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Testing MAP sensor

Old 03-30-2009, 07:01 PM
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Default Testing MAP sensor

Possible indications of a bad MAP sensor.
Late shifting or harsh shift, incorrect/rough idle, stalling, surging, lack of power.

Check the following conditions.
Bad connection at the PCM
Bad connection at the MAP sensor.
Inspect wiring harness.

If these all check out as good, move on to the test.

Manifold absolute pressure sensors have a much different calibration on Supercharged engines than on NA engines. Be sure you are using the proper specifications for the sensor being tested.

Gray wire, 5 volt reference
Black wire, ground
Light green MAP sensor signal

Testing reference wire to PCM

If the reference wire (GRAY wire, pin 16 on PCM) is not supplying the specified voltage, check the voltage on this wire at the computer. If the voltage is within specifications at the computer, but low at the sensor, repair the reference wire. When this voltage is low at the computer, check the voltage supply wires and ground wires on the computer. If these wires are satisfactory, replace the computer.

Testing sensor ground wire

With the ignition switch on, connect the voltmeter from the sensor ground wire (BLACK pin 56 on PCM) to the battery ground. If the voltage drop across this circuit exceeds specifications, repair the ground wire from the sensor to the computer.

Backprobe the MAP sensor signal wire and connect a voltmeter from this wire to ground with the ignition switch on. The voltage reading indicates the barometric pressure signal from the MAP sensor to the computer. Many MAP sensors send a barometric pressure signal to the computer each time the ignition switch is turned on and each time the throttle is in the wide-open position. If the voltage supplied by the barometric pressure signal in the MAP sensor does not equal the vehicle manufacturer'* specifications, replace the MAP sensor.

The barometric pressure voltage signal varies depending on altitude and atmospheric conditions. Follow this calculation to obtain an accurate barometric pressure reading:
  • Phone your local weather or TV station and obtain the present barometric pressure reading; for example, 29.85 inches. The pressure they quote is usually corrected to sea level.
  • Multiply your altitude by 0.001; for example, 600 feet X 0.001 = 0.6.
  • Subtract the altitude correction from the present barometric pressure reading: 29.85 - 0.6 = 29.79.
  • Check the vehicle manufacturer'* specifications to obtain the proper barometric pressure voltage signal in relation to the present barometric pressure.

Barometric pressure voltage signal specifications at different barometric pressures.

If you have a vacuum pump.
To check the voltage signal of a MAP, turn the ignition switch on and connect a voltmeter to the MAP sensor signal wire. Connect a vacuum pump to the MAP sensor vacuum connection and apply 5 in. Hg (127 mm Hg) of vacuum. On some MAP sensors, the sensor voltage signal should change 0.7 to 1.0 volt for every 5 in. Hg (127 mm Hg) of vacuum change applied to the sensor. With 5 in. Hg (127 mm Hg) of vacuum applied to the MAP sensor, the voltage should be 3.5 volts to 3.8 volts. When 10 in. Hg (254 mm Hg) of vacuum is applied to the sensor, the voltage signal should be 2.5 volts to 3.1 volts. Check the MAP sensor voltage at 5-inch (127-mm) intervals from 0 to 25 inches (635 mm).
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