How do I test my alternator?

[OIF1VET]

My damn battery keeps going dead.. Not sure if I have a short or if the alternator is not doing it's job.. If I run it for 10-15 minutes she'll crank right up, but a couple days later it's hard to turn over or dead.. Been having this issue since I bought it, driving me crazy..Thanks!

[BP_Z28]

http://www.aa1car.com/library/charging_checks.htm



Here is what you do, simple car alternator test ;

Make sure all the accessories on your car are turned off (lights, radio etc.)
and then rev up the cars engine to a fast idle (say 2000-2500 rpm).
Set the Voltmeter to the DC scale and measure the voltage across the battery terminals - red lead of the voltmeter on the positive terminal, black on the negative.
You will need someone to rev the engine whilst you connect the meter to the battery.

With the engine at a fast idle, the voltage on the meter should read around 14 volts (13.5 to 14.4). The alternator needs to generate a larger voltage than the battery's rated voltage to overcome the internal resistance of the battery.
The current needed to recharge the battery would not flow at all if the alternator output voltage was the same. The larger the difference between the alternator output and the battery voltage, then the quicker the battery will charge.

So as you can imagine, if your car is quite old and the alternator and the battery are too, problems can occur with the battery getting enough charge and making the car more difficult to start and less reliable.

If the reading on the volt meter is less than 12 volts then you may have a failed alternator


.


One more check ;

Next, turn on the heater, the rear window de-mister, the radio, the headlights and anything else that draws power from the battery.

Now rev up the engine again and look at the the voltmeter.

It should still be reading around 14 volts. If it reads lower than 13 volts the chances are that the alternator is faulty and will need to be replaced before it catches you out.

What to do if the alternator has failed

[BP_Z28]

Or if you dont want to do it.. Autozone does it for free

[OIF1VET]

Thank you!

[BP_Z28]

no prob.

[GETGONE]

If you're only running it for 10-15 minutes at a time that's not enough to recharge the battery. You're draining it more than you're charging it. If the alt is dead/dying your car would die after driving it for a while. How old and what kind is the battery? You could have a dead cell and it's just not holding a charge.

[81DaytonaPaceCar]

All good points above...another indicator is if you start turning major electrical on (defrost, lights, etc.), and for every one you turn on you see a NOTICABLE drop at idle, it means that a problem is developing...not necessarily manifested yet, but developing.

On the plus side, an alternator is one of the easiest things to rebuild, and the kits are something like $30 or less.

[OIF1VET]

If you're only running it for 10-15 minutes at a time that's not enough to recharge the battery. You're draining it more than you're charging it. If the alt is dead/dying your car would die after driving it for a while. How old and what kind is the battery? You could have a dead cell and it's just not holding a charge.

Running a yellow top optima...

[MP81]

Do you have an alarm?

[2000Durahoe]

My money would be on a bad battery. If the alternator out it would keep dying because once the battery is dead its dead. If you can run it for 10-15 and it starts back up your alternator is working. Autozone can run and alternator test and a battery stress test for you for free. My car does the same thing and I know its because my battery is toast. Its a 1998 Camaro with a factory AC Delco battery. I'm afraid to stress test it though because if it gets to weak a stress test can actually push it over the edge and kill it for good.

[srknova74]

Valvoline Instant Oil Change also does the battery/alternator/starter/battery draw test for free. They use the Interstate ED18, which essentially does the exact same thing 'Yoshi' is talking about, it just puts it in neat graphs, pics, and english. Just drive in and ask for it - wait in your car - recieve a full print out of the charging system for free. Takes about 5 minutes. Optima batteries are glass-matt batteries and usually a good charge on a charger (or a decent pushing alternator and a 30 minute drive) will juice 'em right back up. Make sure your battery connections are clean and tight. Although this is a very simple thing to do it is often overlooked and will cause the exact problem you are describing. Good luck brother.

[OIF1VET]

Do you have an alarm?

Yes, something is drawing juice when it sits. If I unhook the battery I have no issues and it can sit forever..

[GETGONE]

It's probably a dead cell in the Optima. My dad had that problem. It wouldn't hold a charge for more than a week or two if it wasn't started. Once it was charged it would start the car and would be fine. My yellow top died last year because after Autorama I didn't take it out and it went dead from sitting in the car. I took it out this year and it went dead from sitting for 3+ months. I haven't tried starting it again since I put it in the car and charged it 2 weeks ago.

[GETGONE]

An alarm will drain the battery too. Get a Battery Tender and just keep it plugged into that. I need to get one for mine and wire up an outlet on that side of the garage.

[lozanoa11]

get a red top. the yellow tops are deep cycles and only work good if you run it dead and fully charge it. many times