Manifold Pressure/Vacuum Sensor: Description and Operation

Map Sensor Circuit (GM Generic):

Manifold Absolute Pressure (MAP) Sensor:

PURPOSE
The Manifold Absolute Pressure (MAP) sensor measures changes in the intake manifold pressure resulting from engine load and speed changes. Under certain conditions, the MAP sensor is also used to measure barometric pressure. This allows the ECM to automatically adjust for different altitudes. The ECM uses the MAP sensor information to control fuel delivery and ignition timing.

OPERATION
The ECM sends a 5.0 volt reference signal to the MAP sensor. As pressure changes in the intake manifold occur, the electrical resistance of the MAP sensor also changes. By monitoring the sensor output voltage, the ECM can determine the manifold pressure. At lower pressure output voltage will be about 1 to 2 volts at idle. At higher pressure or at Wide Open Throttle (WOT) output voltage will be about 4 to 4.8 volts. The higher the MAP voltage output the lower the engine vacuum, which requires more fuel. The lower the MAP voltage output the higher the engine vacuum, which requires lee fuel.
The ECM uses information from the MAP sensor to calculate spark advance as follows:
- Low MAP output voltage results in more spark advance.
- High MAP output voltage results in less spark advance.