brimee

Last weekend I noticed my volt gauge on the dash running low on my way home. This weekend I noticed it was just fine while it was still cool but dropped after heating up. Just got done checking it. When I just turn the car on(cold) I have almost 15v. After heating the engine up I have 12.8v at the alternator and about 12.4v at the battery. Is it the alternator(again) or the battery? The alternator is about 10 months old and has a 1 year warranty. It was rebuilt by a specialty shop. The battery is a litlle over a year old.

Thanks ahead of time for any help.

65Z01

Actually +15Vdc it too high; it shouldn't go above +13.8 (or maybe a little into the low 14s) just after starting the engine. On the low end 12.8Vdc isn't bad.

Try putting a load on at idle, i.e. lights, A/C, etc and check voltage then. If low raise the idle to 2,000rpm and check again.

When my alt went out it's failure mode was such as to not even charge the battery at all.

jfb

My 87 dash voltmeter is about 0.3 volts low compared to the battery post voltage. The alternator is designed to charge at 14.7 volts cold and it drops as it warms up to 13.3 hot. When your engine is idling , the alternator does not turn fast enough to deliver its rated current, so if you are drawing some current with the a/c on , radio , fan, lights, etc and the engine is idling , the voltage will fall. When you get underway, the voltage will rise . If your alternator is not charging the battery, then the battery light symbol will light on the dash and you should turn off every electrical load you can and make a bee line for home or your mechanic. Your system sounds quite normal and nothing to worry about.