Manifold absolute pressure sensor: Notes
The pressure sensor contains two metal-coated ceramic discs mounted close together. The disc closest to the pressure connection is thinner and bends when it comes under pressure. This modifies the capacitance between the metal coatings of the discs as a function of the pressure. Built into the sensor is a circuit that converts the capacitance to an analogue voltage. The pressure sensor is bolted to the engine's intake manifold after the throttle and is supplied with 5 V from control module pin 69 and grounded via control module pin 22.
Depending on the pressure in the intake manifold, the pressure sensor applies a proportional voltage to control module pin 11.
The engine load or air mass/combustion calculated from the mass air flow sensor value can, during rapid load changes, differ from the actual air mass drawn into the cylinder during the intake stroke. The reason for this is the inertia of the large volume of air in the intake manifold. During load changes therefore, the intake pressure is used to correct the air mass/combustion that is used for basic calculation of the fuel quantity.
The pressure is also used to correct the purge PWM ratio. At a high intake pressure, a higher PWM ratio is required to achieve the same purge flow.
At normal barometric pressure, 100 kPa, the voltage from the sensor is about 2.1 V.