Mass Air Sensors (MAF) - Automotive

The Mass Air Flow Sensor is probably the best way to measure
the amount of air an engine takes in (engine load). This sensor
not only measures the volume of air but also compensates for its
density as well. Ford, GM, and many imports are using engine
control systems based around this sensor.

There are two common designs of MAF sensors used in today's
vehicles. One produces a variable voltage output (analog) and
the other produces a frequency output (digital). In either case
their operation is similar. Both outputs can be measured by a
scanner or a digital volt/ohm meter (dvom) that can measure
frequency.

Both designs work on the "hot wire" principle. Here's how they
work. A constant voltage is applied to the heated film or heated
wire. This film or wire is positioned in the air stream or in an air
flow sampling channel and is heated by the electrical current that
the voltage produces. As air flows across it, it cools down. The
heated wire or film is a positive temperature coefficient (ptc) resistor.

This means that it's resistance drops when it's temperature drops.
The drop in resistance allows more current to flow through it in order
to maintain the programmed temperature. This current is changed
to a frequency or a voltage which is sent to the computer and
interpreted as air flow. Adjustments for air temperature and humidity
are taken into consideration since they also affect the temperature
of the heated wire or film.

GM (Bosch) Hot Wire MAF Sensor
Humidity always affects the density of air since humid air is less dense
than dry air. No other compensation is therefore needed for this factor.
Air temperature affects density since colder air is more dense than
warmer air. Many systems use an air temperature sensor to compensate
for this factor since similar amounts of air can enter an engine at
different temperatures. Some MAF sensors use an internal "cold" wire
to send ambient temperature information to the computer. Some use
an intake air temperature sensor in the manifold or the intake piping.
This sensor is almost always ntc in design (negative temperature
coefficient). That is, it's resistance goes up as air temperature goes
down. This "thermistor" works just like a coolant temperature sensor
and usually has identical resistance to temperature values. By the way,
these values are very different from manufacturer to manufacturer
and are available in most repair manuals. They are also programmed
into scanner software.

Ford Hot Wire MAF Sensor
Now, as we discussed, the MAF sensor sends either a variable
voltage or a changing frequency to the computer. The computer
is programmed to accept this information when the car is running
in any mode. For example, idle rpm will send a low voltage or low
frequency and a high revving engine will send a high voltage or
high frequency to the computer along a specific wire (the MAF
signal wire). If the signal is not present when it should be and
within a programmed parameter, say high voltage at high throttle
opening, the computer will set a code.

So, there are several things to consider whenever there is a code
which points to the MAF sensor as the problem:

1. Derive the code(s) by the manufacturer's recommended method.

2. Look up the code(s) in a service manual.

3. Read the explanation(s) carefully!

4. A code that indicates an out of range signal is often an
indication that another sensor, like the throttle position sensor
or the rpm input signal is contradicting the MAF signal. The
cause might be the other sensor or signal being out of adjustment
or faulty.

5. A code that indicates a low MAF signal may be set by various
problems.
These include the following:

1. A bad MAF sensor (internal fault)

2. Any wire on the MAF sensor circuit including:

A. The 12 volt feed wire which connects the MAF to the
battery through the ignition switch or through a relay as
in many GM applications

B. The MAF ground wire

C. The output wire

D. The MAF or computer connectors

E. The computer

Note: The GM Bosch style system used on 5.0L/5.7L Firebirds,
Camaros, and Corvettes have a hot wire "burn-off" feature that
uses a relay to burn any impurities off of the hot wire. This
system will set a code if the ecu controlled side of the relay fails.


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