Manifold Pressure/Vacuum Sensor: Description and Operation

The MAP sensor measures the changes in the intake manifold pressure which result from engine load and speed changes, and converts this to voltage output.
A closed throttle on engine deceleration would produce a relatively low MAP output, while Wide Open Throttle (WOT) would produce a high output. This high output is produced because the pressure inside the manifold is the same as outside the manifold, so you measure 100% of outside air pressure. MAP 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 Engine 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 high pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel. The ECM uses the MAP sensor to control fuel delivery and ignition timing.
A failure in the MAP sensor circuit should set a Diagnostic Trouble Code (DTC) 33 or 34.