If you've been wondering how to test the Alternator on your truck... this is the test to do.

It'll help you to diagnose a BAD Alternator in a pinch. It's a fast test and more importantly an effective test to find out if the alternator is in fact bad.

This test doesn't require any tools, such as a multimeter or anything else. And whether you're driving a Ford, a Dodge, a Chevy, or whatever, this test will work.

Also, you can avoid the most common mistake made when testing an alternator of disconnecting the negative terminal of the Battery while the engine is running. Not to mention that this is a really, really bad way of checking for a bad alternator.

Here are the common symptoms of a BAD Alternator:

The vehicle does not start.

You know it's not the Starter Motor because if you Jump-Start the vehicle it Starts and Runs but after a few minutes or seconds after you disconnected the Jumper Cables from your vehicle's Battery, the vehicle dies.

Or the vehicle may stay running for a while until you start turning on accessories (like headlights, a/c, wipers, etc.).
The Battery light comes on and stays on the whole time the vehicle is running.

At night, when you turn on the headlights, both headlights and dashlights are very dim.


Here is how to test without any tools

Start the vehicle. Either jump-start it or charge the Battery to get it started.

Open the door and leave it open so that the dome light can come on and stay on.

Disconnect the jumper-cables or whatever it was that you used to start the vehicle.

Go back to the inside of the vehicle and eyeball the dome light.

If the vehicle stayed running after disconnecting whatever it was that got it going, then notice the dome light's intensity.

Now turn on the headlights and notice the light's intensity once again.

The light should dim a little and then resume its brilliance to the way it was before the headlights were turned on. If this happens the alternator is doing good.

If the light dims and DOES NOT return to its previous intensity and CONTINUES to dim, the alternator is faulty.

With the vehicle running and with the headlights still on...

Turn on the blower motor on high and notice the light's intensity once again.

The light should dim a little and then resume its brilliance to the way it was before the headlights and the blower motor were turned on. If this happens then the alternator is doing good.

If the light dims and DOES NOT return to its previous intensity and CONTINUES to dim, the alternator is faulty and not charging the Battery or supplying the vehicle's electrical needs.

If the alternator is BAD, as you turn on load after load (headlights, blower motor, wipers, turn signals, etc.) then eventually the dome light and the headlights will get very, very dim till the vehicle stalls.

If the alternator is not BAD, then the dome light and the headlights will stay on and brightly lit (after a slight dim as the load normalizes) no matter what you turn on.

It doesn't matter how discharged the Battery is. After the vehicle is started, either by a jump-start or by charging the Battery, this test applies.

 

Then there is the half a dozen auto parts stores that give free alternator test.

 

And it's easier and more accurate. IMHO

 

What are some signs other then dimming lights that a alternator is going bad? Tonight everything seemed fine with my truck untill i went to start it after leaving barnes and noble. I put the key in and tured, heard a pop then eveyrthing went dead. Nothing worked at all. Called triple A they came and gave me a jump start. It worked but i couldn't believe i needed it. Its a brand new battery purchased in september.

 

You most likely have a loose or a dirty connection.The fastest fix would be charge the battery so it has a full charge than if it starts get the battery and the alternator checked. If it does not start you have a poor connection.

 

They jumped it and it started right up ,,, did it start on it's own since then ??
Like he said it's likely a bad connection but when in doubt have someone do a free alternator test, autozone, oreilly, ....... and have them check the battery too.
First take off both the battery terminals as the side posts like to grow green crap on the side of the terminal contacting the battery. I've had to get in there with a screwdriver and scrape the stuff off as a wire brush isn't strong enough to cut through it.