Honda Accord/Prelude 1984-1995 Repair Guide

Programmed Fuel Injection (PGM-FI) System-1990-95 Models

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GENERAL INFORMATION



The 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.

Troubleshooting is generally based on symptom diagnosis and stored fault codes, if any. Testing requires the use of the diagnostic charts in conjunction with the Honda test harness (pin-out box) and a digital volt/ohmmeter. On the 1995 Accord V-6, a scan tool is also needed to extract the trouble codes.

As useful as the tests found in this section are, the first step in repair or service to engine management systems is still to gain as much information as possible about the problem; when and under what conditions it occurs. At highway speed- At idle only- Only under heavy load or hard acceleration- Wet weather- Defining the problem will eliminate many systems from consideration and possibly point to the affected system. Before diving into an extended electrical diagnosis, take the time to review the basics. Check every vacuum line for cracks or leaks. Check every electrical connector for corrosion or loose pins. Quite often, simply unplugging and reconnecting a connector will break up corrosion on the pins and restore the circuit. Watch out for poor grounds, particularly if the car has experienced major bodywork.

SERVICE PRECAUTIONS





Do not operate the fuel pump when the fuel lines and tank are empty, or when the fuel pump has been removed from the tank.
 
Do not reuse fuel hose clamps.
 
The washer(s) below any fuel system bolt (banjo fittings, service bolt, fuel filter, etc.) must be replaced whenever the bolt is loosened. Do not reuse the washers; a high-pressure fuel leak may result.
 
Make sure all ECU harness connectors are fastened securely. A poor connection can cause a high voltage surge and result in damage to integrated circuits.
 
Keep all ECU parts and harnesses dry during service. Protect the ECU and all solid-state components from rough handling or extremes of temperature.
 
Before attempting to remove any parts, turn the ignition switch OFF and disconnect the battery ground cable.
 
Always use a 12 volt battery as a power source, never a booster or high-voltage charging unit.
 
Do not disconnect the battery cables with the engine running.
 
Do not unplug any wiring connector with the engine running or the ignition ON , unless specifically instructed to do so.
 
Do not depress the accelerator pedal when starting.
 
Do not rev up the engine immediately after starting or just prior to shutdown.
 
Do not apply battery power directly to injectors.
 
Whenever possible, use a flashlight or a shatter-proof drop light. The bulbs used in standard drop lights may shatter on impact, creating a spark which may ignite fuel vapors.
 
Keep all open flame and smoking material out of the area.
 
Use a shop cloth or similar to catch fuel when opening the fuel system.
 
Relieve fuel system pressure before servicing.
 
Always use eye or full-face protection when working around fuel lines, fittings or components.
 
Always keep a dry chemical (class B-C) fire extinguisher near the area.
 

TESTING



Refer to the applicable troubleshooting flow chart (found in this section) for the model and year of your vehicle.

COMPONENT DESCRIPTION



Electronic Control Unit (ECU)

Functions of the ECU include:



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. Data stored in the unit's memory also contains 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.
 
Injector Control-The ECU regulates the injector durations at various engine speeds and loads by precisely controlling the amount of time the injectors are open.
 
Electronic Air Control-The ECU regulates the EACV to maintain correct idle speed based on engine and accessories demand.
 
Ignition Timing Control-The ECU regulates 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
 
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 1300-1500 rpm, contributing to improved fuel economy. Fuel cut-off also takes place when engine speed exceeds the red-line or safe operation rpm limit.
 
Fail Safe-If the ECU notes the loss of or an out-of-range 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 may 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 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.
 
Self-Diagnosis-When an abnormality occurs, the ECU triggers the check engine lamp and stores a failure code in erasable memory. The ECU will display the code any time the ignition is turned ON .
 

CRANK, TDC and CYL Sensors

The CRANK or crankshaft angle sensor signal is used to determine the fuel injection and ignition timing. It is also used to generate the engine speed signal. The TDC sensor signal 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.

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.

Manifold Air Pressure (MAP) Sensor

This 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.

Coolant Temperature (TW) Sensor

This sensor uses a temperature-dependent resistor (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 coolant temperature sensor, but with a reduced thermal capacity 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 then uses this signal to control fuel injector duration, maintaining the stoichiometric air/fuel ratio of 14.7:1. The sensor is located either in the exhaust manifold or in the exhaust piping ahead of the catalytic converter. Some models use 2 sensors.

Oxygen sensors only become efficient when operating at the proper temperature. Some models 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 ECU regulates 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

This sensor 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, usually next to the ECU. On some models, it is contained within the ECU.

Vehicle Speed Sensor (VSS)

The signal from the vehicle speed sensor is used by the ECU as well as the cruise control and automatic transaxle control units.

Accords and Preludes 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 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 according to the signals received from the engine temperature sensors.

Alternator FR Signal

The ECU receives a signal from the alternator to control the system 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.

Automatic Transaxle Shift Position Signal

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 NEUTRAL or PARK.

Power Steering Fluid Pressure Switch

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 located in the underhood fuse and relay box. It detects the presence 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.

 
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