Modes of Operation

Fig. 83 ECU Input/Output:

The PGM-FI Engine Control System is a complete engine management system that controls:
1. Fuel injector timing and discharge duration.
2. Idle speed.
3. Ignition timing.
4. Fuel pump operation.
5. A/C compressor clutch operation.
6. EGR system operation.
The ECU is also equipped with On-Board Diagnostic, and back-up functions to deal with component failures.

The ECU contains memories for the basic discharge timing and duration of the fuel injectors at various engine speeds and loads. The basic discharge values are then modified by signals sent from various sensors to obtain the final injection values.
The ECU controls the idle speed by varying the amount of air that is allowed to bypass the throttle plate. The bypass air is regulated with the Electronic Air Control Valve (EACV). This system is able to compensate for various loads on the engine like the A/C compressor, power steering pump load, transmission in gear (A/T only), etc., to keep the idle speed constant.
The ECU is programmed with basic ignition timing values for various engine speeds and loads. The programmed values are modified by sensor inputs such as coolant temperature.
When the ignition switch is first turned ON, the ECU supplies a ground for the main relay which supplies current to the fuel pump for 2 seconds to pressurize the injection system. The ECU maintains the ground to the main relay as long as the engine is running (as indicated by an ignition signal).
The main relay is used to interrupt power to the fuel pump when the engine is not running (no ignition signal), when the throttle plate is closed and the engine is above 1200 rpm, and when the engine speed exceeds 7000 rpm.
The EGR system is controlled by the ECU. When EGR is required to reduce NOx emissions, (as determined by pre-programmed values in the ECU), the ECU supplies a ground to the EGR control solenoid valve which supplies regulated vacuum to the EGR valve allowing it to open.

ON-BOARD DIAGNOSTICS AND BACK-UP FUNCTIONS

When a signal is sent to the ECU that is significantly out of the expected range, the ECU will light the "CHECK ENGINE" light in the instrument cluster, and store a fault code in an erasable memory. This fault code is displayed by a flashing LED on the ECU as long as the ignition is switched ON. This blinking code will direct the technician toward the probable cause of a component malfunction.
In the case of an abnormal signal from a sensor, the ECU will light the "CHECK ENGINE" light, and assume a pre-programmed value that will allow the vehicle to continue to operate (disregarding the faulty sensor).

NOTE: In this mode, depending on which sensor is faulty, the ECU may not be able to compensate for various running conditions. Continuing to operate the vehicle in this condition may cause additional problems such as poor running, hard starting, etc. The fault should be corrected as soon as possible.
In the case of ECU failure, the fuel injectors will be controlled by a back-up circuit, independent of the ECU in order to allow minimal driving.