Hi Keroppi . . .

You cannot simply connect the old ARM wire to the Bat terminal on the alternator. The Bat terminal on the alternator needs a path to the battery. As we discussed above – that path could be through the Ammeter or via entirely new wire.

I recommend that you spend some time reading and/or studying YouTube videos on how the GM 3-wire alternator functions in its native home a - 64-72 GM car/truck. It is important to get your head around an electrical project before undertaking it. It’s even more important when the project involves adapting a newer component into an older vehicle.

Here’s a great video on where and why the three wires are placed in the native installation: GM 3-Wire Alt Video

Next I would do some reading on 12 volt conversions and Delco alternator wiring. Some of these conversion recommendations seem to specifically avoid working behind the dash board - suggesting that the ignition circuit running from the key to the coil should be involved. I am not a fan of this approach – because it can come with unintended consequences. There are better places to pick up ‘key-on’ 12 volts behind the dash. Moreover, the exciter wire provides the ‘fault’ light feature. There are simple ‘jewel indicator lights’ readily available that can be mounted in or under the dash.

Regards,
Stock49