Electronic Control Module
The electronic control module (ECM) is located in the passenger compartment and is the "brain" of both the EFI and Computer Command Control systems. Locations vary, but the ECM is generally located under the instrument panel behind the glove compartment or behind the passenger footwell kick panel. Information from all data sensors is received and processed by the ECM to produce the proper pulse duration ("on" time) for the injector, correct idle speed and proper spark timing. The ECM performs calculations to control the following EFI operating conditions: Engine start, engine running, fuel enrichment during acceleration, lean fuel mixture during deceleration, fuel cut-off and battery voltage correction.
During engine starts, the ECM delivers an injector pulse for each distributor reference pulse received (synchronized mode). The injector pulse width is based upon coolant temperature and throttle position. The air/fuel ratio is determined by the ECM when the throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5:1 at -33°F (-36°C) to 14.7:1 at 220°F (104°C). The lower the coolant temperature, the longer the injector pulse width (richer air/fuel mixture ratio). The higher the coolant temperature, the shorter the injector pulse width (leaner air/fuel ratio).
If the engine is flooded, the driver must depress the accelerator pedal enough to set the wide open throttle position. At this position, the ECM calculates injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio will be maintained as long as the throttle remains in the wide open position and engine speed is below 600 RPM. If the throttle position becomes less than 80 percent open and/or the engine speed exceeds 600 RPM, the ECM changes the injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).
When the engine is running above 600 RPM, the ECM operates in the open loop mode. In open loop, the ECM calculates injector pulse width based upon coolant temperature and manifold absolute pressure (MAP). The engine will remain in open loop operation until the oxygen sensor reaches operating temperature, the coolant temperature reaches a preset temperature, and a specific period of time has elapsed after the engine starts. When all these conditions are met, the ECM operates in the closed loop mode. In closed loop, the ECM controls injector pulse width based upon oxygen sensor signals to maintain the air/fuel mixture ratio close to 14.7:1.
Fuel enrichment during acceleration is provided by the ECM. Sudden opening of the throttle valve causes a rapid increase in MAP. Pulse width is directly related to MAP, throttle position and coolant temperature. The higher the MAP and the wider the throttle angle, the wider the pulse width (richer mixture). During enrichment, the injector pulses are not in proportion to distributor reference signals (non-synchronized). Any reduction in throttle angle cancels fuel enrichment.
During normal deceleration, the air/fuel mixture must be leaner. The ECM calculates the injector pulse width in a manner similar to that used for fuel enrichment, and fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut-off completely to remove fuel from the engine. This deceleration fuel cut-off overrides the normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.
Battery voltage corrections by the ECM are performed during all operating modes of the EFI system. As battery voltage decreases, the ECM increases the injector pulse width with a correction factor stored in the ECM's memory.
The ECM used on EFI vehicles has a "learning" capacity. If the battery is disconnected, the "learning" process must begin all over again. During this period, a change may be noted in vehicle performance. To "teach" the vehicle, ensure the vehicle is at normal operating temperature. The vehicle should then be driven the vehicle at part throttle, moderate acceleration and idle until performance returns.