those 'spools of wire' are the electro-magnetic coils that cause the pull on the needle, if one or both is internally shorted or open, the gauge is shot - start with a continuity check, no idea what the resistance of each coil would be, but some - you'll prolly find open [infinite resistance]
shouldn't have to ground the gauge to test it, a temp gauge is a meter that reads current flow of the circuit from the hot side to the variable ground of the sender, so you need to be able to put some variable resistance on the sender terminal and attach the ground from the battery to that - I think the only repair you could do is reattach any wire broken free from it's terminal post
DG might try rewinding the coils
OTOH, the repro gauges are modern circuits that don't fry so easily, or you might find a used gauge that works
Bill
