Fast and Easy On Car Bad Alternator Diagnostic Test

This is the EASIEST on car test for testing a bad alternator! If you've been wondering how to test the alternator on your car, 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 ON CAR TEST doesn't require any tools, such as a multimeter or anything else. And whether you're driving a Ford, a Chrysler or a Dodge or a Plymouth, a Nissan, a Mitsubishi, a Volkswagen 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.

You can find this tutorial in Spanish here: La Prueba Más Fácil Para Verificar Un Alternador Fallando (at: autotecnico-online.com).

Common Symptoms Of A Bad Alternator

Here are the basic symptoms of a bad alternator:

1. The vehicle does not start.
2. You know it's not the starter motor because if you jump-start the vehicle, it starts and runs but:
   • A few minutes or seconds after you disconnected the jumper cables from your vehicle's battery, the vehicle dies.
3. Or the vehicle may stay running for a while until you start turning on accessories (like headlights, a/c, wipers, etc.).
4. The battery light comes on and stays on the whole time the vehicle is running.
5. At dark, when you turn on the headlights, both headlights and dash-lights are very dim.

Tests To See If Alternator Is Bad Or Not

OK, let's get started. The test works in this way: You'll be using the vehicle's dome light to diagnose the charging system. You can do this test by yourself or you can have someone helping you.

1. Step 1
   • 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.
2. Step 2
   • 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.

1. Step 3
   • 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.
2. Step 4
   • 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.

Interpreting The Results Of The Alternator Test

Here's an overview of the results from this simple and basic test:

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. And it doesn't matter what type of vehicle it is (whether it's a Chevrolet, Hyundai, Buick, Pontiac, Honda, Suzuki, Oldsmobile or whatever), this simple test will get you going.

If you are a Do It Yourself-er, after you have removed the alternator from the vehicle, have the auto parts store (where you're buying your alternator from) test it on their bench tester to eliminate any other potential cause of your NO-CHARGING condition.

For a more in depth look and test of the alternator, take a look at the second article on TESTING for a bad ALTERNATOR: