GM Astro/Safari 1985-1996 Repair Guide

Differential Pressure (Vacuum) Sensor



The differential pressure sensor measures the changes in intake manifold pressure, which result from the engine load and speed changes, and converts this to a voltage output. The differential pressure sensor operates opposite of a MAP sensor

A closed throttle on engine coastdown will produce a high output, while a wide-open throttle will produce a low output. This low 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 ECM sends a 5 volt reference signal to the 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 sensor to control fuel delivery and ignition timing.


See Figure 1

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Fig. Fig. 1: Differential pressure sensor wiring schematic

  1. Backprobe with a high impedance voltmeter at 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 sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  7. Backprobe with a high impedence voltmeter at MAP sensor terminals B and A.
  9. Verify that the sensor voltage is less then 1.0 volts with the engine not running.
  11. Start the vehicle.
  13. Verify that the sensor voltage is greater than 3.0 volts at idle.
  15. Verify that the sensor voltage drops at Wide Open Throttle (WOT).
  17. If the sensor voltage is as specified, the sensor is functioning properly.
  19. 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.


See Figure 2

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Fig. Fig. 2: Differential pressure sensor mounting

  1. Disconnect the negative battery cable.
  3. Tag and disconnect the vacuum harness assembly.
  5. Disengage the electrical connector.
  7. Release the locktabs, unfasten the bolts and remove the sensor.
  9. Installation is the reverse of removal.