My fuel gauge remains parked on 'full' .... normally, that is a happy sign - but, sadly, it never moves: I have chased this around without success as detailed below, so would appreciate any insight.
I have a spare fuel gauge and tank level sender, so they were pressed into service without any improvement. The level sender ( disassembled) is a simple loop of high resistance wire running the length of the ~14" tube. The total resistance between the top contacts is 75 ohms: a float with a shunt crosses the two vertical lengths of the same wire such that the resistance decreases as the fuel level increases ..... no problem here, as 7/8 full is ~ 10 ohms - so the sender is working correctly.
I changed out the gauge in the dash ... still reads full on the spare ( when there was ~1/4 tank left).
I then shunted a 100 ohm resistor across the two pin contact removed from the sender unit .... still read full ( should be 'empty' as the normal gauge is 75 ohm at empty); however, this clicked on the flashing orange 'low fuel' icon in the gauge .... so that loop works
Any idea what to do next?? - it is possible that both gauges are defective - but unlikely.


As an aside, the oil level indicator in BOTH gauges actually does move off the peg ... and parks in the red 'danger' band: the dipstick level is exactly in the middle where it should be ..... I take it that this valuable level info is as reliable as the voltmeters in 928's!

EDIT: So here it is ... solved 11 months later .... posted below

 

. bump ... drove another 100Km, and the gauge remains on 'full': is there anything else in the circuit I'm missing?

 

I am not sure if this is how a porsche guage funcions, but I do know that most guages function with two windings near the base that magneticly pull the needle in a direction. then check to see if there is anything putting out magnetic interference since the light seems to be working.

 

I now have a working fuel gauge .... and the erratic function of the oil level gauge is gone: the resolution was a reasonably simple swap of the 'Central Informer' module.
Many months ago, I had done the hardware checks noted above - and spent phone time with Geoff (Red Rooster" on the 964 board) as he poured over full sized electrical schematics: everything traced back to the central informer .. whatever that was! Even If it could be found .... what to do with it? So, I let the clock tick on this annoyance - unsure of what to do.

On an unrelated search, I dug out the p/n and location .... and turned up a spare module in my box of tricks: with the typical finesse of a 6'6" contortionist, the module was swapped from its hiding spot above the ignition key - and for the first time, the fuel needle read less than max full.
No more topping off the tank everytime and zeroing the trip odometer to keep track of fuel ... a problem I'm happy to bury.

If you have any gauge issues - this may be a reasonable place to look.