I just had another thought. Since my fuel gauge shows zero, what triggers the low fuel light? Apparently not the sending unit.
Inside the relays electrical junction box (under fuel tank) there is a "Fuel-Warning-Control-Unit". Although it sitting near all other relays, this one is a lot smarter than an ordinary relay (and a lot more $$$).
I never fully opened this fuel-warning-unit, but from the BMW electrical-diagrams, you get these hints (available on CD from BMW, not from CLYMER)
(1) Fuel-Warning-Control-Unit has 5 pins being used by BMW
(2) Fuel level sender (float inside tank) has 1 wire for ground (brown wire) and another wire signal being fed to gauge and also to Fuel-Warning-Control-Unit (yellow-violet wire)
(3) Another pin of the Fuel-Warning-Control-Unit is for ground (brown wire)
(4) Another pin of the Fuel-Warning-Control-Unit is to light up the Low-fuel warning on dash (White-Green wire)
(5) Another pin of the Fuel-Warning-Control-Unit is for power and is fed from FUSE #1 which also feed instruments. Fuse #1 is only valid for 1999-2004 models (Green-black wire)
(6) Finally, the last pin of the Fuel-Warning-Control-Unit goes into pin #3 of the diagnostic connector for dealer's computer or GS911 (Brown-Black wire). You will understand better this part if you read this from the GS911 folks: http://www.hexcode.co.za/techinfo/ta...n-ma2.4-models
So based on this info, my assumption is that:
(a) The Fuel-Warning-Control-Unit is the one controlling the dash "low-fuel" warning lamp.
(b) The Fuel-Warning-Control-Unit has some kind of static memory as it can remember the fuel setting value for "low-fuel" even if Battery is unplugged for weeks.
(c) Electrical diagram shows than none of these input-outputs goes to BOSH-Motronic unit. This makes the assumption in (b) above very plausible.
I hope this helps...