My battery light came on over the weekend and the lights started to dim when I reved the car. I put a multimeter on the battery terminals when the car was off, and it reads 12V. When the car is idle it reads 12V. When I rev it up, it goes down a bit.

Could the battery be weak and not hold a charge or is it more likely a bad alternator? How do alternators go bad anyway?

 

In order for your alternator to recharge your battery, it has to apply a voltage higher than 12V to it. (If the alternator only generator 12V, the battery would never quite get to 12V itself - water runs down hill, if you see what I mean). The PCM is looking for that voltage to be applied to the battery whenever the key is on; if it doesn't see the @14V applied to the battery, the battery light is on to alert you of a problem in the charging system.

So the short answer is that the alternator is most likely bad. The battery is probably OK for now, but you risk wasting it by continuing to drive the car without a functioning charging system. You may be lucky and only have a blown fuse or lose wiring at the alternator; both are important to check before you buy an alternator. If you don't think you will replace it yourself, ask the service adviser if those were in good condition before you have them install an alternator.

Alternators have lots of small copper wires that carry a lot of current as well as a whole slew of diodes and other small electrical components. Any of them are subject to wearing out or failing. With heat and time the conductors wear thin and get brittle and eventually crack. Some (very few) mechanics still rebuild alternators; they'll go into the alternator and test and locate the worn/broken components and replace them one-at-a-time. It's generally more practical and cost-effective to simply replace the entire alternator (most cost-effectively with a factory remanufactured unit). Notice that I am suggesting a "reman" not "rebuilt" unit (and only from reputable sources/brands) as it has had all of the likely bad components replaced, not just the certainly faulty components as in the case of a rebuilt unit.

Good Luck!

Fred

 

Thanks for the detailed reply.

Is the fuse for the alternator located on the dash where the other fuses are? As for loose wiring, is this the main wire thats hooked to the alternator?

Last, I normally buy parts from the local auto parts stores. (Autozone, Advanced Auto Parts, O'Rielly Auto Parts) They tend to sell re manufactured parts. Do you have any recommendations on remanufactured parts?

 

I'm not sure where the fuse is, but I would guess under the hood. Check any fuse that might run a major circuit (engine circuit, etc.). Again for wiring, check all of them; there is likely a main wire connected to a threaded stud by a nut and a plastic clip connecting some control signal wires. Sorry I'm not able to be more specific - my wife has our Pro5 at work and the owner's manual doesn't seem to be any use. (I have it on the computer.)

I work at a repair shop that usually installs Visteon brand reman alternators and starters. (I understand that Visteon is basically the aftermarket arm of Motorcraft.) I also hear that Beck/Arnley is good (Advance has them). They are OE for some manufacturers, I think. Bosch makes high quality parts as well; OE on many European makes. Mitsubishi is OE on a lot of Asian makes. Just make sure you get a warranty with anything you install.