Computers and Control Systems: Description and Operation

ECM is installed to the underside of the instrument panel at the passenger's seat side. ECM is a precision unit consisting of microcomputer, A/D (Analog/Digital) converter, I/O (Input/Output) unit, etc.. It is an essential part of the electronic control system, for its functions include not only such a major function as to control fuel injector, ISC actuator, fuel pump relay, etc. but also on-board diagnostic system (self-diagnosis function) and fail-safe function as described in the following section.

On-board diagnostic system (Self-diagnosis function)
ECM diagnoses troubles which may occur in the area including the following parts when the ignition switch is ON and the engine is running, and indicates the result by turning on or flashing malfunction indicator lamp ("CHECK ENGINE" light).
- Heated oxygen sensor
- Engine coolant temp. sensor
- Throttle position sensor
- Intake air temp. sensor
- Manifold absolute pressure sensor
- Camshaft position sensor
- Ignition fail-safe signal
- Vehicle speed sensor
- EGR system
- ISC system
- CPU (Central Processing Unit) of ECM
ECM and malfunction indicator lamp ("CHECK ENGINE" light) operate as follows.
- Malfunction indicator lamp ("CHECK ENGINE" light) lights when the ignition switch is turned ON (but the engine at stop) with the diagnosis switch terminal ungrounded regardless of the condition of Electronic Fuel Injection system. This is only to check the malfunction indicator lamp ("CHECK ENGINE" light) bulb and its circuit.








- If the above areas of Electronic Fuel Injection system is free from any trouble after the engine start (while engine is running), malfunction indicator lamp ("CHECK ENGINE" light) turns OFF.
- When ECM detects a trouble which has occurred in the above areas, it makes malfunction indicator lamp ("CHECK ENGINE" light) turn ON while the engine is running to warn the driver of such occurrence of trouble and at the same time it stores the trouble area in ECM back-up memory. (The memory is kept as it is even if the trouble was only temporary and disappeared immediately. And it is not erased unless the power to ECM is shut off for specified time above.) ECM also indicates trouble area in memory by means of flashing of malfunction indicator lamp ("CHECK ENGINE" light) at the time of inspection (i.e. when diagnosis switch terminal is grounded and ignition switch is turned ON.)

NOTE:
- When a trouble occurs in the above areas except EGR system and disappears soon while the diagnosis switch terminal is ungrounded and the engine is running, malfunction indicator lamp ("CHECK ENGINE" light) lights and remains ON as long as the trouble exists but it turns OFF when the normal condition is restored. When it is EGR system where a trouble occurs, even if it is only a temporary one and disappears soon, malfunction indicator lamp ("CHECK ENGINE" light) remains ON till the ignition switch is turned OFF.
- When a trouble occurs, ECM causes the malfunction indicator lamp ("CHECK ENGINE" light) to light to indicate occurrence of that trouble but it doesn't for an ISC system trouble (Code No.46). However, as ECM stores diagnostic trouble code in memory for an ISC system trouble, when any abnormality is found with ISC system (i.g., high, low or rough idle) check if Code No.46 is indicated by grounding diag. switch terminal.
- Only ignition circuit trouble (code No.41) is not stored in back-up memory of ECM. (in other words, even if ECM has detected a trouble in ignition circuit, once ignition switch is turned OFF, code No.41 will not be indicated even when diagnosis switch terminal is grounded and ignition switch is turned ON.). Therefore, to check diagnostic trouble code when engine fails to start, crank engine and then ground diagnostic switch terminal with ignition switch ON.
- Time required to erase diagnostic trouble code memory thoroughly varies depending on ambient temperature as follows.

Fail-safe function
Even when a trouble has occurred in such area of Electronic Fuel Injection system that includes the following parts and a failure signal is sent to ECM, control over the injector, ISC motor and others is maintained on the basis of the standard signals and/or back-up program prestored in the ECM while ignoring that failure signal and/or CPU. This function is called "fail-safe function". Thus, with this function, a certain level of engine performance is available even when some failure occurs in such area and disability in running is avoided.
- Engine coolant temp. sensor
- Throttle position sensor
- Vehicle speed sensor
- Intake air temp. sensor
- Manifold absolute pressure sensor
- CPU in ECM





MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP SENSOR)
This sensor senses pressure change in the intake manifold and converts it into voltage change. It consists of a semiconductor type pressure converting element which converts a pressure change into an electrical change and an electronic circuit which amplifies and corrects the electric change. The ECM sends a 5-volt reference voltage to the pressure sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, ECM knows the manifold pressure (intake air volume). ECM uses the voltage signal from the pressure sensor as one of the signals to control fuel injector, shift-up indicator light, EVAP canister purge valve and EGR solenoid vacuum valve.