I searched YouTube on how to test an MAF sensor and a guy said that if you disconnected it while idling, if the car dies, you MAF is working properly. I am dealing with a CEL, and have a P010 code Mass Air Flow sensor "A" Circuit Range/Performance.


Naturally, I tried this and the car kept running. Can anyone confirm or deny the YouTube test?


Thanks for any help.

 

Tried the same yesterday and my engine idled just fine after disconnecting MAF.
Some cars may stall while others may run fine.
Perhaps it has something to do with how the engine and ECU is designed.
If a sensor craps out some ECU's will replace the missing sensor input with data from a table stored in the ECU. This won't be optimal, but it will enable the car to at least limp home.
This youtube guy's statement is strictly only valid for his car and may not apply to the rest of the world.

The best way to test the MAF is to find out which wire belongs to the sensor output and measure the voltage on that wire. If it has a low voltage at idle that steadily climbs upwards as you increase engine RPM then the MAF should be fine.

 

Wow. Thanks for the heads up. MAF only has three pins. Constant 5V, ground, and the output (signal sent to ECU. )


Dusting off the Fluke meter now.

 

Insert a straightened out paperclip into the harness where the signal output wire goes and measure while the harness is connected.
Be super careful not to short the 5 volt supply to ground. That will fry your ECU.

 

Thanks!


Yeah, YouTube.


The heavier gauge red wire to the female connector read 14.3 VDC with the engine running. The outside wire had continuity with the meter lead stabbed into the neg. batt. terminal...obviously ground. The middle wire is green and no doubt the signal (output to ECU). Believe me. I WILL be SUPER careful! Thank you.

 

Sounds like you're good to go.
Even though the output voltage varies with varying rpm the MAF may still NOT be able to correctly measure the amount of air passing through it. If the heated wire(s) are covered (read: insulated) with oil or dirt, more air will enter the engine than what is measured and you'll get a lean condition.

 

well, I measured the sensor output, and it read 0.5 volts at idle and increased when I opened up the throttle. If the MAF works, why did the OBDII read DTC P0101 when I took the car in for smog check? I have cleaned the MAF properly every 35,000 miles with CRC and the MAF wire is clean. Regarding back probing the connector....you can read the signal output with a voltmeter with one lead attached to the paper clip, and the other lead stabbed into the neg. battery terminal to prevent frying your ECU. A lean condition is very noticeable (no guts, rough idle) but my s40 is running fine. Tempted to at least try the BOSCH reject on eBay just to POSSIBLY eliminate P0101.


If I could simply trust either the Stealership or the local yokels to fix my car, I would pay top dollar. However, my experience with the last two garages mirrors some of the horror stories I have read on this Forum. I am an IBEW Electrician, and if I were as incompetent as many auto-technicians, people would get hurt and property would catch fire.

 

Fault codes may indicate something else than what it actually tells you.
The P0101 code may have several possible causes.
I did some googling:

Basically this P0101 diagnostic trouble code (DTC) means that there is a problem with the Mass Air Flow (MAF) sensor or circuit. The PCM detects that the actual MAF sensor frequency signal is not within a predetermined expected range of the calculated MAF value.

Potential causes for this trouble code may include:

• Dirty or contaminated mass air flow sensor
• Failed MAF sensor
• Intake air leaks
• MAF sensor electrical harness or wiring problem (open, shorted, frayed, poor connection, etc.)
• Clogged catalytic converter on some models (GMC/Chevrolet mainly)

Note that other codes may be present if you have a P0101. You may have misfire codes or O2 sensor codes, so it's important to take a "big picture" look at how the systems work together and effect each other when doing a diagnosis.


Possible diagnostic and repair steps include:

• Visually inspect all MAF sensor wiring and connectors to make sure they are intact, not frayed, broken, routed too close to ignition wires/coils, relays, motors, etc.
• Visually inspect for any obvious air leaks in the air intake system
• Visually *closely* inspect the MAF sensor wires or film to see if you can see contamination such as dirt, dust, oil, etc.
• If the air filter is dirty, replace it
• Carefully clean the MAF using MAF cleaner spray is generally a good DIY friendly diagnostic/repair step
• If the air intake system has a mesh in it, make sure that is also clean (VWs mainly)
• Loss of vacuum to the MAP sensor can trigger this DTC
• A low minimum air rate through the sensor bore may cause this DTC to set at idle or during deceleration. Inspect for any vacuum leaks downstream of the MAF sensor.
• Use a scan tool to monitor real-time sensor values from the MAF sensor, O2 sensors, etc.
• Check for Technical Service Bulletins (TSBs) for your particular make/model in case of known issues on your vehicle
• The barometric pressure (BARO) that is used in order to calculate the predicted MAF value is initially based on the MAP sensor at key ON.
• A high resistance on the ground circuit of the MAP sensor can cause this DTC to set
• Do an exhaust backpressure test to determine if the catalytic converter may be clogged

If you do need to replace the MAF sensor, we recommend using an original equipment OEM one from the manufacturer rather than buying an aftermarket part.

As for checking a bad ground connection, try running a wire directly from the battery negative to the ground wire terminal at the harness.
It would be helpful to have an oscilloscope and look at signals. Then a signal not properly referenced to ground would be revealed.
Let's say the MAF is OK but the ground is not in working order. The signal would be present but it would not be properly referenced to ground. Perhaps then the ECU will see a signal that is outside of it's expected range. (Just a theory).

 

Nice work, Brother Emtor!


You are really dialed into some serious knowledge. Must admit it never really occurred to me that a faulty ground might get by my Fluke 332. I think your theory is more factual than just theory. Although math doesn't lie (ever!)....even N. Tesla died knowing mankind doesn't understand everything regarding electronics. Bro Emptor...YOU ROCK!

 

Thanks :-)

I've been wrong before, but I googled "MAF bad ground" after coming up with my theory, and it turns out a lot of people had their MAF issues solved by fixing a bad ground.

Think of it as a one foot stick planted on the floor. Let's say you had a ECU that measured the height of the stick the only way it can, namely by looking at the end of the stick. It would see a one foot stick.
Now, place the stick on a chair and the ECU would see a one foot stick plus the height of the chair,-that's three feet. Same goes for measuring voltage. The ECU sees the correct voltage plus the difference in voltage between the harness ground contact point and battery negative. The voltage drop in the ground wire will never be zero, but if it's too high it will add to the output voltage so much so that the engine gets a screwed up amount of fuel.

Since we're on about bad ground here,-a corroded or loose battery negative terminal would do the exact same thing . . . giving the ECU a faulty MAF-signal.

Perhaps the very first step in diagnosing ANY problen with an engine should be to clean the battery terminal connector and the terminal itself.

 

Most of what's found on the internet are tutorials for dummies. Sometimes I wonder who are the dummies? The ones who writes these things or the ones who are supposed to read them?
Well, here's some stufff on MAFs that is quite informative:

Failure in Spec: What Happens When a Mass Airflow Sensor Lies - Automotive Service Professional


I also found a schematic that shows how MAFs work. Might be useful if one wishes to understand the principle behind these things:



The four resistor thingy is a Wheatstone bridge which is used for measuring the value of an unknown resistor.

https://en.wikipedia.org/wiki/Wheatstone_bridge

The triangle thingy is a differential amplifier or a comparator that compares the signals on the two inputs and outputs a bias volltage to a power transistor which controls the current through the resistor bridge.

https://en.wikipedia.org/wiki/Differential_amplifier

Now we can see what would happen if we inserted a resistance (bad ground) in the ground wire. It would mess up the voltages going to the diff-amp, which makes it bias the power transistor wrongly which in turn sends the wrong current though the bridge. The result is an output signal to the ECU that no longer correctly represents the mass of air flowing through the MAF.