Manifold Pressure/Vacuum Sensor: Description and Operation

DESCRIPTION
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes. The sensor converts the pressure input to a voltage output. Manifold Absolute Pressure is the opposite of manifold vacuum. When manifold pressure is high, vacuum is low.

OPERATION
As the manifold pressure changes, the electrical output of the sensor also changes. A higher pressure, low vacuum (high voltage) requires more fuel while a lower pressure, higher vacuum (low voltage) requires less fuel. By monitoring the sensor output voltage, the ECM determines the manifold pressure and can increase or decrease the "ON" time (or pulse width) of the injectors to allow for the addition or reduction of fuel required. The ECM also uses the MAP sensor to control ignition timing.

The ECM provides the MAP sensor with a five-volt reference voltage, a bias voltage, and a ground. The sensor output signal is scaled by the bias voltage.

BAROMETRIC PRESSURE CALCULATION
The MAP sensor is also used to calculate barometric pressure under certain conditions. This eliminates the need for a separate sensor.

MAP SENSOR CIRCUIT FAILURE
A failure in the MAP sensor circuit should set a Diagnostic Trouble Code (DTC) 31 MAP Sensor Circuit.