How to Troubleshoot a MAP Sensor

by Dan Ferrell
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Your engine’s Manifold Absolute Pressure (MAP) sensor could be the culprit if you're still getting lousy gas mileage after a tune up. A MAP sensor continually compares atmospheric absolute pressure to the intake manifold vacuum and sends the appropriate voltage signal to the vehicle’s computer. The computer then decides whether to increase or decrease fuel supply to the engine and adjust spark advancement accordingly. A bad MAP sensor sending the wrong signal to the computer can disrupt fuel consumption and overall engine performance. But with these simple steps, we will be able to determine if the MAP sensor in your vehicle is giving out.

Step 1

Locate the MAP sensor along the firewall or to the passenger side of the engine compartment.

Step 2

Check the vacuum hose attached to the sensor and make sure it is in good condition and free of obstructions. Make sure there are no loose wires and the sensor’s electrical connector is in good shape.

Step 3

Determine what type of MAP sensor is installed in your vehicle--voltage or frequency type. If you have a voltage type, you will use a voltmeter for this test; if it is a frequency type, you will use a tachometer. Consult your vehicle service manual if you're not sure what type you have.

Step 4

Unplug the electrical connector from the MAP sensor and turn the ignition key on, but do not start the engine. With a voltmeter, measure the reference voltage by touching the reference wire with the meter’s positive lead and the negative lead to ground-black wire, which is at the connector’s end coming from the computer. If the reference voltage--about 5 volts--is out of specification, you have found the source of the problem. Reconnect the MAP sensor and turn off the ignition key. Take your vehicle to a professional auto electrical shop for further testing.

Step 5

Attach the voltmeter or tachometer negative probe to the ground (black) wire and the positive probe to the signal wire. The sensor's connector should be plugged in. The third wire--the reference wire--produces a steady 5-volt signal only. Consult the wiring diagram in your vehicle's service manual if necessary. You might want to insert pins into the wires to attach the probes to for this test.

Step 6

Disconnect the vacuum hose from the MAP sensor and attach a hand vacuum pump.

Step 7

Turn the ignition key on, but do not start the engine.

Step 8

Read the voltage or rpm. With zero inches of mercury (in-Hg) vacuum, you should get between 4.5 and 5 volts, or between 300 and 320 rpm.

Step 9

Apply 5 in-Hg of vacuum. You should be reading now about 3.75 volts, or between 275 and 295 rpm.

Step 10

Apply 20 in-Hg of vacuum now. This time you should be reading about 1.1 volts, or between 200 and 215 rpm. Compare your readings with those specified in your vehicle service manual. If the values are out of specification, replace the MAP sensor.

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