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Volume Air Flow Sensor


The volume air flow sensor is at least as important for the first multi-position injections as is the hot film mass air flow meter for today's engine management. In addition, it is used later on in combination with the electronic programmed injection control with turbocharger. It measures the aerial amount and the temperature of the suction air. These data were so important for the first petrol fuel injection systems that they were named after the device (L- Jetronic).

How it works

The volume air flow sensor features just one movable part, the air vane with attached balance flap. In the figure on top the air is sucked in via the air filter, coming from the right. The air has to pass along the air temperature sensor, which is protected by plastic, and then alters the air vane according to its volume. The degree of the alteration is transmitted - by the way, not proportionally but in segments - via a rotational potentiometer to the control unit. To prevent fluttering of the air vane, the balance flap is positioned in an angle of 90 °. It turns into a closed space and has a damping effect on the air vane.

Completely below there is the channel for the idler mixture. Because this volume air flow sensor is also inserted in systems without Lambda control, the share of 'unregistered' air during idle running can be adjusted. If the set screw is turned outwards, the share is larger and the mixture is more meagre during idle running. Operating the system with lambda oxygen sensor the channel opening is set to a fixed value.

If the volume air flow sensor has more than four effective connections, the circuit for the fuel pump can be integrated, too. For safety reasons it is only operative if air is actually sucked in.
The volume air flow sensor can be tested preliminary by measuring different resistances, yet all of them less than 500 ohms. Whether, however, there is a resistance at every position of the air vane, can be determined only by a so-called 'static check'. For this check, all the values of all possible positions of the air vane are loaded onto an accumulator oscilloscope, indicating any breakdowns afterwards, hopefully.