Manifold Pressure/Vacuum Sensor: Description and Operation

PURPOSE
At start-up, the Powertrain Control Module (PCM) uses sensor to read barometric (atmospheric), pressure, for reference, to calculate air/fuel mixture.

During operation the Powertrain Control Module (PCM) monitors sensor input to adjust spark advance and air/fuel mixture.

OPERATION
PCM supplies 5 volt sensor reference voltage.

Sensor, connected to manifold vacuum at throttle body, converts intake manifold pressure into voltage.

THEORY
Sensor resistance and manifold absolute pressure are inversely proportional (as manifold absolute pressure increases, (low vacuum), sensor resistance decreases and vice versa).

OUTPUT VOLTAGES
Sensor output voltage range is 0.5 to 4.5 volts.
- Output voltages between 0.5 and 1.5 volts indicate a high vacuum situation, such as idle or deceleration.
- Output voltages between 1.5 and 3.0 volts indicate a medium level of vacuum such as a cruise or slight acceleration condition.
- Output voltages between 3.0 and 4.5 volts indicate a low vacuum situation such as hard acceleration or a mechanical failure.
- Any reading of 0 volts or over 5 volts indicates a problem.

CIRCUIT OPERATION
From the Powertrain Control Module (PCM), circuit KS supplies 5 volts to the Manifold Absolute Pressure MAP) sensor. Circuit KS connects to cavity 61 of the PCM.

Circuit K1 delivers the MAP signal to the PCM. Circuit K1 connects to cavity 36 of the PCM.

The PCM provides ground for the MAP signal (circuit K1) through circuit K4. Circuit K4 connects to cavity 43 of the PCM.

Helpful Information
Circuit KS splices to supply 5 volts to the throttle position sensor and NC pressure switch.

Circuit K4 also splices to supply ground for the signals from the following:
- Upstream heated oxygen sensor
- Downstream heated oxygen sensor
- Camshaft position sensor
- Crankshaft position sensor
- Engine coolant temperature sensor
- Throttle position sensor
- Knock sensor
- Intake air temperature sensor
- Vehicle speed sensor