See Figure 1
The manifold absolute pressure sensor measures the changes in intake manifold pressure, which result from engine load and speed changes. The sensor converts this to voltage output. A closed throttle on an engine coastdown will produce a relatively low MAP output, while a wide open throttle will produce a relatively high output. This high output is produced because the pressure inside the manifold is the same as pressure outside the manifold, therefore 100 percent of the outside air pressure is measured.
The MAP sensor signal is opposite of what you would measure on a vacuum gauge. When manifold pressure is HIGH vacuum is LOW. The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the control module to automatically adjust for different altitudes.
The control module 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 control module can determine manifold pressure changes.
By using the MAP output signal, the control module can determine fuel ratio and ignition timing requirements. A high pressure reading would require more fuel, while a lower pressure reading would require less fuel.
Removal and installation of the manifold absolute pressure sensor is covered in Engine & Engine Overhaul of this guide. Please refer there for further information.