[pjmoreland]

If you disconnect the printed circuit from the gauge terminal that the green wire connects to and your gauge still stays pegged, then it would seem your gauge is bad.
If the gauge pegs only if that terminal is connected, then there is likely a short somewhere on the green wire, which follows this path:

Circuit board to gauge cluster connector.
Cluster to firewall bulkhead connector
Firewall bulkhead connector to temp sender

You would need to track down which segment has the short in it and then physically locate the short. You could disconnect all of these connectors and then use an ohm meter to see which segment has continuity to chassis ground.