TEST 4: 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 ECM. Here's a little background information to help you understand what you'll be doing in this test step.
In a nutshell, the MAF sensor measures the amount of air the engine is breathing and converts this measurement into a DC voltage signal the PCM (Powertrain Control Module=Fuel Injection Computer) can use to calculate Fuel Injection. Specifically, the air that the engines breathes at idle (lets say an idle of 800 RPM's) is less than the air the engine needs at 2,700 RPM's. Therefore, the DC Voltage signal the MAF sensor will output to the PCM is less at idle than at 2,700 RPM.
The important thing to know when verifying the MAF signal is not to look for a specific MAF signal voltage reading a specific RPM but to verify that the DC voltage signal rises smoothly when the engine is accelerated and returns to its initial DC readings when the engine returns to its idle.
The two most common results you'll get from a BAD mass airflow sensor (MAF) sensor. are: 1) NO VOLTAGE SIGNAL AT ALL for the MAF signal output or 2) erratic voltages that don't correspond to the amount of air the engine is breathing as you accelerate or decelerate the engine.
In case you're wondering, the working DC Voltage range of the MAF signal is between .5 and 4.5 Volts. At idle the average voltage will be abut .9 to 1.6 volts depending on engine idle conditions. At around 2500 RPMs, you should a Voltage of 1.7 to 2.3 Volts DC.
Start the engine and let it reach it's normal operating temperature. You'll be using the voltage reading you will obtain at idle as a base to diagnose the MAF sensor.
IMPORTANT: The MAF sensor must be connected to its connector to perform this test.
There are the test steps:
- With the key in the OFF position.
- With a suitable tool connected to the RED multimeter lead, probe the D circuit of the MAF sensor connector shown in the photo.
- Put the multimeter in VOLTS DC mode.
- Connect the BLACK lead to the battery (-) negative terminal.
- Start the already warmed up engine.
- Note the Volts 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.
- Accelerate the engine as you watch the multimeter's voltage readings.
- The voltage numbers should correspond to the amount of acceleration.
- Repeat this as often as you need to verify that the voltage numbers on the multimeter rise smoothly every single time.
- If the MAF sensor is good, these readings will not spike up and down crazily but will correspond to the amount of air the engine is breathing at the different RPMs you're accelerating the engine to.
Did the signal rise smoothly with each increase in engine acceleration and stay steady at idle?
CASE 1: The multimeter registered the indicated voltage -The MAF sensor is functioning correctly.
Here's why: If the MAF sensor were defective, your multimeter would have registered a voltage value that would've stayed stuck at one number no matter how much you revved up the engine.
CASE 2: The multimeter DID NOT register the indicated voltage -The MAF sensor is defective. Replace MAF sensor.
If this info really saved the day, buy me a beer!