Having voltage issues with my 93 cadillac seville I've replaced the battery and had the alternator checked and the voltage keeps going down to 12.9 and under until it dies.
It's the alternator. The alternator is the only thing that provides electricity to the vehicle. So if you're losing power it has to be that.
To be more precise. It the voltage regulator. The voltage regulator is built into the alternator.
So if the alternator tested good then you have a charging system wiring problem, make sure the connectors are clean no corrosion. Check main fuses, etc.
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