GM Grand Am/Achieva/Calais/Skylark/Somerset 1985-98

Manifold Absolute Pressure (MAP) Sensor



See Figure 1

The Manifold Absolute Pressure (MAP) sensor measures the changes in intake manifold pressure, which result from engine load and speed changes and converts this information to a voltage output. It is found on all engines except the 3.0L (VIN L) and 3.3L (VIN N). The MAP sensor reading is the opposite of a vacuum gauge reading: when manifold pressure is high, MAP sensor value is high and vacuum is low. A MAP sensor will produce a low output on engine coastdown with a closed throttle while a wide open throttle will produce a high output. The high output is produced because the pressure inside the manifold is the same as outside the manifold, so 100% of the outside air pressure is measured.

The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes.

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Fig. Fig. 1: MAP sensor-1995 2.3L shown

The MAP sensor changes the 5 volt signal supplied by the computer control module (ECM or PCM as applicable), which reads the change and uses the information to control fuel delivery and ignition timing.

Removal and installation of the manifold absolute pressure sensor is covered in Engine & Engine Overhaul of this information. Please refer there for further information.


See Figures 2 and 3

  1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C .
  3. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  5. If the voltage is not as specified, either the wiring to the MAP sensor or the PCM may be faulty. Correct any wiring or PCM faults before continuing test.
  7. Backprobe with the high impedance voltmeter at MAP sensor terminals B and A .
  9. Verify that the sensor voltage is approximately 0.5 volts with the engine not running (at sea level).
  11. Record MAP sensor voltage with the key ON and engine off.
  13. Start the vehicle.
  15. Verify that the sensor voltage is greater than 1.5 volts (above the rec