GM S-Series Pick-ups and SUV's 1994-1999 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 VCM to automatically adjust for different altitudes. The VCM 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 VCM 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 VCM uses the MAP sensor to control fuel delivery and ignition timing.

TESTING



See Figures 1, 2 and 3



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Fig. Fig. 1: Typical Manifold Absolute Pressure (MAP) sensor wiring diagram (wire color, terminal identification/location may vary on certain models)

  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 VCM may be faulty. Correct any wiring or VCM faults before continuing test.
  6.  
  7. Backprobe with the high impedance 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 (at sea level).
  10.  
  11. Record MAP sensor voltage with the key ON and engine off.
  12.  
  13. Start the vehicle.
  14.  
  15. Verify that the sensor voltage is greater than 1.5 volts (above the recorded reading) at idle.
  16.  
  17. Verify that the sensor voltage increases to approximately 4.5. volts (above the recorded reading) at Wide Open Throttle (WOT).
  18.  
  19. If the sensor voltage is as specified, the sensor is functioning properly.
  20.  
  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.
  22.  



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Fig. Fig. 2: Using jumper wires and a high impedance voltmeter test between MAP sensor terminals A and C with the key ON and engine off. The voltage should be approximately 5 volts



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Fig. Fig. 3: Next test between MAP sensor terminals A and B with the key ON and engine off. The voltage should be approximately 0.5 volts

REMOVAL & INSTALLATION



Except 1998-99 2.2L Models

See Figures 4 through 8



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Fig. Fig. 4: Manifold Absolute Pressure (MAP) sensor location-1996 4.3L engine shown, others similar



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Fig. Fig. 5: Manifold Absolute Pressure (MAP) sensor location-1994-97 2.2L engine



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Fig. Fig. 6: Unplug the Manifold Absolute Pressure (MAP) sensor electrical connection



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Fig. Fig. 7: Unfasten the Manifold Absolute Pressure (MAP) sensor retainers ...



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Fig. Fig. 8: ... then remove the MAP sensor

  1. Disconnect the negative battery cable.
  2.  
  3. Tag and disconnect the vacuum harness assembly.
  4.  
  5. Unplug the electrical connector.
  6.  
  7. To remove the sensor, release the locktabs, or unfasten the bolts or simply pull the sensor from the grommet, whichever applies to your vehicle.
  8.  
  9. Installation is the reverse of removal.
  10.  

1998-99 2.2L Models
  1. Disconnect the negative battery cable.
  2.  
  3. Unplug the electrical connection and vacuum hose.
  4.  
  5. Unfasten the bracket retaining screw and remove the bracket.
  6.  
  7. Remove the sensor from the intake manifold.
  8.  
  9. Installation is the reverse of removal.
  10.  

 
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