Computers and Control Systems: Description and Operation

Three specific data sensors provide the VCM with the basic information for fuel management portion of its operation. That is three specific signals to the VCM establish the engine speed and air density factors. The engine speed signal comes from the distributor high resolution circuit. The VCM 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 VCM to control injector pulse width.

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