Caprice V8-265 4.3L VIN W MFI (1995)
Manifold Pressure/Vacuum Sensor: Description and Operation
MAP Sensor
The Manifold Absolute Pressure (MAP) sensor is a pressure sensor that measures changes in intake manifold pressure. The pressure changes as a result
of engine load and speed. The MAP sensor converts this to a voltage output.
A closed throttle on engine coastdown would produce a relatively low MAP output while a wide-open throttle would produce a high MAP output
voltage. This high output voltage is produced because the pressure inside the manifold is the same as outside the manifold, so you measure 100% of
outside air pressure. Manifold Absolute Pressure (MAP) is inversely proportional to 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 Powertrain
Control Module (PCM) to automatically adjust for different altitudes.
The PCM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes the electrical resistance of the MAP sensor also changes.
By monitoring the sensor output voltage the PCM knows the manifold pressure. The PCM uses the MAP sensor to control ignition timing. The MAP
sensor is also used for speed density fuel management. When the PCM detects a malfunction with the Mass Air Flow (MAF) sensor circuit the PCM will
default to speed density.
