A defective manifold absolute pressure sensor may cause a rich or lean air/fuel ratio, excessive fuel consumption, and engine surging. this diagnosis applies to map sensors that produce an analog voltage signal. with the ignition switch on, backprobe the 5-volt reference wire and connect a voltmeter from the reference wire to ground.
Manifold absolute pressure sensors have a much different calibration on turbocharged engines than on nonturbocharged engines. Be sure you are using the proper specifications for the sensor being tested.
If the reference wire 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.
With the ignition switch on, connect the voltmeter from the sensor ground wire 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's 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's 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.
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).
If the MAP sensor voltage is not within specifications at any vacuum, replace the sensor. To check a MAP sensor with a lab scope, connect the scope to the MAP output and a good ground. When the engine is accelerated and returned to idle, the output voltage should increase and decrease.
Trace of a normal MAP sensor. Courtesy of Progressive Diagnostics--WaveFile AutoPro.
If the engine is accelerated and the MAP sensor voltage does not rise and fall, or if the signal is erratic, the sensor or sensor wires are defective.
If the MAP sensor produces a digital voltage signal of varying frequency, check the 5-volt reference wire and the ground wire with the same procedure used on other MAP sensors. The map sequence below shows a typical procedure for testing a Ford MAP sensor, which has a varying frequency.
Follow these steps to test the MAP sensor voltage signal:
- Turn off the ignition switch, and disconnect the wiring connector from the MAP sensor.
- Connect the connector on the MAP sensor tester to the MAP sensor.
- Connect the MAP sensor tester battery leads to a 12-volt battery.
- Connect a pair of digital voltmeter leads to the MAP tester signal wire and ground.
- Turn on the ignition switch and observe the barometric pressure voltage signal on the meter. If this voltage signal does not equal specifications, replace the sensor.
- Supply the specified vacuum to the MAP sensor with a vacuum pump.
- Observe the voltmeter reading at each specified vacuum. If the MAP sensor voltage signal does not equal the specifications at any vacuum, replace the sensor.
If a special map tester is not available, the sensor can be checked with a digital multimeter (dmm) that measures frequency. connect the meter to the map sensor. measure the voltage, duty cycle, and frequency at the sensor with no vacuum applied. then apply about 18 in. hg (457 mm hg) of vacuum to the map. observe and record the same readings. a good map will have about the same amount of voltage and duty cycle with or without the vacuum. however, the frequency should decrease.
Reaction of a good Ford MAP sensor with vacuum applied. Reproduced with permission from Fluke Corporation.
Normally a frequency of about 155 Hertz is expected at sea level with no vacuum applied to the MAP. When vacuum is applied, the frequency should decrease to around 95 Hz.
A lab scope can be used to check a Ford MAP sensor. The upper horizontal line of the trace should be at 5 volts and the lower horizontal line should be close to zero.
A good Ford MAP sensor signal.
Check the waveform for unusual movements of the trace. If the waveform is anything but normal, replace the sensor.
Testing A Ford MAP Sensor
- Remove the MAP sensor's electrical connector and vacuum hose.
- Connect the appropriate connector of the MAP sensor tester to the MAP sensor.
- Connect the remaining tester connector to the MAP sensor's electrical connector.
- Insert the voltage terminals of the MAP sensor tester into the test lead terminals on a DMM; make sure the polarity is correct.
- Observe the MAP sensor barometric pressure voltage reading on the voltmeter.
- Compare this reading to the specifications.
- Connect a hand-operated vacuum pump to the MAP sensor and apply 5 in. Hg to the sensor.
- Observe the sensor's voltage signal on the voltmeter. Compare the reading to specifications.
- Apply 10 in. Hg to the MAP sensor.
- Observe the voltage reading now and compare this to specifications.
- Increase the vacuum to the MAP sensor so that it now has 15 in. Hg.
- Observe the voltage reading now and compare this to specifications.
Art courtesy of Delmar - Thomson Learning.
- Increase the vacuum to 20 in. Hg.
- Observe the voltage reading now and compare this to specifications. If the voltage signals from any of the tests do not match specifications, the MAP sensor needs to be replaced.
- Disconnect the tester and reconnect the electrical connector and vacuum hose to the MAP after you have completed your testing.