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
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
• 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
• 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
• 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.
CONDITIONS THAT AFFECT OPERATION
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
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.
• 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
• 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
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
• 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
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 MAF sensor
• 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