Mercury Repair Questions? Ask a Mechanic for Answers ASAP
Sounds like the alternator is bad. I'd start there. Should have a consistent 13.5 - 14.5 volts when running. If it varies a lot the voltage regulator inside the alternator is probably the issue.
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When the voltage is low or changing rapidly it makes the electronics not able to run. The battery is just to start the vehicle, the alternator runs everything.