I got a weird problem. Need some help. I figured my alternator was going on the vette cause I know they are notorious for this. My voltage was reading around 13-13.3....when I shut the car off it would instantly fall to 13 then 5 seconds later would be at 12.5 then 12 and then down into the 11's. So obviously a problem.

Now the odd part. I change out the alternator and put a new one in. 14.4 right away for about 2 minutes then boom. 12.5 and finally it stays toggling around 11 and won't come back up.

Ok so now Iam guessing hmmm bad battery? So I take mine out and use another backup one we have around the house. Tested it first. Straight voltage from the battery is reading 12.5. I hook it up. No good. It also won't charge. In ends up reading around the same thing ultimately, bout 12 high 11's.

So any ideas? Did I get another bum alternator? How can you effectively test the alternator vs the battery.

The kid (and I use that term cause he was one) the kid at the auto parts store claims he checked my alternator and that it passed fine. But I don't hold much in that.


thanks
Jeff

 

I uploaded this, it may be of some help
http://members.shaw.ca/agent86/Start...g%20System.pdf

 

I believe when the car is turned off, the voltage will steadily drop. As soon as you turn it on, it will come back to 13-14 V. Your car battery is 12V, so if that's what you're reading or close to it, you have no problem. It only goes to 13-14 V when the car is turned on.

 

The alternator voltage regulator attempts to maintain 14.7 volts across the battery, engine running. C4 alternators will show a dash voltage of 14.3 cold and 13.3 hot and there is 0.3 volts drop across the ignition switch contacts, so the battery terminal voltage is 0.3 volts higher than the dash reading. Also, I have observed that the dash voltmeter has error (on the low side) when the engine is not running, so I would not trust the reading when the engine is not running. You should measure the battery terminal voltage with a separate voltmeter and not rely on the dash voltmeter. You can tell the state of charge in a lead acid car battery by measuring its terminal voltage when it has sat for several hours unused. 12.0 volts and lower, discharged. 12.9 volts and higher, fully charged. The % charge is linear between these voltages. I doubt your original alternator had anything wrong with it and maybe the battery too! The most common failure of the alternator is that it stops charging the battery and in this case the red battery symbol will light on your dash.

 

Not quite sure why you figure that my original alternator or battery were ok and that I didn't have a problem.

I was not using my dash for checking the voltage. I have installed a separate voltmeter from summit and in addition to this I used a voltmeter to double check readings.

As stated something is not right. After just 1 day sitting the car can't even start on its own. The voltage is dropping that bad.

With my original alternator in, the car would not go higher then 13.3 volts at any point, whether the car was running or not. When the car was off, as stated earlier, the voltage drops fast and steadily.

With the new alternator in, problem was fixed at first. Then got worst. For the first two minutes it got up to 14.4 when I first put it in. Then plummeted to 12 volts and finally hovered around 11.8-12. This was with the car on and revving the engine.

So Iam just trying to figure out....hmmm another bum alternator or is their something else within the electrical system that could be causeing this. I double checked my serpentine belt. Its not old at all, has no cracks and it nice and tight.

Thanks for the help so far.

Jeff

 

You've got something drawing current such as a short or something is on when it shouldn't be. You're going to have to find what's drawing the power from the battery. You don't want to kill a battery too often so I suggest disconnecting the battery when your not working on it until you find your problem.

 

My battery light is on, on the dash. This happens whenever voltage drops below 12 volts.

 

The battery light tells you that the alternator is not charging the battery! Measure the alternator voltage on its output terminal on the rear of the alternator with the engine running, it should be 14.7 volts alternator cold, then measure the voltage across the battery terminals, it should be the same as the alternator output voltage. I suspect that the fusible link in the wire that goes between the alternator output and the battery is defective and needs to be replaced. The link could be badly burned and be intermittent or has resistance that causes it to get hot during charging to the point that it disconnects until it cools down again. This resistive link can cause voltage drop in the alternator charging path and may be the reason why your battery voltage is so low (13 volts) with the engine running.
Or, you may have a large current drain and you can find this out by disconnecting the negative cable and placing an ammeter in series between the battery terminal and the negative cable. After the courtesy lights time out, you should not see more than 50 milliamps of leakage current. My 87 draws 27 ma.
Charge you battery up with a battery charger so you can start your engine and make tests. Even if you fix the problem, don't use the alternator to charge your battery up, charge it with a battery charger!

 

In addition to others suggestions. It's possible for a bad battery to cause charging problems. I think getting both batteries "load" tested may be in order. As well as doing a current drain check on your car.

Good luck,
Chris