Understanding Manifold Absolute Pressure (MAP) Sensors

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Purpose

The manifold absolute pressure (MAP) sensor is essentially an electronic version of the vacuum advance diaphragm used in mechanical distributors. The MAP sensor measures the difference (or absolute pressure differential) between the air inside the manifold and the ambient air pressure.

The sensor sends a voltage or frequency signal representing engine (intake) vacuum to the vehicle’s computer. The voltage or frequency increases as engine vacuum decreases. The computer adjusts air/fuel ratio and ignition timing according to changes in engine vacuum. For vehicles that use a "speed density" type of fuel injection, the vehicle’s computer also uses the MAP sensor signal as an aid in estimating airflow.

Location

The map sensor is generally located on the air cleaner, inner fender, firewall, or other locations in engine compartment.

Operation

The MAP sensor reads engine vacuum and manifold pressure through a vacuum hose connected to the intake manifold. A pressure-sensitive ceramic or silicon sensing element, and an electronic circuit in the sensor, generate a voltage or frequency output signal that changes in direct proportion to engine vacuum. The voltage or frequency increases as engine vacuum decreases.

Drivability Symptoms

The following symptoms may indicate a damaged or defective MAP sensor, or trouble in a related system:

• Surging
  
• Stalling, stumbling or rough idle
  
• Hesitation during acceleration or while driving under load
  
• Air/fuel ratio problems (fuel mixture is excessively "rich" or excessively "lean")
  
• Increased vehicle emissions
  
• Detonation
  
• Poor engine performance or loss of power
  

Drivability symptoms due to a bad MAP sensor can be similar to symptoms caused by a bad throttle position sensor. Be sure to investigate both areas before parts replacement.

Testing

MAP sensors that provide a voltage output signal can be checked using a voltmeter and hand vacuum pump as follows:

CAUTION: Do not use this procedure to check MAP sensors that provide a frequency output signal (generally used on Ford vehicles). The sensor's internal electronics will be damaged. Refer to the vehicle's service manual for proper testing procedures.

NOTE: It may be necessary to disconnect the wiring harness from the MAP sensor and jumper the POWER and GROUND pins from the harness to the sensor in order to facilitate access to the SIGNAL pin for testing. Be sure to jumper the pins properly.

• Set the voltmeter to the 20V scale.
• Connect the black lead of the voltmeter to the battery negative terminal (ground). Connect the red lead of the voltmeter to the MAP sensor’s SIGNAL pin.
• Turn the ignition key on. DO NOT start the engine.
  
  Verify MAP sensor SIGNAL voltage is within specification for the current altitude (elevation).
  
• Disconnect the vacuum line from the MAP sensor and connect a hand pump to the vacuum fitting.
  
• Observe the voltmeter, and slowly apply vacuum (approximately 18 in./Hg) to the MAP sensor.
  
  Verify MAP sensor SIGNAL voltage decreases as vacuum is applied.
  
• Disconnect the hand pump and voltmeter from the BARO sensor. Reconnect the vacuum line to the BARO sensor vacuum fitting.
  

Troubleshooting

If the sensor test results are within limits, check for a loose or leaking vacuum hose between the MAP sensor and the intake manifold, or other vacuum leak.A MAP sensor failure may also be the result of defective wiring, damaged connectors, or other electrical failures.

• If sensor SIGNAL voltage is higher than specified at no vacuum, check for a short circuit in the sensor wiring. If no short is present, the sensor may be defective. Refer to the vehicle’s service manual for additional testing and troubleshooting procedures.
  
• If sensor SIGNAL voltage is lower than specified at no vacuum, check for a short or open circuit in the sensor wiring. If the wiring is in good condition, a defective sensor or a malfunction in the vehicle’s computer may be at fault. Refer to the vehicle’s service manual for additional testing and troubleshooting procedures.
  
• If sensor SIGNAL voltage is within specifications at no vacuum, but fails to change (decrease) when vacuum is applied, the sensor is defective.
  

Repair/Replacement

The MAP sensor is a sealed unit. If you determine it is defective, it must be replaced. No repair or adjustment is possible.

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