Engine Control Module: Description and Operation

Fuel Control:

FUEL INJECTION CONTROL
Basic Injection Time
Basic fuel injection time is calculated from the mass air flow sensor signal and engine speed signal. Injector opening time and duration is then modified using other input signals regarding the engine's operating conditions.

Cold Start
During cold start operation a temperature related quantity of fuel is injected once per crankshaft revolution in parallel for up to five crankshaft revolutions. The quantity of fuel is in excess of the normal injected quantity. After five revolutions, the high initial fuel quantity is reduced on an rpm-related basis. This is accomplished in the control unit by start-up recognition.

Start-up recognition is permanently programmed in the electronic control unit and is temperature and speed dependent. After the start-up recognition threshold (600 - 800 RPM) has been reached, injection takes place only once every other crankshaft revolution.

Warm-up Operation
The electronic control unit uses the after-start warm-up function, which is mapped and controlled up to a coolant temperature of 158°F, to determine the fuel injection quantity and timing during the warm-up phase. The control unit compares and evaluates input signals from the coolant temperature sensor and the engine speed signal against fixed values in it's program. When these values are combined, the proper fuel quantity is injected. The system also recognizes a repeat start and reduces the injected quantity of fuel to avoid an over-rich condition in the event of a restart.

Normal Operating Temperature
When the engine reaches normal operating temperature (above 158°F) the basic fuel injection time is calculated from the mass air flow sensor signal, the engine speed signal, and the oxygen sensor signal. The oxygen sensor signal is used as a correction factor to obtain the optimum air/fuel mixture for reduction of pollutants and maximum engine performance.

FUEL DELIVERY CONTROL
Operation of the main fuel pump and in-tank pump is controlled through a relay which is energized by the Motronic control unit. The relay is energized only during engine cranking or while the engine is running. The electronic control unit uses the engine speed signal as an input for fuel pump control.

Ignition Control:

IGNITION CONTROL
The primary side of the coils are connected with the positive battery terminal via the ignition coil relay. During the primary current-flow time (coil saturation) the coils are connected to ground through the power output transistors (power stages) via the electronic control unit.

Based on load conditions received by the electronic control unit in relation to various engine speeds a three dimensional ignition performance characteristic "Basic Ignition Map" is used to optimize the ignition to achieve the most favorable exhaust gas and fuel consumption performance for any given load or speed condition.

When the throttle valve is in the closed position, the bottom line of the basic ignition map is selected as the idle/coast characteristic.

For engine speeds below the nominal values, the ignition timing is advanced to obtain idle stabilization.

For coasting, ignition timing is programmed to obtain minimum exhaust gas pollutants and optimum driveability performance.

For the wide open throttle operation, the top line of the basic ignition map is selected. This full-load characteristic is programmed with engine knock limits taken into consideration.

For very high ambient temperatures the ignition is retarded as a function of engine and intake air temperature.

IDLE SPEED CONTROL
The idle speed is electronically controlled by the electronic control unit with relation to the respective engine coolant temperature and load signals.

ON-BOARD DIAGNOSTICS
In the event of a system malfunction, circuit information is stored in the internal memory of the electronic control unit. Intermittent failures are stored with a record of the failure frequency.

The electronic control unit is able to store up to eighteen malfunctions, and will give priority to those items which can cause the vehicle to exceed exhaust emission standards. In addition to being stored in memory, an emissions-related failure will cause the "Check Engine" light to illuminate on the instrument cluster.

ADAPTIVE CONTROL
The electronic control unit has adaptive idle speed, throttle position, and air/fuel mixture capabilities. The control unit continuously compares the programmed data, for idle control and air fuel mixture, with the input values.

If a deviation occurs, the new input values are superimposed and stored in the programmed data map. As a result, new output signals are sent to the idle control valve, and or to the injectors. This allows the electronic control unit to correct the idle speed, or injector pulse width for break-in wear or slight air leaks which might occur in the systems.

If power is interrupted to the control unit, new adaptation takes place within several minutes of engine operation.