So let's say that we measure the current going through the two parallel lines (the 12 gauge shunt wire and the 10 gauge ammeter wire) on a 30 amp load, or a 30 amp charge, which will occur right after starting your car after a couple of long cranks, and 25 amps are going through your shunt wire and 5 amps are going through your ammeter wire. That would tell us there is too much resistance in your ammeter wire. That could be from a bad connection, a faulty wire or the ammeter itself. If you then put a short wire with two alligator clips across your ammeter and the two current values even out then that would tell you the resistance is in the gauge itself. Now, OTOH, if just putting a shunt wire across completely stops the charging circuit, but doesn't illuminate the idiot light in the process, then on that I'd have to noodle.
