The Technical Website for Vehicle Technicians  

This is a peculiar little device which we don’t hear much about so having found out what it does, thought I’d pass it on. 

It is used in some, but not all, versions of the Toyota TCCS engine management system [Toyota Computer Control System]. This can be found on a lot of Supras, in the 1987 to 1992 age bracket, and also the Lexus V8. The only difference between the meters fitted to the different engines is the size of the two main passageways or chambers used in the air flow meter. The Lexus meter allows more air flow through the meter at the same Karman vortex square wave frequency.

The advantage of this type of meter rather than a vane air-flow type sensor is that it causes less restriction. It is simpler and more reliable as it doesn’t suffer contamination of the heated wire or filament.  It is also able to respond very quickly to changes in air flow and so information sent to the ECU is almost instant. 

It’s called a Karman Vortex sensor because that’s the principle it uses. When air flows past a stationary object it creates a swirl, or turbulence behind the object. This turbulence is called a vortex which is a bit like the swell created behind a boat as it moves through the water. The greater the air flow, the greater the turbulence. The sensor measures the amount of turbulence behind a small object which is placed in the path of the incoming air to generate an air-flow signal.

The turbulence is measured electronically either by passing light or sound waves through the air to detect the changes in pressure, or by counting the frequency of the pressure changes (air turbulence). This allows the sensor to generate a signal which is proportional to air flow. 

In the Toyota models which employ this type of sensor, the connector is five-pin. A LED, mirror and photo receptor are used to count the pressure changes. 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 (this bit is the vortex generator). Every time a vortex forms, the drop in pressure wiggles the spring which makes the reflected light from the LED flicker as it is picked up by the photo receptor.  The vibrations of the mirror produced by the vortices makes the light flicker on and off in proportion to the air flow. 

The photo receptor inside the sensor creates an on-and-off digital signal which varies in frequency in direct proportion to the air flow. At idle, when air flow is low, the signal frequency is also low (around 30 Hz). As the air flow increases, the frequency of the signal increases. At high speed the signal may get to at least 160 Hz. 

Some early Mitsubishi models also used the KV sensor. Ultrasonics (sound waves with a frequency higher than the human ear can hear) are used 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 which indicates air flow. 

The 1983 – 86 Mitsubishis have a four-pin connector and the 1987 to 1990 versions have a six-pin connector. The early units also contain an integral air-temperature sensor while the later ones also have a built in barometric-pressure sensor. From 1991 onwards Mitsubishi started using a redesigned KV sensor with an eight-pin connector which replaces the ultrasonic generator with a pressure sensor which measures fluctuations in air pressure directly. 

The air flow meter is just that; it’s not an air mass meter - it needs other sensors to gain information on air density and temperature so the ECU can make the necessary compensations.  An air temperature sensor is fitted inside the Karman vortex air flow meter, but it is absolutely nothing to do with the meter itself.The system also uses a High Altitude Compensation (HAC) sensor. The frequency of the Karman vortex square wave varies according to the density of the incoming air and the TCCS ECU compensates for this using the data from the HAC sensor. 

If the KV air flow meter packs up completely the TCCS will go into fail safe mode.  In this state injection pulse width and ignition timing will be set to the fail safe fixed values based on other engine sensor conditions. This is the normal injection pulse width and ten degrees fixed ignition timing. 

The TCCS uses the variable frequency square wave signal from the Karman vortex sensor to work out the volume of air entering the engine. The signal is a five volt signal switching on and off, faster and faster as the volume of air passing through the air flow meter is increased. The frequency of the signal will increase – so you’d see a narrower pulse width as air flow intensifies.  Frequency should also increase smoothly and steadily with rpm.

If the ECU picks up an air flow meter square wave frequency value equal to a pre-programmed value stored in the memory of the ECU then it will shut off the fuel injector pulse to avoid detonation. This is called ‘Hitting the Fuel Cut’. There is some slack beyond the point of fuel shut off, but the ECU is determined to avoid detonation. 

The Karman Vortex airflow sensor is quite a sensitive device and suffers with age. Its outputs can become erratic which can in turn produce some funny driveability problems. These include poor idle, surging, hesitation, cutting out and non-start; most noticeable at normal operating temperature or if it is running hot. Emission levels will also be raised.

On early Mitsubishis the sensor is fitted inside the air cleaner. Problems arise if the air filter is badly fitted as the air cleaner lid needs to fully seat against the sensor.  Air leaks in the inlet pipe work or manifold cause similar troubles. 

Check the fault code first and if this indicates the sensor is at fault still check the integrity of the wiring. You could use VIxEN’s DMM to check the square wave output (connect across the sensor wiring). This should be a simple square wave (frequency proportional to engine load).  One of the main signs of trouble is a rounding of the normally sharp corners of the wave. 

© Vanessa Guyll, August 08