It's an amp gauge. The movement responds to the voltage difference between the junction block, by the battery (battery voltage), and the horn relay (alternator output voltage). When the voltage is higher at the J-block the battery is discharging. When the voltage is higher at the horn relay the battery is charging. The gauge should indicate discharge with the headlights, wipers, heat or A/C blower on/running.
The connection at the J-block is a black wire, the connection at the horn relay is a black/white/dkblue both are 14 gauge wire. To check you can pull the connector from the back of the gauge and check for voltage on both wires. If either wire has no voltage on either wire there is an open in the circuit. The wires are routed through the bulkhead connector, the open circuit may be there.
If you have power on both wires, Use an OHM meter to check for continuity through the gauge-terminal to terminal. You can use an AAA battery, connect it to the gauge, the gauge should peg in one direction. Reverse the wires the gauge should peg in the other direction. Don't leave the battery connected very long.
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