Engine Repair

How to tell if a Cars Alternator is going Bad

Mo Bruce's image for:
"How to tell if a Cars Alternator is going Bad"
Image by: 

The alternator could be considered as the heart of your car.   It provides the current to operate and regulate almost every part on the car, either directly or indirectly.   From starting your car, to powering your radio, without the alternator, nothing works.   So how do you tell if a car’s alternator is going bad?  

First of all, assess the symptoms, and work through the possibilities logically.   You need to eliminate each possible cause to rectify the fault.   So if your battery is not holding a charge, your alternator may not be providing a charge at the correct output.  

Taking a reading using a millimetre attached to the battery posts will provide the necessary information.   You must do this test while the engine is running.   The meter should show a reading over 13 volts if your alternator is ok.

There is a simple test you can do as part of your regular maintenance.   This test will give you and idea of the state of both your battery and your alternator.   It is also a handy elimination test if you think your alternator is going bad.

Start your car and put the car dip beam headlights on.   Your car should be facing a handy wall or door, so that you see the headlight beams clearly.   If you now rev your engine, your headlights should brighten slightly.   If they do, your alternator is still producing an acceptable charge.

To eliminate a faulty battery, you should switch your headlights onto full beam, without starting your car.   After about ten or fifteen minutes, your headlights will start to dim if your battery is on its way out.   If they don’t, your battery is holding its charge.

If your battery warning light on the dash remains illuminated when the engine is running then give your engine a rev.   If the light doesn’t go out, then there is no charge going to the battery.   Your alternator brushes may be worn, or it could be a problem with your voltage regulator.  

Unfortunately most regulators are integrated into the alternator, you have to bite the bullet and change the whole thing.   It could be worth checking the integrity of the wiring from the alternator.   If there is a break in any of these, your charging system will not work.   Check the warning bulb while you’re at it.   It could be that your system is actually fine, but the bulb is blown.

There is a small wire, which can be easily overlooked, and it runs from the alternator to the positive post on the battery.   Without this little wire, the charge the alternator is pumping out cannot get to the battery.

It also won’t hurt to give the fuse box a check over.   It isn’t just a blown fuse that can stop a car in its tracks.   Corrosion on the rear of the fuse box can reduce the efficiency of the current through the system.   Loose connectors or fuse fittings will also affect the integrity of the system.

Always check the connections before blaming the more expensive part.   Once these are eliminated, then it’s time to replace the alternator.   You could try a repair yourself, but you could also end up really botching it.   It is much easier to get a new one, especially since most auto parts shops offer an exchange system.   Just hand in your old alternator in part exchange, and get a new one.   A much more simple road to go down.

More about this author: Mo Bruce

From Around the Web