I was wondering if it the Alternator light still flashes even if the alternator is still good?? Could a bad battery cause the alternator light to come on? Because I just put in a USED battery today from a friend and its still doing the same thing. The battery wont hold a charge...I had it get tested and the guy said the battery is junk. He said the alternator is fine. But could the battery have been good, but as drove it up there to get it tested, could the Battery have been going bad driving it up there with a bad alternator not charging the battery??

86 Trans Am 4bll 305

 

If you're friend's battery turned you're engine over rather easily, only to lose it's power later on, then I'd say it's you're alternator (or a bad or loose ground).

What does you're volt gauge read when the car is idling, and then what does it read when it's in gear???

 

My volt gauge reads at about 10-11 volts when at idle.....and when im driving it gets close to 13 volts.....when I come to a stop or slow down, the gauge goes down real quick...so I dont think the battery is charging..... I dunno

 

You will always get fooled if you don't start with a known good, fully charged battery. With a good battery measure across the terminals with a voltmeter. Average reading should be 13.5-14.8 volts with accessories off. A bad battery can and will cause the alt to have low output, or worst case cause it to fail.

 

If you're reading that low of voltage at idle, then that's way too low (especially if you have you're head-lights, wipers and A/C off).

Also, if you're gauge fluctuates like that when coming to a stop.... then it really can't be you're battery, since the alternator can run the car on it's own without the battery (after it's started of course).

You might wanna buy a new batt. from the auto-parts store. Before you bring it home, have them check the batt. before you leave, so you know you have a good piece (keep the receipt).

When you install the batt., and if that gauge reads and does the same thing.... then it's you're alternator, or a loose connection somewhere in the charging system, absolutely.