E30 Help with fuel gauge!

[Mike Stukalin <mikest@xxxxxxx>]

The fuel gauge in my '92 325iC (e30) behaves rather strangely. It shows way
past full on fill-up, then as the tank gets empty it goes toward 1/2 mark
and turns the yellow light at the right time.
I inspected the sender unit, even tried another one. Inspected the wires.
Yesterday I yanked the instrument cluster out, ringed out the wires. All to
no avail.
Everything seems normal. Sounds like a bad gauge. 
Another strange thing is that OBC shows readings similar to the gauge,
always thinking that there is 25.5 liters of gas left when the tank is
empty, but only 54 liters when the tank is full.
So, any help is appreciated. Does anyone have an extra cluster they're
willing to let me try? Does anyone know if the gauge can be replaced in the
cluster (Bentley is mum, I hate that book).
Oh, someone mentioned that there should be a bolt with ground wires on the
back of the gauge cluster. I did not see it. Am I missing something here?
Finally, under the rear seat, on the driver's side there is a ground
connector. Can someone tell me what grounds go there (I have a black wire
connected there and not connected to anything else. I verified that it has
nothing to do with fuel sender unit, but I'm curious where it belongs).
TIA. Mike.S

'92 E30 325iC, BMWCCA #159367
http://www.gis.net/~mikest

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