GM Astro/Safari 1985-1996 Repair Guide

Manifold Absolute Pressure (MAP) Sensor



See Figure 1

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Fig. Fig. 1: Common GM style MAP sensor

The Manifold Absolute Pressure (MAP) sensor measures the changes in intake manifold pressure, which result from the engine load and speed changes, and converts this to a voltage output.

A closed throttle on engine coast-down will produce a low MAP output, while a wide-open throttle will produce a high output. This high output is produced because the pressure inside the manifold is the same as outside the manifold, so 100 percent of the outside air pressure is measured.

The MAP sensor reading is the opposite of what you would measure on a vacuum gauge. When manifold pressure is high, vacuum is low. The MAP sensor is also used to measure barometric pressure under certain conditions, which allows the ECM to automatically adjust for different altitudes.

The ECM sends a 5 volt reference signal to the MAP sensor. As the manifold pressure changes, the electrical resistance of the sensor also changes. By monitoring the sensor output voltage, the ECM knows the the manifold pressure. A higher pressure, low vacuum (high voltage) requires more fuel, while a lower pressure, higher vacuum (low voltage) requires less fuel.

The ECM uses the MAP sensor to control fuel delivery and ignition timing.


  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 ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  7. Backprobe with the high impedence 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 recorded reading) at idle.
  17. Verify that the sensor voltage increases to approximately 4.5. volts (above the recorded reading) at Wide Open Throttle (WOT).
  19. If the sensor voltage is as specified, the sensor is functioning properly.
  21. If the sensor voltage is not as specified, check the sensor and the sensor vacuum source for a leak or a restriction. If no leaks or restrictions are found, the sensor may be defective and should be replaced.