warning lamp will be lit by the ECU for 2 seconds during the initial ignition ON sequence. After this test sequence has ended, the ECU will illuminate the lamp anytime an electrical fault is noted within the system.

When a fault is detected, the dash warning lamp will stay on for the duration of the ignition ON cycle. Once the ignition is switched OFF , then recycled, the dashboard warning lamp will not light unless the original fault or some other (newer) fault is still present. Although the lamp does not re-light, the stored code for the original fault will remain in memory with the ignition OFF . For this reason, diagnosis should be based on the presence of fault codes, not the illumination of the warning lamp.

Electronic Air Control Valve (EACV)

The EACV controls the amount of air bypassing the throttle plate in response to signals from the ECU. During conditions of cold engine operation, A/C compressor engaged, automatic transaxle in gear, or alternator charging, the EACV is energized to control the idle speed correctly. Function of the EACV is controlled by the ECU proportional to engine coolant temperature. Integra engines incorporate an additional vacuum operated air boost valve to provide more intake air during cranking.

All vehicles except Integra with manual transaxle also employ a fast idle valve. The Integra with automatic transaxle controls this valve electrically through the ECU; all others are controlled by a temperature-sensitive thermowax valve reacting to the coolant being circulated through the valve body. On a cold engine, more air is admitted into the engine, raising the idle.

Note that in all cases, the external idle adjustment screw acts on an air-bleed passage, not on the throttle mechanism.

Air Conditioning Compressor Clutch Relay

When the engine control unit receives a request for air conditioning from either the A/C switch or the compressor control unit, the signal to the compressor clutch relay is momentarily delayed until the ECU can enrich the air/fuel mixture. This allows a smoother engagement of the compressor with less effect on driveability.

Purge Cut-off Solenoid Valve

When the coolant temperature is below a predetermined level, the ECU controls the function of the purge cut-off solenoid or purge control solenoid. This eliminates vacuum to the purge control valve. The exact coolant temperature limit or engagement point is different for each engine but is approximately 150-160 °F.


Although part of the ignition system, the igniter is triggered by a signal from the ECU. The timing of the spark, including advance, is handled by the engine controller.

EGR Control Solenoid Valve (EGR CSV)

When EGR function is necessary to provide control of NOx emissions, the ECU grounds the control solenoid valve. The solenoid allows regulated vacuum to pass to the EGR, thus precisely controlling EGR function. The system is used on all vehicles except Integra with manual transaxle.

Pressure Regulator Cut-off Solenoid Valve

Used on Integra and Legend, this valve is activated by the ECU during hot restarts if the intake air and/or coolant temperature are above preset limits. When energized, the valve cuts off vacuum to the fuel pressure regulator for 60-80 seconds, allowing the pressure to build within the fuel rail. This reduces the occurrence of fuel boiling and improves hot-restart characteristics.

Bypass Control Solenoid Valves (BPCSV)Chamber Volume Control Solenoid Valve

Used only on the Legend and Vigor engines, this system provides 2 or even 3 air intake paths within the manifold. By selectively switching between them, both high torque at low rpm and high power at high rpm can be maintained.

On 1990 and earlier Legend models, the BPCS valves are designated A and B. When engine speed is below 3200 rpm, the valves are activated by the ECU and intake air is flowed through the longer intake path. Above 3800 rpm, the valves are de-energized and the shorter port is opened.

1991-1993 Legends identify the valves as Bypass Low Control Solenoid and Bypass High Control Solenoid valves. When the engine speed is below 3100 rpm, both valves are energized by the ECU, causing the airflow to travel through an additional chamber. Between 3200 and 3800 rpm, the Bypass Low valve is deactivated, allowing air to flow through a shorter intake path. Above 3900 rpm, both valves are turned off, allowing air to flow through the shortest intake path.

The 5-cylinder Vigor engines use one BPCSV directing air into the longer intake path below 4900 rpm and through the shorter intakes above 4900 rpm. When the engine rpm is below 4800, the chamber is opened, creating a longer intake path for the air. Above this rpm, the control solenoid is de-energized by the ECU and the air is forced through a shorter intake path.

Fan Operation

On all vehicles except Integra and 1990 and earlier Legend, the ECU either communicates with the fan control unit or timer or controls the function of the cooling fan relays.

Resonator Control Solenoid Valve

Used only on the 1986-1990 Legend, this ECU-operated valve allows selective control of airflow into the intake air resonator chamber. The solenoid is placed in the vacuum line controlling the resonator diaphragm. The system decreases intake noise during operation below 3800 rpm.

Fuel Cut-Off Function

All ECUs are designed to shut off current to the fuel injectors when the vehicle is decelerating with the throttle closed. The cut off point varies with each engine , generally in the vicinity of 1000 rpm.

Additionally, the ECU will shut off current to the fuel injectors when the safe rpm or red-line is exceeded. Again, the cut-off point varies with each engine.

Air Suction Control Solenoid Valve

This valve is used on 1990-93 Legends. The system is designed to reduce exhaust emissions by introducing fresh air from the air cleaner into the exhaust manifold. When the solenoid valve is activated by the ECU, manifold vacuum raises the diaphragm valve of the air suction valve. The pulses in the exhaust stream cause a reed valve-within the air suction valve-to admit fresh air.

Ne Signal

The output signal to the tachometer connector on 1991-93 Legends is controlled by the ECU.

Oxygen Sensor Heaters

Operation of the heating elements used in the oxygen sensors for 1991-93 Legend and Vigor is controlled by the ECU.

Alternator Control

The Vigor ECU controls the voltage output of the alternator. Output is varied according to electrical load and drive mode, reducing engine load and improving fuel economy.

VTEC Spool Valves

The engine control unit oversees the operation of the spool valves; they are energized or released depending on the engine rpm.


Fuel Pump

The fuel pump is installed inside the fuel tank, thereby saving space and simplifying the fuel line system.

The fuel pump is comprised of a DC motor, a circumference flow pump, a relief valve for protecting the fuel line systems, a check valve for retaining residual line pressure after the pump stops, an inlet port, and a discharge port. If the flow of fuel is obstructed at the discharge side of the pump, an internal relief valve will open to bypass the fuel to the inlet port.

The pump assembly consists of the impeller (driven by the motor), the pump casing (which forms the pumping chamber), and cover of the pump.

Pressure Regulator

The fuel pressure regulator maintains a constant fuel pressure at the injectors. The regulator is located on one end of the fuel rail. The spring chamber of the pressure regulator is connected to intake manifold vacuum to constantly maintain the fuel pressure at a specified pressure higher than the pressure in the manifold. When the difference between the fuel pressure and manifold pressure exceeds 36 psi (2.55 kg/cm) on Integra, Legend and Vigor or 50 psi (3.5 kg/cm), the diaphragm is pushed upward, and the excess fuel is fed back into the fuel tank through the return line.


The injector is of the solenoid-actuated constant-stroke pintle type consisting of a solenoid, plunger, needle valve and housing. When current is applied to the solenoid coil, the valve lifts up and pressurized fuel fills the inside of the injector and is injected near the intake valve. Because the needle valve lift height and the fuel pressure are constant, the amount of fuel injected is determined by the length of time (duration) that the injector is open. The injector is sealed by an O-ring and seal ring at the top and bottom. These seals also reduce operating noise.


See Figure 9

All Acura fuel injected engines employ a resistor in series between the power supply and the injector. The injector timing, which controls the opening and closing intervals, must be very accurate since it dictates the air/fuel mixture ratio. The injector must also be durable. For the best possible injector response, it is necessary to shorten the current rise time when voltage is applied to the injector coil. Flow of current in the coil is restricted by a resistor installed in series between the electric power source and the injector coil.

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Fig. Fig. 9: A resistor is used in the injector circuit on all models-6 cylinder shown

Main Relay

See Figures 10 and 11

The relay is found at the extreme left side of the dashboard, above and behind the fusebox on Integra and Legend or behind the left front kick panel on Vigor.

The main relay unit contains two separate relays within one case.The first relay is energized whenever the ignition is switched ON . Power is supplied to ECU, the injectors and to the second relay. The second relay circuit supplies power to the fuel pump. It is engaged for 2 seconds any time the ignition is switched ON and is always engaged when the engine is running.

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Fig. Fig. 10: Main relay circuit schematic

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Fig. Fig. 11: Main relay location-Legend and Integra, Vigor location is behind the left kick panel