The powertrain control module (PCM) for a fuel injected
engine needs to know how much air is entering the engine at any
given instant in time so it can vary the pulse width of the
injectors to maintain a properly balanced air/fuel mixture. On
engines with "speed-density" type fuel injection systems, air
flow isn't measured directly but is estimated using inputs from
the throttle position, manifold air temperature and manifold
absolute pressure (MAP) sensors. But on engines with "mass
airflow" fuel injection systems, airflow is measured directly
with a sensor: either a vane airflow (VAF) sensor, a hot wire
mass airflow (MAF) sensor, or in the case of some rather unique
Japanese applications, a "Karman-Vortex" airflow sensor.
The advantage of using a Karman-Vortex airflow sensor instead
of a vane airflow sensor is that it causes less restriction. And
compared to a MAF sensor, it is simpler and more reliable
(contamination of the heated wire or filament in MAF sensors is a
big problem). What's more, a Karman-Vortex airflow sensor can
respond more quickly to changes in airflow than a typical mass
airflow sensor, which allows the PCM to maintain better control
over the fuel mixture.
HOW IT WORKS
This type of airflow sensor is named after the Karman-Vortex principle that says turbulent swirls or "vortices" will form behind an object if it is placed in the path of a moving stream or column of air. As the air bumps into the object and passes around it, little swirls form behind the object much like the wake behind a boat. The number or frequency of these vortices will vary in proportion to the velocity of the airflow.
So how can this be used to measure airflow? The number of vortices are counted electronically by one of several means. Each time a vortex is formed, it causes a slight drop in air pressure. So the sensor actually counts the number of pressure changes that occur. This tells the PCM how much air is flowing through the sensor so it can adjust the fuel mixture accordingly.
TOYOTA & LEXUS APPLICATIONS
Karman-Vortex airflow sensors are used on 1987 and later turbocharged Toyota Supras, and all Lexus engines except the ES 250 and ES 300. The sensor on these applications have a 5-pin connector and an integral air temperature sensor. A light emitting diode, mirror and photo receptor are used to count the pressure changes in these applications. The mirror is mounted on the end of a very weak leaf spring which is placed over a hole leading directly to the area in the sensor where the vortices form (the "vortex generator"). Every time a vortex forms, the drop in pressure wiggles the spring which causes the reflected light from the LED to flicker as it is picked up by the photo receptor. The vibrations of the mirror produced by the vortices thus causes the light to flicker on and off in proportion to airflow.
The photo receptor inside the sensor generates an on and off digital signal that varies in frequency in direct proportion to airflow. At idle when airflow is low, the signal frequency is also low (about 30 Hz). But as airflow increases, the frequency of the signal increases. At high speed the signal may go to 160 Hz or higher.
Karman-Vortex airflow sensors are also used on 1983 and later Mitsubishi's with turbocharged engines, and 1987 and later fuel injected applications. A different technique is used in these sensors to measure the vortices. The early sensors use ultrasonics to detect the pressure changes. A small speaker sends a fixed ultrasonic tone through the vortex area of the sensor to a microphone. The greater the number of vortices, the greater the turbulence and the more the tone is disrupted before it reaches the microphone. The sensor's electronics then translate the amount of tone distortion into a frequency signal that indicates airflow. The 1983-86 Mitsubishi sensor has a 4-pin connector while the 1987 to 1990 versions have a 6-pin connector. The early units also contain an integral air temperature sensor while the later ones also contain an integral barometric pressure sensor. In 1991, Mitsubishi changed to a redesigned Karman-Vortex sensor with an 8-pin connector that replaces the ultrasonic generator with a pressure sensor that measures fluctuations in air pressure directly (much like a MAP sensor).
Both Toyota/Lexus and Mitsubishi Karman-Vortex airflow sensors put out two signals: an airflow frequency signal and an air temperature or barometric pressure voltage signal. The Karman-Vortex signal should be a square wave signal that flips back and forth from zero to five volts. The frequency of the signal will increase (narrower pulse width) as airflow increases. Frequency should increase smoothly and steadily with rpm. Driveability problems such as surging, hesitation, stalling and elevated emissions may indicate a sensor failure. Most sensor problems are caused by a loose or corroded wiring connector.
On the early Mitsubishi applications, the sensor is mounted inside the air cleaner. A poorly fitting air filter may prevent the air cleaner lid from fully seating against the sensor and cause problems. Similar problems can be caused by air leaks in the intake plumbing or manifold.
Codes 31 & 32 indicate no signal from the airflow sensor on the Toyota & Lexus applications. Code 12 refers to a fault in the airflow sensor circuit on Mitsubishi applications.