Just note on the stock original "old ammeter gauge", in good shape, with correctly tightened/maintained connections, undamaged insulators, they don’t just fail for no reason whatsoever over time. The factory original ammeters are a simple device, stoutly constructed, with good connections, can handle a great deal of current. Problems begin with poor servicing of the stud nuts, left loose or oven-tightened (crushing the fiber insulators, leading to loose nuts-high resistance). Loose connections will build resistance and the resulting heat build up will cause even more resistance and insulator damage. Allowing a stud to twist while removing a stud nut will damage the internal stud head to bus connection and will contribute to even more resistance/heat build-up at the ammeter connections.
Another common cause of ammeter and charging system connection issues, for these cars still running an ammeter, is adding aftermarket loads directly to the battery/starter relay (or anywhere on the battery side of the ammeter). This creates a false charge indication as the added current is pulled though the ammeter, and all other charge circuit connections, from the alternator while in operation. This can place the entire charge circuit well outside of the original design current limits. For an original stock charging system configuration, all vehicle loads need to be on the alternator side of the ammeter, as are all factory original loads, no exceptions.
