Electronic control system (Honda Accord 3)

Control system

To ensure that fuel is supplied to the engine cylinders at the right time and in the right amount, the control system for this process must be able to perform a number of individual functions. The brain of this system is the electronic control module ECU (on-board computer) based on an 8-bit processor. The module consists of a CPU (data processing unit), a memory device and input/output ports. Based on the basic data stored in the computer's memory and taking into account the actual signals coming from various sensors, the module calculates the correct fuel mixture that the engine may require.

Electronic Control Unit (ECU)

The main, first task of the module is to control the process of fuel supply to the engine. Maintaining precise compliance of the supplied fuel with the engine's needs reduces the level of harmful gases in the exhaust to a minimum, and maintains maximum efficiency. The ECU can control the fuel injectors only by exciting them. Since the fuel pressure in the injector is maintained constant, and the injector size is unchanged, the exact amount of fuel supplied to the combustion chamber is regulated by the duration of the injection and its time.



The module stores in its memory the values of standard injection durations for different engine speeds and vacuum manifold pressures. The injection duration value read from the memory is specified by signals coming from various sensors. Other functions of the control system include:

• Engine start control: The fuel mixture must be changed according to different operating modes. For example, at the moment of start-up the mixture must be enriched. Therefore, the injection duration read from the computer memory is specified by signals about the position of the starter, engine speed, coolant temperature, which ensures the supply of additional fuel at the moment of engine start-up.
• Fuel Pump Control: When the engine speed drops below a certain limit, the power supply to the fuel pump is cut off, which stops fuel injection from the injectors.
• Fuel cut-off function: During braking, with the throttle valve almost completely closed, and when the engine speed exceeds 900 rpm, the power supply to the injectors is interrupted, which increases the efficiency of the engine. The fuel cut-off function also operates when the engine speed exceeds the maximum safe value (red line). This occurs regardless of the position of the throttle valve.
• Safety System: A special system for maintaining uninterrupted operation monitors all sensors and the correct operation of the ECU. It ensures safe driving, even if several sensors fail simultaneously or the ECU malfunctions. In the event of failure of the ECU itself, all control signals are replaced by a fixed set so that the car can safely reach the parking lot. When this function is activated, the driving characteristics of the car are significantly reduced.

Crankshaft angle sensor

The ignition distributor and its sensors are designed as a single module to reduce size and weight. It consists of two rotors - BMT (TDC) and CIL (SUD - and two sensors for each of them. The rotors are connected to the camshaft and rotate with it as a single unit. The CYL sensor measures the position of cylinder No.1 as the starting point of the sequence, and the TDC sensor determines the ignition timing of each cylinder. The TDC sensor also measures the engine speed, based on which the initial fuel injection duration is determined. After that, the injection duration is specified for specific driving conditions.

Manifold Pressure Sensor (MAP Sensor)

The sensor converts the measured manifold air pressure into an electrical voltage signal that is sent to the ECU. This data, together with the ignition timing sensor data, is used to read the initial fuel injection duration from the ECU memory.

Atmospheric pressure sensor (PA sensor)

Like the MAP sensor, this sensor converts the measured atmospheric pressure into a voltage signal that is sent to the ECU. This signal is used to adjust the initial injection duration with the atmospheric pressure compensation circuit.

Coolant Temperature Sensor (TW Sensor)

This sensor measures coolant temperature variations using a temperature sensor (thermistor). The signal from this sensor is used by the ECU to adjust the initial injection duration. The resistance of the thermistor decreases as the coolant temperature increases.

Incoming air temperature sensor (TA sensor)

This sensor is also a thermistor placed in the air intake manifold. It works like a liquid temperature sensor, but with a lower thermal capacity for faster response. The data from this sensor is also used to adjust the injection duration to compensate for changes in driving conditions.

Throttle Angle Sensor

This sensor is essentially a variable resistor. It works as follows. The rotor shaft is connected to the throttle valve shaft. Therefore, with the movement of the throttle valve, the resistance of this connection changes, which causes a change in the output voltage signal sent to the ECU.

Oxygen sensor

The oxygen sensor, measuring the oxygen content in the exhaust gases, maintains the air-fuel mixture ratio. During operation, the ECU receives signals from this sensor and changes the fuel injection duration accordingly. The oxygen sensor is located in the exhaust manifold or in the exhaust pipe before the catalytic converter.

The sensor is made in the form of a zirconium cup. The inner and outer surfaces of the cup are coated with a layer of platinum, forming platinum electrodes. The inner surface of the cup opens to the outside atmosphere, and the outer surface is blown by the exhaust gas flow.

When there is a difference in the oxygen content on the inner and outer surfaces of the sensor, a voltage is induced on the platinum electrodes. The principle of operation of this device is that when the electrode is heated above a certain temperature, the induced voltage tends to change sharply if the stoichiometric proportion in the air-fuel mixture is exceeded.

Idle Mixture Adjustment Sensor (IMA Sensor)

This sensor is located in the control box and its purpose is to maintain the correct mixture proportion in idle mode. This sensor does not require adjustment of its operation.

Starter switch

At the moment of engine start, the air-fuel mixture should be enriched. During the starter operation, the ECU receives a signal about the position of the starter switch and increases the amount of injected fuel according to the engine temperature. After the starter is turned off, the amount of injected fuel gradually decreases.