How do you know when your alternator is bad?

**SinnerG** · 10-30-2003, 01:35 AM

Today I noticed a light on my dash of a 'little battery.' I also noticed the the normal voltage of ~14V dropped to ~12.5V. I have a 1998 Chevy Blazer 4.3L 4X4 with 105000 miles on it.

I went to a friend's house and he put some water in the battery resevoir, and said to wait a day or two to see if that improved my voltage reading.

I have a feeling that the alternator is dead, or getting there. Fortunately the alternator on my car is easy to get to. Should I just replace it, or is there any sure fire way to rule out any other potential problam.

I took my multimeter out and tried to get a reading off of the alternator while the car was running. The wierd thing is, I see the red wire (+) coming off of it, but the black wire (-) is grounded to the engine block where the negative side of the battery is also grounded. Basically the (+) sides are tied together, and the (-) sides are also tied together. How can I get a reading off of the alternator alone without the battery tainting the reading?

Forgive my mechanical ignorance. I can change the oil, brake pads and flush my radiator. Anything more than that I leave to the 'pro's.'

Also, my local parts place wants to charge me $149 for a new alternator. Does that sound about right? The refurbs are only $10 less, so I'll take a new one over the refurbed one for a $10 hit.

Sorry for the long post, but I'd hope some of you who are more savy to this may have some insight. Thanks in advance.................

**jol** · 10-30-2003, 02:39 AM

I dno what it's called in english but the relay that controls the charging may me dead

**cproaudio** · 10-30-2003, 03:06 AM

It's call the regulator and it's built inside the alternator. SinnerG, your alternator is shot. There's no other possible problems. Adding water to the battery aint gonna help. Alternator puts out 13.5+ volts when the engine is running. If your battery light is on and the voltage drops to 12v while the engine is running, it means that the alternator is charging and it's dead. replace it. Also while you're at it, see if they have a high output alternator for your blazer since you're gonna be putting a system with DD5.1 in it.

**SinnerG** · 10-30-2003, 03:14 AM

No high output alternators aren't availible at the local AutoZone, Kragen, or Pep Boyz. I want to plug and chug. I know of places that have HO alternators, but you usually have to accomodate them to stock. They usually don't fit like the OEM's. 105A should be plenty for my setup.

**SinnerG** · 10-30-2003, 03:19 AM

If your battery light is on and the voltage drops to 12v while the engine is running, it means that the alternator is charging

I'm assumming you meant that the alternator is not charging.

**cproaudio** · 10-30-2003, 03:44 AM

my bad, its not charging.

**frodobaggins** · 10-30-2003, 04:24 AM

sure fire way to test:

start car
remove positive battery cable.
(if car dies, alternator definately bad)
If car doesn't die:
check voltage between positive from alternator and ground.

If too low, then alternator bad.