TEST 1: Testing The Power (12 V) Circuit
The very first thing that we'll do is check that the MAF sensor is receiving 12 volts. The procedure I recommend for you to use (to accomplish all of the tests below) is to use a test probe that pierces thru' the wire's insulation (Wire Piercing Probe). DO NOT insert anything into the female terminal.
Whatever method you use, the key here is not to damage the female terminal or the wire. Again, be careful. Use common sense and take all necessary safety precautions.
- Put the multimeter in VOLTS DC mode.
- Do not disconnect MAF sensor connector from the MAF sensor.
- With the red multimeter test lead and an appropriate tool, probe the MAF sensor connector's C circuit as shown in the photo.
- With the black multimeter test lead probe the battery (-) negative terminal.
- Turn Key On with the engine Off.
You should see 12 Volts on the multimeter. Do you have 12 volts?
CASE 1: The multimeter registered 12 Volts -All is good in the neighborhood, GO TO TEST 2.
CASE 2: The multimeter DID NOT register 12 Volts -The MAF sensor is not the problem. Without this voltage the MAF sensor will not work.
TEST 2: Testing The Ground Circuit
In the previous test we checked that the sensor was receiving 12 Volts. Now we'll check that the MAF sensor is getting a good ground.
As you can see in the photo, this is the middle wire of the MAF sensor's connector. You can test this circuit with the connector connected to MAF sensor or not. I prefer to test it with it connected and with a wire-piercing probe.
- Turn key to the OFF position.
- Multimeter in VOLTS DC mode.
- With the black multimeter test lead and a wire-piercing-probe, probe the MAF sensor connector's B circuit as shown in the photo.
- Connect red lead to the battery (+) positive terminal.
You should see a voltage of 12 Volts. Do you have that?
CASE 1: The multimeter registered 12 Volts -All is good in the neighborhood, GO TO TEST 3.
CASE 2: The multimeter DID NOT register 12 Volts -The MAF sensor is not the problem. Without this path to ground, the MAF sensor will not work.
TEST 3: Testing The MAF Signal
Now that the basics have been checked, we'll check the MAF signal coming out of the sensor and going to the fuel injection computer.
Start the engine and let it reach it's normal operating temperature. You'll be using the Hertz reading you will obtain at idle as a base to diagnose the MAF sensor.
The MAF sensor must be connected to its connector to perform this test.
- With the key in the OFF position.
- With a suitable tool connected to the RED multimeter lead, probe the A circuit as shown in the photo.
- Put the multimeter in frequency (Hz) mode (don't have a digital multimeter that can read Hertz frequency? Click here to see my recommendations: Buying A Digital Multimeter For Automotive Diagnostic Testing).
- Connect the BLACK lead to the battery (-) negative terminal.
- Start the already warmed up engine.
- Note the Hertz reading on your multimeter at idle. This reading may be stable (with only small fluctuations) or unstable with very extreme fluctuations. No matter what the instability in the reading, this will be your base reading.
- Manually accelerate the engine from the engine compartment as you watch the multimeter's frequency readings. The Hertz Frequency readings should increase.
- When you let go off of the throttle and the engine returns to idle, the Hertz reading should come down to the base Hertz reading you observed in step 6 of this test.
- Repeat this as often as you need to verify that the Hertz numbers on your multimeter rise and decrease smoothly every single time.
- If the MAF sensor is good, these readings will not spike up and down crazily but will smoothly increase as you manually accelerate the engine and smoothly decrease as you let the engine return to idle.
Did the signal rise smoothly with each increase in engine acceleration and stay steady at idle? Oscilloscope users- the waveform should not have missing parts or erratic patterns at idle or at acceleration. (see photo 2 of 2 to see MAF sensor oscilloscope waveform)
CASE 1: The multimeter registered the indicated Hertz values -The MAF sensor is OK and functioning correctly.
CASE 2: The multimeter DID NOT register the indicated Hertz values -The MAF sensor is BAD and needs to be replaced.
If this info really saved the day, buy me a beer!