Manifold Pressure/Vacuum Sensor: Description and Operation

DESCRIPTION AND OPERATION
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. A closed Throttle on Engine coast down would produce a relatively low MAP output, while a Wide Open Throttle would produce a high output. MAP is the opposite of what would be measured 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 higher pressure, low vacuum (high voltage) requires more Fuel, while a lower pressure, higher vacuum (low voltage) requires less Fuel. A failure in the MAP Sensor Circuit should set a DTC 33.