alternator?
 

Ok, I'm about 95% sure that my alternator is going. Just wanted to get a second opinion other than from someone selling me one...

with the car running and either at idle or higher rpm's, if I have a dome light on and use the window, seat warmer, or rear defrost, the dome light gets much dimmer. Also, when the car is idling between 700 and 1000 rpm, the intensity of the light varies directly with the current idle rpm (idle up=light brighter).
I feel like this could also have an effect on having a slight rough idle when it drops down, maybe electronics are taking voltage from the ignition system which might cause less spark to occur and not run as smooth?

Either way, before I spend $150 on an alternator...any other thoughts?

It's either an alternator, or a bad connection somewhere. How are your battery cables, particularly at the battery terminals and the ground? Make sure they aren't loose, dirty, corroded, etc.

If you have access to a voltmeter, you can measure the voltage at the battery with the engine running. That will tell you for sure if it's an alternator. (Should be running ~14.2v, and not vary much with a load.)

+1 on that ^ and check the belt tension too

There's an easy way to test to see if it's your alternator.

1.) Start your car

2.) Carefully disconnect your battery (yes, while your car is running)

**If your car doesn't shut off, your alternator is fine.

Clean the battery terminals in baking soda and water with a toothbrush, rinse it clean with water afterwards, wait till it dries and coat the posts and the terminals with as much Vaseline as they will hold, put the terminals are back on and tightened down.

Vaseline blocks Oxygen from reaching the metal in the terminals and battery posts.

DONT DISCONNECT THE BATTERY CABLE WHILE THE ENGINE IS RUNNING!
You could possibly sent a surge of voltage through your system and that could be nasty.
That used to be an old test to do but with the new electronics now a days --dont do it.
Yall do know that some of the voltage at certain times is regulated by the pcm--not the alternator?.

Do you have aftermarket pulleys?

Don't mess around with this without some care. It sounds to me that either the voltage regulator (wherever it is) or a rectifier diode (usually mounted to the alternator) has failed. Given that, the system voltage may climb proportionately to engine rpm enough to start frying very expensive stuff as well as the battery. Make sure the system voltage doesn't go over 15v as the rpm increases. Otherwise it could also be a shorted cell in the battery or belt or possibly bad connections. If the battery is good, charging it overnight should make the dimming issue go away for a while. Sears/Autozone and the like will test batteries and unmounted alternators for semi-free.

There is no voltage regulator, or not one in the usual sense. The RX-8 alternator is PCM controlled. It is not your usual "dumb" alternator. While all the internal components are replaceable and available through Mazda, they are so expensive that it usually makes more sense to replace it rather than to rebuild or repair. I think I calculated all the internal parts at over $1000 or something ridiculous like that.


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Yikes! Makes one yearn for the days of points, plugs, and condenser.