GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Manifold Absolute Pressure Sensor

Print

OPERATION



The Manifold Absolute Pressure (MAP) Sensor responds to changes in intake manifold pressure which result from engine load (intake manifold vacuum) and RPM changes; and converts these to a voltage output. The PCM sends a 5 volt reference voltage to the MAP sensor. As the manifold pressure changes, the output signal voltage of the sensor also changes. By monitoring the sensor output voltage, the PCM knows the manifold pressure. A lower pressure (low voltage) output voltage will be about 1-2 volts at idle. While higher pressure (high voltage) output voltage will be about 4-4.8 volts at Wide Open Throttle, (WOT). The MAP sensor is also used under certain conditions, to measure barometric pressure, allowing the PCM to make adjustments for different altitudes. The PCM uses the MAP sensor to control fuel delivery and ignition timing.

If the PCM detects a voltage that is lower than the possible range of the MAP sensor, a DTC will be set. A signal voltage higher than the possible range of the sensor will also set a DTC. The PCM will recognize an intermittent lower or higher voltage than possible as a fault and will set a code. The PCM can also detect a shifted MAP sensor. The PCM compares the MAP sensor signal to a calculated MAP based on throttle position and various engine load factors. If the detects A map signal that varies excessively above or below the calculated value a DTC will be set.

TESTING



The MAP sensor voltage reading is the opposite of a vacuum gauge reading. When manifold pressure is high, the MAP sensor value is high and vacuum is low.

  1. Using the accompanying chart as a guide, determine the height above sea level and the comparative voltage range.
  2.  
  3. Now, backprobe the MAP connector between terminals A and B.
  4.  
  5. Key On, Engine Off, the voltage reading obtained should coincide with the chart; voltage and altitude should match.
  6.  

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: MAP Sensor specifications-voltage and altitude

  1. Now apply 10" Hg (34 kPa) vacuum to the MAP sensor, it should cause the voltage to be 1.5 to 2.1 volts less than the voltage at Step 1. Upon applying vacuum to the sensor, the change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
  2.  
  3. Check the voltage between terminal C and ground, you should read reference voltage (about 5 volts).
  4.  
  5. If the reference voltage is right, and the sensor voltage is not within specs, replace the sensor.
  6.  
  7. If the sensor and the circuits are functional, the PCM may be faulty.
  8.  
  9. Be sure to test the vacuum hose for leaks or restrictions. This source must supply vacuum to the MAP sensor only.
  10.  

See Figure 2

Click image to see an enlarged view

Fig. Fig. 2: MAP Sensor wiring schematic and vacuum lead

REMOVAL AND INSTALLATION



  1. On early model vehicles with a TBI unit, the MAP sensor plugs into the side of the air cleaner.
  2.  
  3. On later models, the MAP is bolted in, on top of the engine.
  4.  

See Figure 3

Click image to see an enlarged view

Fig. Fig. 3: MAP Sensor service on 3.1L Engine

  1. Disconnect the negative battery cable.
  2.  
  3. Detach the electrical connector from the sensor.
  4.  
  5. Unplug the vacuum line.
  6.  
  7. Remove the locking clip and remove the sensor.
  8.  

See Figure 4

Click image to see an enlarged view

Fig. Fig. 4: MAP Sensor location and service shown 3.4L Engine

 
label.common.footer.alt.autozoneLogo