GM Full Size Vans 1987-1997 Repair Guide

Manifold Absolute Pressure (MAP) Sensor

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OPERATION



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 coastdown 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 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.

TESTING



See Figure 1

  1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C.
  2.  
  3. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  4.  
  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.
  6.  
  7. Backprobe with a high impotence voltmeter at MAP sensor terminals B and A.
  8.  
  9. Verify that the sensor voltage is approximately 0.5 volts with the engine not running.
  10.  
  11. Start the vehicle.
  12.  
  13. Verify that the sensor voltage is greater than 1.5 volts at idle.
  14.  
  15. Verify that the sensor voltage increases to approximately 4.5. volts at Wide Open Throttle (WOT).
  16.  
  17. If the sensor voltage is as specified, the sensor is functioning properly.
  18.  
  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.
  20.  



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Fig. Fig. 1: Typical Manifold Absolute Pressure (MAP) Sensor wiring diagram

REMOVAL & INSTALLATION



  1. Disconnect the negative battery cable.
  2.  
  3. Tag and disconnect the vacuum harness assembly.
  4.  
  5. Disengage the electrical connector.
  6.  
  7. Release the locktabs, unfasten the bolts and remove the sensor.
  8.  
  9. Installation is the reverse of removal.
  10.  

 
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