Vane (Volume) Air Flow sensor question.

[firewalker]

On older vehicles, it was common to use Vane (Volume) Air Flow (VAF) sensors. It is just a vane attached to a potentiometer. Reading the position of the pot, we know the position of the vane, and from the geometry of the sensor, the amount of air going to the engine.

On some VAF sensors, the potentiometer, has a weird "geometry". It uses several trimmed resistors. The resistor between the wiper and a terminal will give a resistance that fluctuates from a value (that will rise with the position of the wiper) and a fixed low value (diagram bellow).

Is there a advantage to do so?

          

Alexander.

[Gregg]

Automotive engine controls have come a long way from the early days of fuel injection. I think what you are seeing is an attempt to increase reliability by separating the resistors from the mechanical switching surfaces.  The discreet resistors probably have better thermal and mechanical characteristics than a plain pot.
I remember replacing throttle position sensors in older vehicles that were essentially carbon printed on a ceramic with a metallic wiper (like a cheap trimmer pot).  The wiper would wipe away enough of the carbon to change the overall resistance and sometimes wipe the resistive carbon clear of the ceramic backing resulting in an open circuit.  Similar open circuits in fuel tank gage sensors also were common; but they more often broke the wiper arm.

[james_s]

I think the hot wire air mass meters were a nice improvement over the old mechanical vane airflow sensors. I'm not sure why anyone thought a mechanical pot was an adequate solution for something like that, I've seen those TPS's fail that way too. A magnetic or capacitive position sensor would have been far superior and probably not much more expensive.

[firewalker]

Thanks you.

[Dr. Photon]

I always assumed it was done that way so that they could easily create a very customizable taper/curve to the relationship between flapper position and air flow. While I agree that the vane meters suck compared to the more modern mass air flow meters, I have found the old flapper meter, and the rest of the old Bosch analog L-jet system, very reliable for what it was.