Manifold Absolute Pressure Sensor

Manifold Absolute Pressure (MAP) sensor is located near throttle body or near front of intake manifold. See Figure , Figure , Figure or Figure . The MAP sensor contains a silicone based sensing unit to provide information on manifold pressure. The PCM uses this input to calculate fuel injector pulse width and ignition spark advance. When manifold pressure is the same as barometric pressure, fuel injector pulse width is adjusted to maximum. The PCM supplies the MAP sensor with a 5-volt reference signal. There is a return voltage signal from the MAP sensor which indicates manifold pressure. A zero pressure reading equals .5 volt and full scale is 4.5 volts. Voltage swing is 4 volts. This indicates a pressure swing of zero to 15 psi.

The PCM uses MAP sensor input as the number one contributing factor for adjusting fuel injector pulse width. Air density changes with altitude. The PCM must know air density to properly adjust air/fuel mixture. This input also helps PCM adjust for changing barometric pressure. Altitude and barometric pressure have an inverse relationship. As altitude increases, barometric pressure decreases. When altitude decreases, barometric pressure increases. After engine start, the PCM looks at MAP sensor input every 12 milliseconds. The PCM compares this input with what the input was at key-on. The difference between current input and input when key-on is current manifold pressure.

When ignition is first turned on with engine off, PCM updates barometric pressure. If vehicle is started and driven to a different altitude, barometric pressure needs to be updated. The update takes place in the MAP sensor memory cell. When barometric pressure is updated, the PCM can adjust air/fuel mixture more efficiently. During vehicle operation, if PCM sees Wide Open Throttle (WOT) (based on throttle position and engine RPM), barometric pressure will update. The PCM uses MAP sensor input to calculate the following.

• Automatic Transmission Shift Point Strategies (Certain Models)
• Barometric Pressure
• Deceleration Fuel Shutoff
• Engine Load
• Fuel Injector Pulse Width
• Idle Speed
• Ignition Spark Advance
• Manifold Pressure

The MAP sensor produces a voltage from a single piezoresistive element located in the center of a diaphragm. The diaphragm and element are both made of silicone. When manifold pressure changes it causes the diaphragm to deflect which stresses the silicone. Resistance in the silicone changes when stressed. The MAP sensor also contains electronics to compensate and condition the signal for temperature.