Manifold Pressure/Vacuum Sensor: Description and Operation

Manifold Absolute Pressure (MAP) Sensor:

PURPOSE
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes and converts this to a voltage output.

OPERATION
A closed throttle on engine coastdown would produce a relatively low MAP output, while a wide open throttle would produce a high output.
The MAP sensor is the OPPOSITE of what you would measure on a vacuum gage.

When manifold pressure is high, vacuum is low.

The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the Electronic Control Module (ECM) to automatically adjust for different altitudes.

The ECM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes.

By monitoring the sensor output voltage, the ECM knows the manifold pressure.

A higher pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.

Once a trouble code is set, the ECM will use a default valve for MAP.
See Diagnostic Charts. Testing and Inspection