Electronic Control Module (ECM)

The ECM is located in the passenger's compartment and is the "brain" of the EFI system and Computer Command Control system. 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 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 flooding, engine running, fuel enrichment during acceleration, lean fuel mixture during deceleration, fuel cutoff 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 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 ratio). The higher the coolant temperature, the shorter the injector pulse width (leaner air/fuel ratio).

During engine flooding, 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 wide open and engine speed is below 600 RPM. If the throttle position becomes less than 80 percent and/or engine speed exceeds 600 RPM, the ECM changes 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, coolant temperature reaches a preset temperature and a specific period of time elapses after engine start. When all these conditions are met, the ECM operates in the closed loop mode. In closed loop, the ECM controls the injector pulse width according to oxygen sensor signals to maintain the air/fuel ratio at 14.7:1. In either mode, the injector is pulsed once for each distributor reference.

Fuel enrichment during acceleration is provided by the ECM. Sudden opening of the throttle plate causes a rapid increase in MAP. Pulse width is directly related to MAP, throttle position and coolant temperature. The higher the MAP and 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 similar to that during fuel enrichment. Fuel output is reduced due to fuel remaining in the intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cutoff completely to remove fuel from the engine. This deceleration fuel cutoff overrides the normal deceleration mode. During either deceleration mode, the 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.