The fuel level sender broke and started to leak fuel. Turns out the early model k100 fuel level senders are now NLA. Felt like this was a good time to upgrade to a newer tank so I can install a fuel level gauge. Didn’t feel like installing just a single gauge, so I also installed a coolant temp gauge.
Step 0, buy a late model tank that has the fuel level sensor under the rear of the tank.
Step 1, remove stock 4 pin connector to fuel pump/level sender.
Step 2, extend the wires to reach closer to the rear of the tank and wire in new connector to connect to the new fuel pump/level sensor. 3 wires come from the stock connector wires (power to pump, ground, fuel warning). The 4th wire gets run to your new fuel level sensor.
Step3, wire up your new instruments. It helps if you get the BMW “harness for additional instruments”. Use the wire mentioned in above step as the signal for the fuel level sensor.
Step4, mount instruments wherever you want.
Step5, make mounts for the rear of the tank since it mounts differently than the early tanks.
Check wiring diagrams for wiring details.