A shunt is a wire/component for resistance, I believe in the sensing circuit. It is probably a brown wire with white stripe on your truck. Some look like a terminal block. Forces a per centage of the current to run thru the gauge. You need to get a schematic for your truck. 60-66 had different wiring systems for curtain years. There may be football shaped fuse holders in the ammeter circuit (small, hard to find) that you need to find that are probably blown. Check all fuses in fuse block also. You could have an open circuit from broken wire or blown fuse. Check to see if there is voltage at meter. One side of meter is batt hot, other is alt output. There may be a wire or two for the gauge light. Get educated on this stuff from internet search if you continue. Dozens of hits on 65 Chevy ammeter problems.

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