Manifold Pressure/Vacuum Sensor: Description and Operation

PRINCIPLE OF OPERATION, MANIFOLD ABSOLUTE PRESSURE SENSOR






Manifold Absolute Pressure Sensor--contains a ceramic pressure sensitive element, an amplifier and a temperature compensation circuit. It is connected by means of a short special hose to the intake manifold of the engine after the throttle and is supplied with 5 V from pin 43 and grounded via pin 67 of the control module.

Depending on the pressure in the intake manifold, the pressure sensor applies a proportional voltage to pin 22 of the control module.

The control module uses intake manifold pressure and temperature to calculate the engine load, i.e. how much air mass is sucked in by each cylinder. Fuel injection duration is proportional to the air mass sucked in by the cylinders. At barometric pressure (100 kPa), the voltage from the pressure sensor is about 1.9 V. Information from the pressure sensor is also used for calculating ignition timing and controlling boost pressure.

If the hose works loose, if the sensor is unserviceable, or if there is no continuity in the circuit, the control module will use the throttle position as a default value. The CHECK ENGINE malfunction indicator lamp (MIL) in the main instrument display will light up.

Engine Load Signal--is determined on the basis of the pressure and temperature in the intake manifold.

The control module emits an engine load signal from pin 35. This is a PWM signal which varies in frequency. The frequency is the same as the engine speed signal, i.e. 2 pulses per crankshaft revolution. It is the width of the pulse which conveys information on the engine load, however.

At idling speed the pulse width is about 25 us. Cars equipped with electronic throttle system (ETS) have a load signal of about 60 us at idling speed. The engine load signal is used by the ETS.