i have a 91 na fc.
i have a quick question... simple..
Would a bad alterator drain a battery after a few cranks? even if the battery was new and had 12.70 volts ?

 

The alternator isn't doing anything during cranking. The battery is the only power source at that time.

 

I grounded my alternator by accident, safe to say its toast or is there a way to test it?

 

If your battery shows voltage before cranking but dies after a few cranks then it'd have to be a bad battery. If the battery recovers then there's probably a short in the system, possibly the alternator.

Any auto parts store will test both your alternator and battery for free. Sometimes they do it in the car, sometimes you remove one or both. Just ask them to test it and go from there. Removing the alternator is a piece of cake, even in an auto store parking lot (most don't mind, some might). You need a mallet, wrench and a rag to remove it, plus a pry bar (crow bar) to pry on the alt to re-tighten the belt after you put it back in. The center of the belt should deflect 1/4"-1/2" ish when you press it with your finger. The Haynes manual has more info if you can't figure it out. Removing the battery is just a couple bolts. Everything you might need is right at their store, which is why they happily do this for free.

A fully charged battery in good shape can let you drive up to 30 minutes-ish even with the alternator removed, which would help you get your car to the store. Or if you can borrow a car and bring in the alt and battery that's probably a better way to do it. When a short or weak battery could make your car stop at any time, things could get scary.

New alternators are notorious for being a gamble. If you know a reputable shop, go there. If not, oh well. Either way save the warranty info in case it fails after a few days/weeks/months. If it does, just keep returning them and retrying until you get a good one. It's easy to replace. For this reason, don't buy online or anywhere where it would be hard to return.

 

grounding my alt. also did blow the 100amp fuse , which i replaced with another 100amp fuse.
thank you for the info.

 

Definitely keep this part in mind. When I replaced my alternator, I had to go through 2 defective "new" ones from Kragen before I got fed up and bought a $200 one from Napa. The Napa one finally worked properly.