99 Grand Prix - alternator

[Tom7227]

Kids car had check engine light blink three times. Now she said that the lights dim imtermittently while driving at speed. Charger only went up to 2 amps so the battery doesn't seem drawn down. Any way to tell if it's the alternator short of removing and testing it? I'm DIYer with a volt/ohm meter but not much more to diagnose auto electrical problems. 99 Grend Prix with about 160K and a rebuilt engine.

Thanks for any help you can give.

Tom

[jeffreyd]

put your meter on dc volts start the engine put the red lead on positive cable at battery black on neg at the battery. read the meter it should be showing around 13 volts or so if you turn lights on it should go up. if it is below 12.7 i would say you have a bad alt. you could also take it to a parts store and they can check the whole system for free.

[Eric Wettschreck]

What I would do........

As stated, put voltmeter on dc and check battery while running. If not at least 13 or so volts it's not getting a charge. If below 13 volts I'd check what I call the primary wire attaching to the alternator. With the motor running it should show voltage (12 vdc). This means (In my non-ASE tech language) the field in the alternator is excited.

If you have voltage to the primary wire, and less than 13 volts on the battery, good chance the alternator is weak or dead. Don't know if it's a good idea on newer motors, but on the old school stuff I used to remove the positive cable from the battery while the car is running. If the car stayed running I knew the alternator was good. IF the car died the alternator was kaput. Once again, I don't know if this is a good thing to do on the more electronic controlled motors.

[Jeremy airjer W]

Not a good idea!

Some GM's you'll need a scan tool to turn on the "exciter" wire while the engine is not running.

[Whoaru99]

Might be tough if it's an intermittent.

If you have not checked all connections on starter, alternator, battery, any auxilary battery feeds/fuse blocks, that's where I'd start. Given it was worthy of mention the engine has been rebuilt, I think there's at least an average chance of a loose connection somewere.

[Jeremy airjer W]

Rebuilt motor?

Maybe check the ground cable that bolts to the transmission bolt down by the starter. Wouldn't be the first time I left....I mean saw one loose. They can cause some really funky issues.

[jeffreyd]

as noted above, check the ground cable from battery to frame, the alt should be case grounded. DO NOT remove the positive cable while the engine is running. You will cause all sorts of issues with the computer. You could do this years ago on non computer engines but not any longer. Is the battery low in the am? you might let it sit overnight and prior to starting the engine, touch the alt to see if it warm. Could be a leaky diode but the battery would be low or dead in the am.

[Tom7227]

Bad bearings. Replaced it and the light was gone. Couple of hours of fun, even got to break out the metric tools and the magnet when I dropped stuff. Great evening with the kid.

[Whoaru99]

Yup...that can do it too. Last alternator I had bearings go out on went pretty quick. Noticed an occasional squeak but didn't think much about it. Next thing I knew, the volt gauge went nuts and when I looked under the hood, sparks were flying out of the alternator!