TEST 3: Testing The MAF Signal Circuit
Before you jump into this last test, let's go over some basic working theory of how the mass air flow (MAF) sensor works that'll help you to breeze thru' it.
The MAF sensor's job is to measure the amount of air the engine is breathing at any given RPM and to convert this measurement into a Hertz frequency reading (as measured by a digital multimeter that can read Hz frequency) the PCM can use to calculate fuel injection. Therefore the more air the engine breathes, the higher the Hertz frequency that the MAF sensor will output to the PCM.
So keeping this in mind, the Hertz frequency reading will be higher at 2500 RPM's than at 800 RPM's. On your multimeter, this Hertz (Hz) reading will progress in a smooth way as you accelerate the engine and decrease in the same way as the engine decelerates. Now, in testing the MAF sensor, you won't be looking for a specific Hertz (Hz) number at a specific RPM, but for crazy fluctuations in the signal that don't correspond to the amount of air entering the engine or no signal at all. OK, crash course is over, let's start testing.
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 a suitable tool connected to the red multimeter test lead, probe the wire identified with the number 3.
Put the multimeter in Hertz frequency (Hz) mode
Don't have a digital multimeter that can read Hertz frequency? See my recommendations here: Buying A Digital Multimeter For Automotive Diagnostic Testing.
Connect the black multimeter test lead to the battery negative (-) terminal.
Start the already warmed up engine
Note the Hertz reading on your multimeter at idle. Now, to give you a reference point, this Hertz value usually hovers around 10 to 14 Hertz 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. At around 2,500 RPM's this Hertz reading will oscillate around 70 Hertz.
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 4 of this test.
Every time you rev up/down the engine, the Hertz numbers on your multimeter should rise/decrease.
If the MAF sensor is defective, the Hertz value will stay stuck in one number.
Let's take a look at what your test results mean:
CASE 1: If the Hertz (Hz) signal rose smoothly and decreased smoothly as the engine was accelerated and decelerated respectively, then this indicates that the mass air flow (MAF) sensor is working correctly.
CASE 2: If the Hertz (Hz) signal DID NOT rise smoothly nor decreased smoothly as the engine was accelerated and decelerated respectively, then this indicates that the mass air flow (MAF) sensor is bad. Replace it.
TEST 4: Testing The MAF Reset Signal
The last part of the Mitsubishi volume air flow sensor (mass air flow sensor) is the verify that it's producing a Reset Signal the fuel injection computer can use.
This is just an On/Off type DC voltage signal that turns ‘On’ when the throttle opens and turns ‘Off‘ when the throttle closes.
OK, let's get started:
Place the multimeter in Volts DC mode.
Connect the red multimeter test lead to the wire identified with the number 7 (see photo above) using an appropriate tool.
Connect the black multimeter test lead to a good Ground point on the engine or to the battery negative terminal.
Turn the key to the ON position and start the your Mitsubishi car (or Chrysler Sebring or Dodge Stratus).
Manually open and close the throttle from the engine compartment as you observe the multimeter. Now, since the engine will be running; take all necessary safety precautions.
With the engine at idle and the throttle closed, your multimeter should register around 1 Volt DC or less. This is the ‘Off’ voltage reading.
When you open the throttle about 1/3 or more, your multimeter should register 6 to 9 Volts. This is the ‘On’ voltage reading.
When you let go off the throttle and it closes (causing the engine to return to idle) the multimeter should register the voltage you observed in step 6 of this test.
Let's analyze your test result:
CASE 1: The multimeter registered the ‘On’ and ‘Off’ voltage readings: This indicates that the mass air flow sensor is creating the Reset Signal. Your MAF sensor is working properly.
CASE 2: The multimeter DID NOT register the ‘On’ and ‘Off’ voltage readings: Recheck all of your multimeter connections and retest. If the indicated voltages are still not present then the MAF sensor on your Mitsubishi vehicle is not functioning correctly. Replace the MAF sensor.
TEST 5: Mitsubishi MAF Sensor Signal Wave Form
If you have access to an oscilloscope, this is what the mass air flow (MAF) sensor waveform looks like at idle.
If the MAF sensor is good then at idle and at any RPM, the waveform will stay perfectly formed. Also, as you accelerate the engine, the wave-length will become shorter while the wave amplitude stays the same.
Now, if the MAF sensor is bad, the waveform will have missing pieces or no waveform will be formed at all.
If this info really saved the day, buy me a beer!