Electronic Control Module (ECM)

The ECM is located behind instrument panel and is the "brain" of EFI and Computer Command Control systems. Information from all data sensors is received and processed by the ECM to produce proper pulse duration for each 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 first signal sent to injectors is a "prime" pulse. This pulse charges intake manifold with fuel during or just prior to engine starting. This pulse width (injector "on" time) is not synchronized with HEI distributor reference pulses. Prime pulses will be delivered for a length of time dependent upon coolant temperature.

After delivering prime pulses, ECM will deliver a pulse for each distributor reference pulse received. Air/fuel ratio is determined by ECM based upon throttle position and coolant temperature. 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, accelerator pedal must be depressed enough to set wide open throttle position. At this position, ECM calculates injector pulse width equal to an air/fuel ratio of 20:1. This air/fuel ratio will be maintained as long as throttle remains wide open and engine speed is below 600 RPM. If throttle position becomes less than 80 percent and/or engine speed exceeds 600 RPM, ECM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).

When engine is running above 600 RPM, ECM operates in open loop mode. In open loop, ECM calculates injector pulse width based upon coolant temperature and manifold absolute pressure (MAP). Engine will remain in open loop operation until 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, ECM operates in closed loop mode. In closed loop, ECM controls injector pulse width according to oxygen sensor signals, maintaining air/fuel ratio of 14.7:1. In either mode, injectors are pulsed alternately for each distributor reference pulse.

Fuel enrichment during acceleration is provided by ECM. Sudden opening of throttle plates causes a rapid increase in MAP. Pulse width is directly equal 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, injector pulses are not in proportion with distributor reference signals. Any reduction in throttle angle will cancel fuel enrichment.

During normal deceleration, air/fuel mixture must be leaner. ECM calculates 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 at preset levels, fuel flow is cut off completely to remove fuel from the engine. This deceleration fuel cutoff overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.

Battery voltage corrections by ECM are performed during all operating modes of EFI system. As battery voltage decreases, ECM increases injector pulse width with a correction factor stored in ECM's memory.

Hood Louver Control (HLC) system is controlled by ECM. When engine coolant temperature and throttle position meet preset specifications, ECM activates HLC relay which opens hood louver to allow additional air into engine.