not sure about reading 12V across the terminals, should be like checking 2 points on the same wire
maybe a reading from the sender terminal to ground would show 12V on a good gauge because that would be completing a circuit as if the sender were shorted
if grounding the sender wire moves the gauge that'd indicate the gauge is good, sender bad, if it doesn't move the gauge, the gauge is bad - you can check the resistance of the sender [post to threads] cold, then dunk it in boiling water for a minute or 2, pull it out and check again, or have wires attached to the sender that you can meter as the water heats .... also do not use any thread sealer or teflon on the sender when installed to isolate it from ground .... rad level needs to be below the level of the sender, but you'll get a bit of water out the sender hole when you pull it regardless just from water in the block
Bill
