Operation

OPERATION

The MAP serves as a PCM input, using a silicon based sensing unit, to provide data on the manifold vacuum that draws the air/fuel mixture into the combustion chamber. The PCM requires this information to determine injector pulse width and spark advance. When MAP equals Barometric pressure, the pulse width will be at maximum.

Also like the cam and crank sensors, a 5 volt reference is supplied from the PCM and returns a voltage signal to the PCM that reflects manifold pressure. The zero pressure reading is 0.5 volt and full scale is 4.5 volt. For a pressure swing of 0 - 15 psi the voltage changes 4.0 volt. The sensor is supplied a regulated 4.8 to 5.1 volts to operate the sensor. Like the cam and crank sensors ground is provided through the sensor return circuit.

The MAP sensor input is the number one contributor to fuel injector pulse width. The most important function of the MAP sensor is to determine barometric pressure, which the PCM uses along with ambient air temperature to calculate air density. Air density decreases at higher altitudes, and increases at lower altitudes. As air density will be affected by the weather, the PCM can also use the MAP and Intake Air Temperature (IAT) to fuel injector timing to correct for varying weather conditions. When the ignition switch is turned to the run position, the PCM powers up, and monitors the MAP voltage. Based upon the MAP voltage, the PCM can monitor the current barometric pressure relative to altitude. Once the engine starts, the PCM monitors MAP voltage continuously every 12 milliseconds, and compares the current MAP voltage to what the MAP voltage was at key on. The difference between current MAP voltage and what MAP voltage was at key on is manifold vacuum.

The PCM uses the MAP sensor to calculate the following:

- Barometric pressure
- Engine load
- Manifold pressure
- Injector pulse width
- Ignition timing
- Idle speed