there is no such thing as a 6V sending unit, sending units are simply variable resistors, identical for any voltage, for our gauges they are 0-30 ohms, check it with an ohmmeter .... the test you did shows the gauge is working but your sending unit appears defective

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics