Part 2/3 - Triumph Spitfire 1500 - How To Change A Voltage Regulator / Stabiliser - Fuel Gauge Issue 

The original one probably still works, bad information is why your here. the original voltage stabilizer never had a steady 10v output as that is a bad interpretation of how it works . its a slow switcher meaning if battery voltage is 12.5v it will switch on for 80% of the time. the switch pulse is roughly a second and the slow response of the gauges integrates the pulses so it reads steady. a multimeter will read erratically anywhere between 6-8v but connect a 3w bulb between I terminal and 0v should see it flash for roughly 75% on to 25% off. B means battery, I means instrument, the stabiliser's can must be 0v which you can check by measuring the voltage between the can and 0v. If your can is at 12v the stabiliser isn't doing anything as its not 0v. Solid state stabilisers will be steady 10v and are often fitted because people expect to read 10v from the old one. Also, the gauge is 60ohms resistance, 2.5v is where these start to read on the scale and full scale is 7.5v. the fuel sender is 240 ohms empty and 20ohms full but some senders I've replaced are rubbish.