FINALLY I have some great news to report on this thread - my fuel gauge works
Today I arranged an appointment with Italian Auto Centre - perhaps the only folks to have spare instruments from a 156 made after 2003 - it was a 135km drive away, so I put in ten bucks of petrol (to ensure I wouldn't run out), and drove there this afternoon (after replacing a split brake hose; slight delay!)
I had discovered that if I unplugged one of the speedometer's plugs, the fuel gauge dropped to zero, and if I reconnected that plug and unplugged the other plug, the fuel gauge went past 'full'. So clearly there was some relationship, as I had read on a thread here.
But when I swapped the speedometer, there was no change (other than the odometer reading 78,324km instead of 231,450km
). The fuel gauge still read 'full'.
I tried swapping the fuel gauge cluster, and even the tachometer - no change.
I began to wonder if the tank was possibly still quite full. So I removed the back seats for a third time, and unplugged the sender. No change.
I even unplugged the infocentre on the top of the dashboard (the plugs are down near the glovebox). No change.
I was really scratching my head by now, and clutching at straws - wondering if wiring was damaged under the back seat. I scrutinised it closely, and spotted some un-factory-looking insulation tape just under the carpet-holder trim, left-rear. I removed the trim... and discovered something truly amazing...
As seen in the first pic below, some large red and blue wires were connected to a small orange-white wire... the orange-white wire had been cut... and at the sender, there was an orange-white wire... Ripping off various items of trim, I followed the wires to the front-left footwell, where I discovered the scene you see in the second picture. LOTS of tape, a relay, a killswitch, a cut-off multiplug... obviously, some BOZO had fitted an alarm. Their intention was to have the immobiliser relay cut the supply to the fuel pump, but instead, it cut the supply to the fuel gauge!
So after all that, nothing wrong with the instruments and electronickry... my poor much-maligned Alfa 156 and its troublesome electrics, when really it is the result of someone else stuffing around... I cut those wires and twisted together - gauge working fine... Tomorrow I will remove ALL the extra wiring...
At least while at Italian Autos, I bought something to thank them for their trouble - I bought an air filter and pollen filter, which came to about $200 (79 pounds...)