Speed Density System

The Sequential Fuel Injection (SFI) system is a speed and air density system. The system is based on "speed density" fuel management, so let's define their basis of operation.
Three specific data sensors provide the Powertrain Control Module (PCM) with the basic information for the fuel management portion of its operation. That is, three specific signals to the PCM establish the engine speed and air density factors.

Speed
The engine speed signal comes from the 3X crankshaft position sensor is conditioned by the electronic Ignition Control Module (ICM). This signal is input to the PCM on the crank reference high, CKT 430. The PCM uses this information to determine the "speed" or RPM factor for fuel and ignition management.

Density
Three sensors contribute to the density factor, the Intake Air Temperature (IAT), the Manifold Absolute Pressure (MAP) and the Engine Coolant Temperature (ECT) sensor.

The IAT sensor is a 2-wire sensor that measures the temperature of the air entering the intake manifold. The IAT sensor is a thermistor that changes its resistance as an inverse function of temperature. When the temperature is low, the resistance is high, and when the temperature is high, the resistance is low.

The engine coolant temperature sensor is also a 2-wire sensor that measures temperature. It is mounted in the engine coolant stream and operates the same way as the IAT sensor.

The IAT and ECT sensor work together to assure that proper temperature information gets to the PCM.

The Manifold Absolute Pressure (MAP) sensor is a 3-wire sensor that monitors the changes in intake manifold pressure which results from changes in engine loading. These pressure changes are supplied to the PCM in the form of analog electrical signals.

As intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. The MAP sensor sends this pressure information to the PCM and the PCM increases the amount of fuel injected by increasing the injector pulse width. Conversely, as manifold pressure decreases, the amount of fuel is decreased.

These four inputs MAP, IAT, ECT sensor, and RPM are the major determinants of the air/fuel mixture delivered by the fuel injection system.

The remaining sensors and switches provide electrical inputs to the PCM which are used for modification of the air/fuel mixture, as well as for other PCM control functions, such as Idle Air Control (IAC).