How do you get to alternator?

This is an easy answer. The alternator is hooked (through wires blue and black) to the battery. So every time you start an engine the alternator kicks on and charges the battery. That's practically...

The alternator generates power to run the car and charge the battery

The devices that develop electric power are a generator (for direct current), or an alternator (alternating current). In a car, an alternator is used, with a rectifier, because of size and weight...

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Yes, one is a 90amp unit the other is 120, the mounts and connections are the same. better to put in the 120 in place of the 90 then to to go down in size if you have a lot of accessories.