Hi folks,
I don't know if it was removing the tank for a "johnny blanket" intallation or just being nearly 30 yrs but I have developed a slow gasoline leak from the electrical connection under the tank.
I couldn't find any reference to the part in any manual of motobins catalog other than the 4 prong electrical connector so I removed the 27?mm nut and pulled the sensor.
What a stupid way to send  wires into a gas tank! A plastic nut holding a small ferule like plug inside a 2 piece tube????
The fuel level sensors (2 on my 85) are just small transistors that seem to sense the absence of fuel at two different levels. On a side note one was broken so I soldered it back together.See if that helps.
I wish I could include a picture or exploded drawing here but every google + search came up dry but it appears that the leak is between the inner plastic tube which holds the 4-prong socket and the outer plastic tube assembly that contains the sensors. The only gasket is between the outer 27mm? nut and the screw fitting on the tank itself.
I am used to seeing plastic crap on Japanese bikes but I expected something more robust from der vaterland.
Anybody seen or had this problem?
Cheers 
Rob

I have seen it before a few times. I would recommend using a later model fuel tank but even that can have drawbacks because of the different rear mount.
What I would be inclined to do would be cap it off and use a digital fuel gauge where you stick one digit into the tank to see if there is fuel there.
I was talking about this sort of thing to the mate who has the local auto shop about BMW being so good in a lot of areas and yet doing some things so badly that it just leaves me agape and thinking WTF are these idiots thinking.

Thanks for reply Rick.
I was a little angry and mostly just venting!
I have been using the digital gauge since day one and the broken wire may fix the idiot light but it won't replace the 10 litres of fuel on my garage floor.
I,m thinking of just plugging it up with some "petrol" resistant gasket maker and see if that works.
Rob
p.s I apologise for posting in the wrong section (again)!

You might be able to get a dome nut from a refrigeration place or similar to cap that hole.  I never trust gauges I have a low fuel light and a gauge and still have a look for a real assessment.