If you connected power to the sender lead on the rear of the gauge, you might have damaged the gauge.
DADS50 has posted a video and info regarding one of the standard/common tests for a gauge. (maybe he will post a link AND maybe he or others will correct/clarify the instructions below)
This is a "bench test" of sender and gauge - the sender is not in the tank and the gauge could be in or out of the gauge-assembly housing:
Put the 12v-6v resistor on the "E" post of the gauge (if you have a 6v gauge).
This is a one-way resistor (it has an 12v-in side and a 6v-out side).
Connect the 12v power lead to the 12v-in of the resistor on the gauge.
Or, if you have a 12v gauge and no resistor, attach the 12v power lead to the "E" post on gauge.
Ground the gauge-assembly housing.
Attach the sender to "F" post on the gauge.
- the gauge should read Full.
Ground the sender at an attachment hole on the sender unit.
Move the sender's fuel-level bar/bulb up and down.
- the gauge should give a reading between F and E, as you move the bar/bulb up (F) and down (E).
Here is a list of diagnostics:
http://www.fifthaveinternetgarage.com/tech_tips_gas_gauge.php
