When was the last time anyone saw an aftermarket amp gauge for a car/truck that had an external shunt like HCB3200 shows above? It is a good idea for less current flowing around up under the dash.
Stewart-Warner used to have a separate Marine Gauge section in their catalog that included External Shunt Ammeter Gauges. A Internal Shunt gauge on a boat could require very long and very large wires, which would be expensive and dangerous.
60 - 72 Chevy Trucks used External shunt gauges, but they didn't use an actual External Shunt. They utilized existing harness wire to serve double purpose as both charging wire between the Alt and Battery and also as the Shunt.
The first image is an actual External Shunt. The 2nd image shows the segment of the 12R that serves as the Shunt as pointed out in Blue. The 18B with fuse and the 18 B/W with fuse, connect through the bulkhead connector to the dash External Shunt Ammeter gauge.
The 3rd image shows 3 Internal shunt gauges, even though the shunts are mounted on the back of 2 of them. Both are 60 Amp gauges and would generate too much heat if the shunt were inside the gauge. The small Ammeter is a 20 Amp and does have the shunt inside the body of the gauge.
