Engine Control Module: Description and Operation

The ECU is located under the drivers seat on 1985-87 Accord and under the front passenger seat on 1985-87 CRX Si and 1986-87 Civic. On 1986-87 Prelude models, the ECU is located behind the driver's seat within the door pillar assembly.
The ECU controls fuel delivery to provide proper air/fuel mixtures for different operating conditions. The ECU monitors operating conditions through a group of sensors in order to properly control injection timing and duration. The ECU memory is programmed with basic injector timing and discharge durations which will provide proper air/fuel mixtures for various engine operating conditions. The ECU modifies these basic settings based on signals sent from various sensors to provide the proper injection timing and discharge duration to maintain ideal air/fuel mixtures throughout the range of engine operating conditions. Other functions of the ECU include starting control, fuel pump control, fuel cut off control, control of ignition timing certain emission control devices, back-up and fail safe modes, and a self-diagnostic function.
During starting signals from the starter switch and coolant temperature sensor allow the ECU to increase injection duration to provide additional fuel necessary for startup and cold engine operation. When engine speed falls below a predetermined limit, electronic current to the fuel pump is cut off by the fuel pump control, preventing injectors from discharging fuel. During deceleration with throttle valve nearly closed or when engine speed exceeds a predetermined limit, regardless of throttle valve position, electric current to the injectors is cut off by the fuel cut off control.
On all 1987 models the ECU controls vacuum applied to the distributor vacuum advance through the Ignition Control Solenoid Valve (ICSV). The ECU controls the solenoid ECU ground circuit, completing the circuit to allow vacuum to be applied to the advance diaphragm and opening the circuit to block vacuum signals. When coolant temperature is 140-212°F, the ECU operates the ICSV to provide maximum performance and economy while preventing engine detonation.
On 1987 Accord and Prelude, the ECU controls vacuum applied to the EGR valve through the EGR Control Solenoid Valve (EGR-CSV). The ECU controls the solenoid ground circuit in order to regulate vacuum applied to the EGR valve.
On Civic and CRX the ECU controls the evaporative emission purge control solenoid ground circuit. When the engine is cold, the ECU completes the circuit and purge vacuum signals are blocked. When the engine is at normal operating temperature the ECU opens the purge solenoid circuit and purge vacuum is directed to the charcoal canister.
On all models a fail safe circuit within the ECU monitors sensors and detects system electrical malfunctions. In this mode the ECU ignores aberrant signals from malfunctioning circuits and assumes a preprogrammed value that allows the engine to continue to run. In addition, when an abnormality occurs in the ECU, fuel injection is controlled by a back-up mode that is independent of the main fuel control circuitry and allows minimal operation.
The ECU contains a self diagnosis function which stores data pertaining to system malfunctions and lights a series of indicators to aid in troubleshooting. When the ECU senses an abnormal sensor signal, the ECU lights the warning lamp, stores the failure code in erasable memory, and the failure code is indicated on the ECU LED display whenever the ignition is on. In addition, the ECU provides a ground to light the warning lamp for approximately 2 seconds as a bulb check when the ignition is turned on.