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. As intake manifold pressure increases, the air density in the intake manifold also increases and additional fuel is required. Under certain conditions, the MAP sensor is also used to measure barometric pressure. This allows the control module to automatically adjust for different altitudes.

The PCM uses information from the MAP sensor to calculate spark advance and fuel as follows:
- Low MAP output voltage (low pressure) results in more spark advance.
- High MAP output voltage (high pressure) results in less spark advance.
- Low MAP output voltage (low pressure) results in less fuel.
- High MAP output voltage (high pressure) results in more fuel.

OPERATION
The control module supplies a 5 volt reference signal to the MAP sensor and the sensor provides a path to ground through its variable resistor. The control module by monitoring the sensor output voltage can determine the manifold pressure. A closed throttle on engine coastdown will produce a relatively low MAP output, while a wide open throttle will produce a high output. 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. At lower pressure output voltage will be about 1 to 2 volts at idle.


A failure in the MAP sensor circuit should set Diagnostic Trouble Codes (DTC) 33 or 34 with PCM/ECM systems and DTC P0107 or P0108 with VCM systems.