Dakota 4WD V8-318 5.2L (1991)
Manifold Pressure/Vacuum Sensor: Description and Operation
Manifold Absolute Pressure (MAP) Sensor
The Manifold Absolute Pressure (MAP) sensor is used to monitor manifold vacuum (engine load). It sends voltage signals to the engine controller that
represent the engine's varying load conditions. The MAP sensor is connected to a manifold vacuum source via a vacuum hose. Changes in manifold
absolute pressure are determined by driver demand (amount of throttle). Changes in manifold absolute pressure are sensed by a silicon crystal, via the
vacuum hose, in the MAP sensor. This crystal changes the resistance of the sensor depending upon the manifold absolute pressure acting upon it, and
the change in resistance affects the amount of voltage that the sensor allows to flow back to the engine controller. The engine controller uses this
information and other input signals to make decisions as to what output devices need to be adjusted to improve the driving conditions or emission levels.
The relationship between the resistance of the MAP sensor and the manifold absolute pressure is inversely proportional (as the manifold absolute
pressure increases, low vacuum, the resistance of the sensor decreases and vice versa). The relationship between the manifold absolute pressure of the
MAP sensor and the voltage returned to the SBEC is directly proportional (as manifold absolute pressure increases, low vacuum, the voltage returned to
the engine controller increases and vice versa).
The voltage supplied to the MAP sensor from the engine controller is 5 volts, and its input voltage range is .5 to 5 volts. .5 to 1.5 volts indicates a high
vacuum situation such as idle or deceleration. 1.5 to 3.0 volts indicates a medium level of vacuum such as a cruise or slight acceleration condition. 3.0
to 4.5 volts indicates a low vacuum situation such as hard acceleration or a mechanical failure. Any reading of 0 volts or over 5 volts indicates a
problem.
The MAP sensor is also used by the engine controller during engine start-up. The engine controller looks at the MAP reading during
key-ON-engine-OFF and compares it to a reading received once the vehicle is running and operating within +/- 64 rpm of its specified idle rpm. If there
is not a significant difference between the two readings, then a MAP sensor code will be stored in the engine controllers memory.
