How the MAF sensor works...

The MAF or Mass Air Flow sensor is an ingenuous device that measures incoming air to the engine. This device uses a hot platinum wire to detect minute changes in the incoming air. The more air coming in the more the MAS sensor has to cool the wire to maintain a specific temperature. For you, the general consumer and DIY mechanic there are specific and easy things you can do to correct a large percentage of issues, saving loads of money.

Theory of Operation

The Mass Air Flow (MAF) sensor senses the amount of incoming air (Volume) into the engine. This sensor does not regulate the incoming air, this is done by the engine throttle plates. The MAF merely senses incoming air and relates a signal to the ECM. Air flow sensors come in three types. The Vane Air Flow sensor, Hot Wire MAF sensor and Hot Film MAF sensor. They all perform the same function but their operation is quite different.

The VAF sensor measures the amount of air flow into the engine with a spring-loaded air flap/door attached to a variable resistor (potentiometer). VAF sensors measure air volume and not mass. The incoming air strikes or pushes against the internal air flap on the VAF sensor, which also moves the variable resistor’s sensing arm (wiper arm). As air flows into the engine the mechanical air flap rotates further, causing the wiper arm to contact a series of resistors, changing the voltage signal output.

NOTE: The output signal on some systems is directly proportional (Motronic) to the incoming air, while in others the signal is inversely proportional (L-Jetronic). In other words, if the VAF electronic signal is directly proportional, then as the air flow increases the voltage output also increases and if the system is inversely proportional then as the air flow increases the output voltage signal decreases. Directly proportional systems (Motronic) output a low voltage at idle with voltage increasing as air flow increases. Inversely proportional systems (L-Jetronic) output a high voltage at idle with the voltage decreasing as air flow increases. The ECM is programmed to the particular system and increases injector pulsation as more air enters the intake manifold.

• The VAF sensor has an air-fuel adjustment screw, which opens or closes a small air passage on the side of the VAF sensor. This screw controls the air-fuel mixture by letting a metered amount of air flow past the air flap, thereby, leaning or richening the mixture. By turning the screw clockwise the mixture is enriched and counterclockwise the mixture is leaned. In addition to the regular air flow measuring function, some VAF sensors also employ an air temperature sensor (IAT sensor) and a fuel pump switch.

The IAT sensor is found inside the VAF casing and has the same electrical characteristics as a regular air temperature sensor. The VAF sensor flap also closes a set of contacts that activate the fuel pump relay coil (circuit opening relay). The contacts are actually closed as soon as the smallest amount of air pushes on the air flow flap. Once this happens the fuel pump starts running and the engine starts.

One of the main drawbacks of the VAF sensor is that it measures volume of air and not weight. As air temperature changes so does its weight. There are more air molecules present when the air is colder than when it is hotter. As air temperature decreases, more air is absorbed by the engine, so there are drastic changes needed in the air fuel ratio (depending on the temperature of the air). The air temperature sensor inside the VAF somewhat compensates by signaling the ECM of any changes in air temperature.

• The HOT WIRE MAF sensor is a fully electronic unit. It senses the amount of air flow into the engine by measuring the amount of current needed to maintain a constant temperature through a very thin (70 micrometers) platinum hot wire. Hence the name hot wire MAF sensor. It also measures air by weight, since it takes into consideration the air temperature as well.

This sensor works as follows. As the air enters the intake manifold through the hot wire MAF sensor it cools down the platinum wire, which is heated at a very precise temperature. When the MAF circuitry senses the platinum wire cooling down it increases the amount of current flow through the hot wire trying to maintain a specific temperature. This varying current flow is then converted to a voltage output signal by the MAF electronic circuitry and is used as an air flow indicator by the ECM. Hot wire MAF sensors have a signal that is directly proportional to air flow. So as air flow increases so does its voltage signal output.

This sensor sometimes employs a mixture screw, but this screw is fully electronic and uses a variable resistor (potentiometer) instead of an air bypass screw. The screw needs more turns to achieve the desired results. A hot wire burn-off cleaning circuit is employed on some of these sensors. A burn-off relay applies a high current through the platinum hot wire after the vehicle is turned off for a second or so, thereby burning or vaporizing any contaminants that have stuck to the platinum hot wire element.

• The HOT FILM MAF sensor works somewhat similar to the hot wire MAF sensor, but instead it usually outputs a frequency signal. This sensor uses a hot film-grid instead of a hot wire. It is commonly found in late 80’s early 90’s fuel injected vehicles. The output frequency is directly proportional to the amount of air entering the engine. So as air flow increases so does frequency. These sensors tend to cause intermittent problems due to internal electrical failures. The use of an oscilloscope is strongly recommended to check the output frequency of these sensors. Frequency distortion is also common when the sensor starts to fail. Many technicians in the field use a tap test with very conclusive results. Not all HFM systems output a frequency. In some cases, this sensor works by outputting a regular varying voltage signal.

NOTE: A little known type of MAF sensor is the Karman-Vortex. Inside this sensor is an LED (light source), phototransistor, and a mirror (mounted on a spring base). The light beam from the LED is reflected back-and-fourth from the mirror and is picked up by the phototransistor. As the air flows though the sensor, turbulence or vortices are created inside it. These vortices cause the mirror to vibrate depending on the flow of air. It is these vibrations which are picked up by the phototransistor and converted into a frequency, which is sent to the ECM as a measure of air flow.


VAF sensors are mechanical in nature. Their measuring element (wiper contact, pivot bushings and sensor resistors) get worn out over time. A binding air flap door is also a major problem with these sensors. The air flap mechanism is extremely precise and does not tolerate any misalignments. Always make sure that the air flap can travel freely all the way to its full open position. A broken air duct pipe will also render the VAF useless, since most of the air will be bypassed and enter though the broken duct hole. A thorough air duct check is always a good idea. The resistors also tend to wear out over time, sending the wrong voltage signal to the ECM. This will certainly throw off the air-fuel ratio.

 NOTE: It is important to remember that most VAF systems do not have a potentiometer type TPS. These systems use a throttle switch sensor as an input for throttle position. Throttle switch sensors only signal the ECM of a closed or WOT throttle, but nothing in between. Therefore, the VAF sensor is the main indicator of throttle opening and load demand. Without the VAF signal input the engine will start and idle but will probably not be able to do much else, since the ECM does not know how much air is entering the intake manifold.

The air temperature sensor and the fuel pump switch are the other reasons for VAF failures. This fuel pump switch activates the fuel pump relay and its contacts also wear down over time, causing a no start-no no-fuel pressure condition. A simple continuity test will quickly reveal a bad fuel pump switch. The air temperature sensor also follows the same electrical characteristics of a normal IAT sensor and the same ohms to temperature tables could be used for diagnostics.

Hot Wire MAF sensors are very prone to sensing wire element contamination. A condition referred to by many technicians as “growing hairs” happens when debris, dirt from cheap air filters and outside air stick to the sensing wire element, shielding it from the incoming air. This shielding effect prevents the MAF sensor from correctly measuring the air flow and mass causing severe air-fuel ratio control problems. An ECM not in control while at pre-load is a strong indication of a dirty MAF.

In any fully electronic device, the electrical connections and circuitry fails after a certain lifespan of operation. An output signal voltage test will surely reveal a bad MAF sensor. Hot Film MAF sensors tend to get electrical damage more often that the other type of sensors. The tap test ,as mentioned before, is a useful and simple procedure that usually reveals a bad hot film MAF sensor. Contamination or a broken air duct is also a problem for this sensors.


• The first step common to all MAF or VAF sensors is to perform a thorough visual check of the air duct to detect any breaking of the rubber air duct. Secure and tight clamps are a must. Do not overlook this simple procedure as it is common to find broken ducts that are hard to see at first sight.

• The second common ailment to check for is a vacuum leak. Vacuum leaks have a big effect on MAF operation, since it provides a way for the air to enter the engine through a passage other than the throttle bore. This illegal air is never measured by the MAF and never compensated for by the ECM with extra fuel added. A lean mixture is usually the result. Check the power feed and grounds going to the air flow sensor.

VAF sensor

• Check for an air flap binding. The air flap should travel free through its entire travel range. Stick your fingers through the air inlet opening and push the air flap, sensing for any binding or mechanical problems.

• With KOEO the fuel pump contact should be checked by pushing the air flap very lightly and probing with a VOM on the fuel pump contact output pin to verify the output voltage. A failed continuity test of the fuel pump contacts is also a good failure indicator.

• With KOEO, verify the wiper arm output voltage (VAF output) with the air flap closed through the full open position. Compare readings to the correct specifications. Start the engine (KOER) and measure the output voltage at idle. Compare the measurement to correct specification. This test will detect a misadjusted air flow sensor. Remember that vacuum leaks or a broken air duct will derail the readings, making it seem like the air flow sensor is out of adjustment.

• Verify proper IAT sensor reading and compare to correct specs.

Hot Wire MAF sensor

• Check the voltage signal output KOEO and at idle.Compare to proper specifications. Off specs readings are common

• Accelerate the engine and look for a steadily raising voltage output.

• If the voltage output is wrong or no voltage at all, remove MAF and inspect the hot wire element. Look for a broken or contaminated hot wire element. Clean or replace as necessary. If MAF is dirty or contaminated check the operation of the burn-off circuit for correct operation. The burn-off relay should be activated for a second or so after the engine is shut off. Do not expect to see a RED HOT wire all the time. The hot wire element can be very hard to see, so use a multi-meter instead.

NOTE: A strong indicator of a dirty hot wire MAF sensor is the BARO scanner PID. The BARO or barometric sensor was eliminated between the late 1980’s and early 1990’s. Newer systems deduce the barometric pressure from the MAF sensor reading at WOT. The ECM simply assumes the correct barometric reading at WOT, since that is the time when the atmospheric and intake pressure are equalized. A dirty MAF sensor will be reflected on the BARO PID, making the ECM operate as if at higher altitudes. The results are disastrous to the A/F ratio control, with usually a lean condition as a result. Newer systems also update the BARO reading during brief acceleration periods. In some systems, a quick check can be performed when the engine is not starting. Simply disconnect the MAF and start the engine. A non-starting engine that starts when the MAF is disconnected reveals a defective MAF unit. These systems will start on TPS alone and substitute the MAF reading with a good pre-programmed value.


• Hot film units usually output a frequency. Check the frequency at KOEO and at idle. Compare to correct specifications. A KOEO off-calibration MAF sensor is most likely defective, since it already started out with the wrong frequency setting.

• Measure the output frequency throughout the entire RPM range. Be careful not to over-rev the engine.

• The outside casing of some of these sensors tend to melt or deform when defective. Perform a visual inspection and look for traces of melted material. In no other automotive sensor is the TAP test more important than with the HFM sensor. A missing of stalling engine when the HFM is tapped points to a defective unit.

• Using a scope, probe the output frequency and verify correct MAF sensor waveform integrity. A slight rounding off of the square waveform edges is sometimes normal. Experience and a good known waveform database are very useful. These guidelines and a careful understanding of the particular system you are working on should point you in the right direction.




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