Help Needed: Electronic Dipstick

I don't know how the new Porsche 911 (A.K.A. 997) does it, but the air cooled 911 cars had a float system very much like those in your gas tank. The float sits in an auxillary oil tank (the 911 is a dry sump engine) and is connected to a gauge on the dash. The car's oil temperature must be fully warmed up to open the recirculation valve and running at idle on level ground for about 60 seconds to get an accurate read.

The newer 911 takes the level reading once upon engine start up, so the oil temperature can be cold and is displayed on some sort of bar graph on the dash. I think they can do this because the water cooled engine is not a true dry sump engine and all of the oil pools to one spot, I believe.

You can find out more if you search for web forums for cars that have this feature and ask the gear heads how their system works.

Let me know what you learn. My guess is all use a float.

I remember looking at how a automobile fuel gage worked (some 30 years ago - things may have changed since). It was an incredibly simple system. A reduced voltage circuit includes the fuel sender unit, which is just a float on an arm with a wiper contacting spiral wound resistance wire such that as the float swings up and down the resistance in the circuit is increased or decreased (if they used the exact type sensor for oil level, oil would tend to interfer with the wiper contact creating an open circuit - but it would not be that hard to seal the circuit part of the sender).

The sender is connected in series to the dash gage sensing element, which is a tiny bi-metal conductor which deflects as it is heated up by the current going through it. A small inductor coil in series with the bi-metal helps dampen the oscillating current as the gas sloshes around in the tank. The needle of the gage is hooked on the bi-metal and is anchored at a pivoting point such that the needle swings as the bi-metal deflects.

I have not had a lot of confidence in fuel gages since. Do I really have a quarter tank or is it just a hot day?

If you have a conventional sump you will have a position on the side of the sump wall that will always be below the normal oil level regardless of engine speed. If you install a liquid sensing sensor at that point and baffle the inside to compensate for braking and turns then whenever the oil drops below "normal" level and exposes the sensor you could activate a light or audible alarm. In a dry sump system you would be more accurate if you monitored the scavange pump output pressure. Since there will always be drops during braking and turns you would have to count and time the duration of the gaps to establish a baseline in order to predict a scavange shortfall. As an additional measure you would want to monitor the minimum liquid level in the oil reservoir (swirl pot, post cooler)so that reaching minmum level would force progressive engine shut down. i.e. cut injection and spark to alternating cylinders in the firing order simultaneously, force aux. fans to highest speed if available, and program a "limp" mode so that you can safely move the car off the road. Depending on whether you have a two or three stage oil pump you should already have a pressure warning device in the high pressure feed to the motor.