How the MAF Sensor works:
In this page you will learn about MAF ( Mass airflow ) sensors, how they work, how to diagnose problems related to their operation, and a step by step guide to replace them.
- What is a MAF sensor?
A Mass airflow sensor is a device used by the engine control computer to measure the amount of air entering the engine through its air intake system.
- How exactly does it work?
There are two kinds of MAF sensors, platinum wire ( hot wire ), and nickel foil grid ( hot film).
They vary slightly in their design, but their working principal is the same; they use electrical current to measure airflow, the computer sends electrical current to the MAF sensor's sensing element to heat it and keep its temperature higher than the temperature of the incoming air.
In a hot-film MAF sensor, the film is heated 170 degrees F warmer than the incoming air temperature.
In a hot-wire MAF sensor, the wire is heated to 210 degrees F above the incoming air temperature.
As air passes through the element, it cools it and thereby increases the amount of current needed to heat it up again.
Because the necessary current varies directly with the temperature and the density of the air entering the intake, the amount of current is directly proportional to the air mass entering the engine.
This information is fed into the computer and the fuel mixture is directly controlled according to the conditions.
- Can a MAF sensor be tested with a digital voltmeter?
Yes, the MAF sensor can be tested with a voltmeter, you will need to know which terminal is the correct one to probe, some MAF sensors only have three wires, making them the easiest to test, for example:
In a three wire MAF sensor, one terminal will be the negative wire, the next terminal will be the signal wire, and the last terminal will be the current wire, the way to know which terminal is the signal, is to connect the negative terminal of your voltmeter to a good ground in your vehicle, next make sure that there are not any wires that may get tangled in the fan or belt of the car and start the engine,, probe the terminals in the MAF sensor,the ground terminal will show nothing , one of the remaining two will have constant 5 volts, and the signal wire will vary its current depending on the engine speed, between 0.6 volts at idle to around 4.5 volts at wide open throttle.
Once you have the positive prove of your voltmeter in the signal wire, proceed to increase and decrease the RPM's of your vehicle, if the signal increases as you increase the engine speed, the sensor is working, if it doesn't change, the sensor is bad.
- MAF sensors with 4 or five terminals usually have the air temperature sensor combined in a single unit, we will show you how to test the air temperature sensor in a different page.
If you continue reading, you will learn that just because the MAF sensor changes as the RPM increases, it doesn't mean that it is not malfunctioning, we will show you why, in the mean time we want to let you know what the most common symptoms of a bad MAF sensor are:
1- Sluggish engine performance
2- Hesitation under acceleration
3- Lean running condition.
4- If stuck on high signal the car will run rich
Below are a few samples of problems we ran in to with different vehicles, The first one for us to mention in this page is a 1993 VW Jetta with a 2.0 L engine.
The girl who owns it brought it to us complaining that the car was surging when taking off, she told us that the car was acting like if it had a clogged fuel filter, plus it didn't seem to have the power it once had.
Our scanner starts on 1996 and newer for the European vehicles, making it harder for us to diagnose her problem.
The car didn't have the check engine light on either, so as far as the computer was concerned the problem was not bad enough, but it was bad enough for the owner to notice it, and for us too once we took it for a test drive.
We suspected a bad MAF sensor being the problem based on its symptoms, even though the sensor was working, it seemed sluggish, so we decided to pull it off to inspect it, we know by experience that the MAF sensors are prone to get contaminated once the air filter is not doing its job correctly, or as we see it often by the lack of filtration offered by aftermarket air intake systems.
These are the steps involved in removing a MAF sensor in a VW Jetta:
- Push on the tabs and unplug the MAF sensor electrical connector
2- Remove the two tamper proof torx screws that attach the MAF sensor to the air intake tube and remove the sensor from the vehicle.
- As you can see in this image, the MAF sensor is contaminated with engine oil, it was cause by the back-pressure in the engine due to high mileage, when a vehicle has higher back-pressure than normal, the PCV valve is not able to recycle the crankcase pressure back in to the engine, causing the excess crankcase pressure to exit through the breather tube that leads to the air intake tube, because its proximity to the MAF sensor, the oil expelled by the engine coated the film inside the sensor causing it to have a very sluggish performance.
- We removed all the oil from the MAF sensor by spraying it with carb cleaner, (you can also buy MAF sensor cleaner at your local auto parts store )once it was clean, we allowed it to dry as we cleaned the electrical connector, once the sensor was dry, we installed it back on the car and took it for a test drive, the problem was gone, no more hesitation under acceleration and no more lack of power, it felt like we could pass everybody if we wanted to.
As you just witnessed, MAF sensors can malfunction when they get contaminated, but you can't go to the store and buy a new sensor every time, we just showed you how to clean it, and if the problem was just contamination, once is clean it will work as good as new.
In the next page we will show you the steps needed to replace an air-flow meter ( MAF sensor basically) in a Nissan pick-up.
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