I decided to replace the malfunctioning fuel gauge on Mr K (1987 LT) with a voltage gauge ; who needs a fuel gauge when not only is there a red then yellow warning fuel light but also option of 0000 the trip when you fill up.

I spent a considerable length of time on-line sourcing a reasonably priced product. I got this one... it fits perfectly.

http://rspec.co.uk/performance-gauges/black-face-waterproof-series/52mm-clear-black-voltage-gauge

I used thicker wiring than the usual minute original BMW wiring and did not add a light to the gauge as I dont ride at night much. I wired it directly to the battery so it is always showing the voltage output, which I hope will not run down the battery unless not used for ages and ages.

It works fine . Good tidings to know alternator is working well (just over 13.5v) and battery is ready ( just over 12.5v ) for starting.

John QT

You are best to use a power source that is off when not in use. One wire you can tap into is the green and black wire that feeds the power to the clutch override switch on the left handlebar, you will need to remove the tank.