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If you have a Lexus vehicle 1995 and older, chances are, you will have not an MAF< but an AFM. There is really no difference in the two in what they do, but the technology is much different. The MAF is all electronic, and the AFM uses hot wires to detect air flow and intake manifold pressure.
This AFM is also known as the hot wire sensor.
The AFM measures the mass of air entering the cars air intake system. It works just like the hot wire anemometer, (which determines air velocity). The AFM works by heating the hot wire with electrical circuit from the vehicles air stream, very much like a toaster wire. The wires electrical resistance increases as does the while the wires temperature increases, which, in doing, limits electrical current flowing through the circuit. When air flow passes the wire, the wire cools, decreasing it's resistance, until the resistance reaches Equilibrium again. All of this information is sent directly to the ECU, which is the engine computer controller.
If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass air flow. Unlike other sensors, the hot wire responds directly to air density. This components capability is made for the gasoline run engines, which fundamentally responds to air mass, not air volume. (Or Stoichiometry)
***COPYRIGHT 2013 by Jonathan H. (No plagorizing information)
Deeper into the subject...
This AFM is also known as the hot wire sensor.
The AFM measures the mass of air entering the cars air intake system. It works just like the hot wire anemometer, (which determines air velocity). The AFM works by heating the hot wire with electrical circuit from the vehicles air stream, very much like a toaster wire. The wires electrical resistance increases as does the while the wires temperature increases, which, in doing, limits electrical current flowing through the circuit. When air flow passes the wire, the wire cools, decreasing it's resistance, until the resistance reaches Equilibrium again. All of this information is sent directly to the ECU, which is the engine computer controller.
If air density increases due to pressure increase or temperature drop, but the air volume remains constant, the denser air will remove more heat from the wire indicating a higher mass air flow. Unlike other sensors, the hot wire responds directly to air density. This components capability is made for the gasoline run engines, which fundamentally responds to air mass, not air volume. (Or Stoichiometry)
***COPYRIGHT 2013 by Jonathan H. (No plagorizing information)
Deeper into the subject...
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.
Some of the benefits of a hot-wire MAF compared to the older style vane meter are:
responds very quickly to changes in air flow
low airflow restriction
smaller overall package
less sensitive to mounting location and orientation
no moving parts improve its durability
less expensive--(I don't agree)
separate temperature and pressure sensors are not required (to determine air mass)
There are some drawbacks:
dirt and oil can contaminate the hot-wire deteriorating its accuracy
installation requires a laminar flow across the hot-wire
Last edited by SC400slide on Wed May 29, 2013 11:38 am; edited 1 time in total (Reason for editing : FIX)
