General System Description

The Electronic Control Module provides all computation and controls for the DEFI system. Sensor inputs are fed into the ECM from the various sensors and processed to produce the appropriate pulse duration for the injectors, the correct idle speed for the particular operating condition and the proper spark advance. Analog inputs from the sensors are converted to digital signals before processing. The ECM is mounted under the instrument panel and consists of various printed circuit boards mounted in a protective metal box.
The ECM receives power from the vehicle battery and when the ignition is set to the ON or CRANK position. The following information is received from the sensors:
a. Engine coolant temperature.
b. Intake manifold air temperature.
c. Intake manifold absolute pressure.
d. Barometric pressure.
e. Engine speed.
f. Throttle position.
The following commands are transmitted by the ECM:
a. Electric fuel pump activation.
b. ISC motor control.
c. Spark advance control.
d. Injection valve activation.
e. EGR solenoid activation.
The desired air/fuel mixture for various operating atmospheric conditions are programmed into the ECM. As the above signals are received from the sensors, the ECM processes the signals and computes the engine's fuel requirements. The ECM issues commands to the injection valves to open for a specific time duration. The duration of command pulses varies as the operating conditions change.

The DEFI System is activated when the ignition switch is turned to the ON position. The following events occur at that moment:
1. The ECM receives ignition ON signal
2. The Fuel Pump is activated by the ECM. (The pump will operate for approximately one second only, unless the engine is cranking or running.)
3. All engine sensors are activated and begin transmitting signals to the ECM.
4. The EGR solenoid is activated to block the vacuum signal to the EGR valve at coolant temperatures below 110° F.
5. The ``Check Engine'' and ``Coolant'' lights are illuminated as a functional check of the bulb and circuit. The ``Check Engine'' light is replaced by ``Service Now'' and ``Service Soon'' telltale lights.
6. Operation of the fuel economy lamp begins.

The following events occur when the engine is started:
1. The fuel pump is activated for continuous operation.
2. The ISC motor will begin controlling idle speed (including fast idle speed) if the throttle switch is closed.
3. The spark advance shifts from base (bypass) timing to the ECM programmed spark curve.
4. The fuel pressure regulator maintains the fuel pressure at 10.5 psi by returning excess fuel to the fuel tank.
5. The following sensor signals are continuously received and processed by the ECM:
a. Engine coolant temperature.
b. Intake manifold temperature.
c. Barometric pressure.
d. Intake manifold absolute air pressure.
e. Engine speed.
f. Throttle position changes.
6. The ECM alternately grounds each injector, precisely controlling the opening and closing time (pulse width) to deliver fuel to the engine.
The ECM's control of fuel delivery can be considered in three basic modes: cranking, part throttle and wide open throttle.
If the engine is determined to be in the cranking mode by the presence of a voltage in the cranking signal wire from the ignition switch, the starting fuel delivery consists of one lone ``Prime'' pulse from both injectors followed by a series of ``Starting'' pulses until the cranking mode signal is no longer present.
In addition, there is a ``clear flood'' condition where smaller alternating fuel pulses are delivered if the throttle is held wide open and cranking exceeds five seconds.
Once the engine is running, injector pulse width is then adjusted to account for operating conditions such as idle, part throttle, acceleration, deceleration, and altitude.
For wide open throttle conditions, which are sensed by matching the MAP and BARO sensor inputs, additional enrichment is provided.
Engine ignition timing is controlled by the ECM. The two basic operating modes are ``Cranking'' (or bypass) and ``Normal Engine'' operation.
When the engine is in the cranking/bypass mode, ignition timing occurs at a reference setting (distributor timing set point) regardless of other engine operating parameters. Under all other normal operating conditions, basic engine ignition timing is controlled by the ECM and modified, or added to, depending on particular conditions, such as altitude and/or engine loading.
The Idle Speed Control system is controlled by the ECM. The system acts to control engine idle speed in three ways: normal idle (RPM) control, as a fast idle device, and as a ``dashpot'' on decelerations and throttle closing.
The normal engine idle speed is programmed into the ECM and no adjustments are possible. Under normal engine operating conditions, idle speed is maintained by monitoring idle speed in a ``Closed Loop'' fashion. To accomplish this loop, the ECM periodically senses the engine idle speed and issues commands to the ISC to move the throttle stop to maintain the designed speed.
For engine starting, the throttle is either held open by the ISC for a longer period (cold) or a shorter time (hot) to provide adequate engine warm-up prior to normal operation. When the engine is shut off, the throttle is opened by fully extending the ISC actuator to get ready for the next start.
Signal inputs for transmission gear, air conditioning compressor clutch (engaged or not engaged) and throttle open or closed are used to either increase or decrease throttle angle in response to these particular engine loadings.