How to Test Your Car Alternator for Power

Often times when a car battery isn't providing enough power to start a car the first thought is that the battery is bad and should be replaced. Before buying an expensive new battery that you may not need at all, it would be wise to check the alternator first to see if it is providing proper power to the battery.The job of the alternator is to charge up your battery, and if the alternator is doing it's job providing voltage, then your battery just may be the problem and simply cannot hold the power provided from the alternator. However, if the alternator is not working properly and charging the battery, the battery may be fine but a new alternator may be in order. There is a very simple test you can do yourself to check out that alternator.

Difficulty: Moderately Easy

Instructions

Things You'll Need:

• Volt meter tester. Sold at most auto parts supply stores.

1. 
   Alternator in view, the silver and black rounded part.

   The alternator can be tested while the engine is not running and also while the engine is running. The test method is the same but you will get two different readings.

2. 
   Close up of alternator

   Locate your alternator, which will have a pulley belt attached to it. Picture shows you what a typical alternator looks like.

3. 
   Testing the belt for tightness

   While the engine is "off" check the belt that turns the alternator pulley for tightness. The belt should be tight enough so that when pressed down with your thumb in a center section away from the pulleys, that you have no more than an inch or so of slack in the belt. Also check the condition of the belt looking for any breaks or serious cracks, in which case a new belt may be in order because in order for the alternator to create energy to the battery the alternator pulley must turn smoothly.

4. 
   Typical Volt Meter Tester

   Set your volt meter to the 20 V (volts) setting. See picture for proper setting and to view a typical volt meter tester.

5. Start the engine and be very cautious that you keep your hands, clothing and jewelry away from the moving engine parts, such as fan blades, pulleys and belts. Be "very" aware of these areas and stay clear. Now observe the alternator pulley and be sure that the belt is running straight and turning the pulley without missing or skipping. If it is not, chances are that your belt is damaged or too loose which should be corrected before you can perform a proper test. Sometimes you may be able to create more tension on the belt by adjusting the alternator back further in it's bracket.

6. Assuming the belt is turning the alternator pulley you can now set the volt meter down out of the way safely and yet still have a good view of it with while both of your hands are busy.

7. 
   Red to Alternator, black probe to ground

   You want to touch the "red" positive cable probe from the volt meter onto the red terminal connector coming out of the rear of the alternator. This terminal would typically have a heavier wire attached to it. Now with the black cable probe coming from the volt meter, which is your grounding source, find a nearby ground which could be a nut or bolt on the frame or firewall of the car. Try and find a ground that is bare metal and paint free. Do not ground to the alternator itself.

8. Your volt meter should now give you a reading. If you are not getting a reading chances are that you are not touching the red terminal firmly enough or have not found a proper ground. The meter may jump around with readings for the first few moments but if the alternator is working properly you should end up with a reading of 14 volts or more with the engine running.

9. If your readings for your alternator are not satisfactory than that would mean that the alternator is not providing proper power to your battery and a new alternator may be in order or possibly the wiring at the alternator may be loose or corroded or maybe you have a bad pulley belt or possibly the voltage regulator may be faulty in which case you may want to have a professional check out for further confirmation.If you are confident with your test, simply replace the alternator.

Tips & Warnings

• A weak battery can burn out an alternator before it's time since it would be constantly working hard trying to keep power in the battery.
• Always check that you have sound corrosion free connections at the alternator before performing the electrical test.
• ALWAYS be very cautious when you are working on the engine while the car is running. Double check that there are no rags or tools lying around within the engine compartment and remove any necklaces, bracelet's, ties or loose clothing that may get caught up in a moving part.

[bluueone]

on 6/27/2009 as the above states, testing the alternator alone doesn't necessarily give you a full picture of the alternator/battery duet. a good way to go about this is to test the voltage at the BATTERY, not the alternator. this is SIMPLE and will make you feel like a pro even if you've never done anything under the hood before:

***car batteries contain very dangerous chemicals - always wear proper eye/skin protection when dealing with them and NO SMOKING!!***

ok, start with the engine OFF and test the voltage of the battery (red meter probe to the positive battery node, black meter probe to the negative battery node) Doing this tests to make sure your battery is holding a charge even when voltage is not being supplied by the alternator. the average battery should read in at about 12 volts. a little higher's ok... but if you're down below 10, that's trouble and may mean your battery's no...

[DIYdaddyDawg]

on 12/23/2008 Does anyone know when testing the alternator on the Motomaster Nautilus Intelligent Battery Charger, if the reading should be 100? My current reading is 75 with the engine running.

[DIYdaddyDawg]

on 12/23/2008 Does anyone know when testing the alternator on the Motomaster Nautilus Intelligent Battery Charger, if the reading should be 100? My current reading is 75 with the engine running.

[nomader]

on 7/27/2008 "To test with the engine off..."
If you are testing with the engine off, you are not directly testing the alternator at all -- you are just reading the battery's voltage. And if it is lower than it should be, then you don't really gain any information about the nature of your problem, be it battery or alternator.