For 1990-2009 cars

Three specific data sensors provide the PCM with the basic information for fuel management portion of its operation. That is three specific signals to the PCM establish the engine speed and air density factors. The engine speed signal comes from the distributor high resolution circuit. The PCM uses this information to determine engine speed (RPM). Air density is derived from IAT and MAP sensor inputs. The IAT sensor measures the air temperature that is entering the engine. The IAT signal works in conjunction with the MAP sensor to determine air density. As the intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. This information from the IAT and MAP sensors is used by the PCM to control injector pulse width.

The speed density system is only needed when there is a Mass Air Flow (MAF) sensor malfunction. If the PCM detects a malfunction with the MAF sensor circuit, the PCM will default to speed density fuel management.