Bench testing rally gauges

I = input, for this gauge thats 12 volts, A=altered or adjusted voltage , S=sender or signal.
The 2 post gauges use I and A. This input voltage was altered or adjusted elsewhere.
If you had put 5 or 6 volts at the I post of the fuel gauge, it wouldn't harm anything. That mechanical limiter wouldn't function. Voltage through it to the A post? I never tried/tested. It wouldn't be much.
The solid state regulators need a minimum of 9 volts input to function.