90 Bronco assuming original alternator with a new Optima Red Top Battery. According to my CD player I am only getting about 12.5 to 13 volts while running down the highway with the lights on radio and subs loud and the ac on low. I am thinking my alternator is letting go. Would this be a correct assumption? Or did I buy a battery that is too small?

 

probably runnin the subs and a/c and lights could be a pretty big load on the alternator...try not runnin the subs so loud or look into gettin a capasitor (i kno i cant spell) but in my buddies grand am his lights were really dim and when the subs hit they almost went out and then he hooked up the capasitor and now the subs hit harder and the lights work perfect

 

A Capacitor will store up power and make delivery to your equipment smoother. It does not however alleviate the load, only the burst load during more powerful pieces of music. If they're constantly dim you need a bigger Alternator. It's not that your alternator is necessarily dieing (it could be), but me thinks you need a bigger one to feed the battery faster. If you're getting more than 12v but less that 14 that's a good sign your alternator is working, but can't keep up.

 

ya it doesnt take a load off of the alternator but it could help the dimming light situation possibly

 

What makes me think it is the alternator is that at idle it will drop below 12 volts with just the ac on high.

 

Yeah that sounds like your alt. I would consider buying a bigger one than stock if you're going to run alot of equipment.

 

i think the stockones on broncos are 95 amps...maybe bump it up too 120..

 

The alternator (second generation) on an 87 was a whopping 55 amps. You could pull the volts down running the blower. Swapped it out to a 3rd generation that is for a 95 mustang V8 that takes it up to 110 amps. A little bracket grinding, rewiring and fusing needs to take place as the twin ten gauge wire won't take that amperage.

If your alternate has the plug-in for the power wire then it is the 2nd gen.