Idle voltage

Chris

Chris

YARRR SUBY MONSTER!!
So at idle my headlights have always dimmed considerably. And I always attributed it to low idle, but now that I have a scangauge2 I know that my car idles at between 685 and 700rpms. Right where it should. (my tach actually reads about 300rpm low...)

But at idle my voltage drops to about 12v. If it sits and idles for a while, esp with radio and high beams, it drops down to about 11.5.

The voltage at the battery, 12v socket, and on my scangauge all agree by about 0.1v. (I checked at the battery and 12v socket with a digital meter)

When I rev up the engine the voltage jumps back up though, and never goes above 14.4 or 14.5 no matter how high I rev.

Even if the battery is weak the alternator should always keep it above 12.5 or something.

The battery actually did die this winter (friggin parking light switch...). It went down to about 2v, but after charging it up, it seemed to recover.

So what's up? Alternator? Regulator? Battery?

I'm tending to think it isn't the battery because I never had a problem starting even in the coldest -20

 
Are all of the connections clean of corrosion? That could cause the voltage drop. An easy thing to check anyways...

 
Alternator starting to go. Does it leak down and if you press on gas pedal they will come back to life. I would go to AutoZone and see if they can check it for free.

 
Alternator starting to go. Does it leak down and if you press on gas pedal they will come back to life. I would go to AutoZone and see if they can check it for free.
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How do they check it? Anything I can do, or do they have some special alternator tester?

I think the closest autozone to me is like 25 miles :/

If it is the alternator, I can probably get a used one pretty cheap though. And I have time to look before it fails.

Since I started this thread, Murphy's law says it will fail tomorrow
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I think napa tests the alternators. Or Morins Auto in Brunswick
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Yeah, I'll ask napa. I have some stuff to bring back anyway. So I may as well ask then.

If that is the problem I'll replace it ASAP. Knowing my luck, it would fail at the most inconvenient time possible

 
GRRRR, I'm so friggin sick of cars .... I'm building a motorized bicycle that gets 150mpg, has about 5 moving parts, and is extremely easy to fix...

Sorry for the rant, cars piss me off, and there is still something wrong with it after some certain mechanic fixed my CV boot...

 
I know when my old cars alternator went. First one night my lights started dimming. I got home fine. Next day I drove around town next thing you know car was almost shutting off all by itself. Then I stopped at my friends shop he charged my battery up. Thought I had enough juice to get home but just missed it. Brother came out we hooked cars up and charged again. Got home pulled aternator. Was complety locked up. Had guy in town rebuild it for me.

 
Knowing my luck it would happen when I was in acadia... I'll get it tested ASAP.

 
have you check the voltage while the car is running and the battery is disconnected to see if the voltage still drops below 12v again

 
have you check the voltage while the car is running and the battery is disconnected to see if the voltage still drops below 12v again
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Do you mean to disconnect the battery while the car is running, and test the cables?

I thought doing that was bad for the alternator/charge controller.

But if it isn't, I'll go give it a try.

 
what? how does your car run with the battery disconnected? The alternator is only there to keep the battery charged not to run the car and charge the battery.

 
I'm not sure... It would probably cause a surge and blow things. I'd rather not find out.

 
no your car is run by your alt. your battery is there pretty much just to start it and for when your car isn't running for acc. and i've own 100's of cars and it is fine to disconnect the battery while it's running just don't let the pos. terminal touch any grounds and you'll be fine

 
Gabe is right, the car's electrical system is run from alternator.

Check all grounds first make sure there are no problems there. If grounds are good and voltage at battery is low at correct idle rpm then the alternator is bad. End of story.

 
I have read on many places that on 1980's cars and above taking the battery off of your car with it cranked can cause some serious issues.

I never knew taking the battery off the car could you still run the car. I guess I didn't know this because all cars I have owned were or are newer than 1980s

 
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