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Can a bad alternator run a battery down?

I came out from work this afternoon to a dead battery in my '99 Taurus. I saw nothing left turned on. I got a jump and went to Autozone two miles away. By that time there was enough charge to restart the car. My battery checked good, but they said the car had a 4-1/2 amp draw when it was shut off. The alternator still put out over 60 amps when reved up. That’s good.

Can a bad alternator on a modern car run the battery down? I know the old ones did sometimes, but I thought they “fixed” that aspect of them several years ago.

Yes, but so can many of the electronic modules. Did you narrow the parasitic drain down to being the alternator, or are you just guessing?

Yes, a bad alternator can run down a battery. They still have diodes to convert the AC to DC and the regulators can fail, leading to a battery-killing draw. These, of course, can be checked and tested.

However, there are other modules and relays that can also fail and can cause a parasitic draw. Finding them is sometimes a time consuming task. Good luck.

I pulled a 30 amp fuse from the panel under the hood and the draw went away. Unfortunately I don’t have an owner’s manual to tell me what that fuse is for. Ford is too cheap to lable the fuses like most car makers do. It might cost a nickel. When I started the car again, the battery non-charge light came on. Sorry I can’t remember what the fuse number is, and the car is not near me and my computer tonight.

If you are asking “can some (you did not specify) defect with my alternator cause a 4.5 amp draw”, I would say, not the first place I would look.

You may have difficulty fixing this yourself if you had difficulty doing this:

Doesn’t even cost a nickel.

Looking at some data for your vehicle it shows a 30 amp fuse between the alternator and the battery. I assume it is the main output lead of the alternator. The fuse number is 26 and it is the fifth fuse down in the row furthest from the relays in the same panel. From what you say I think you may have found the trouble and will need to replace the alternator if that is correct. Blocking diodes can go bad inside the alternator and cause this kind of trouble.

Yes. In fact, it’s very common.