It’s hard to think of anything more crucial to a car’s health than its alternator. It’s like the car’s beating heart, pumping not blood but electricity. Without it, a car will die – slowly, sometimes – but die nevertheless.
You see, a car may have a big heavy battery full of electricity but unlike a battery in a TV remote, it soon runs out of the stuff.
Yes, you could go to the shops and replace the battery with a new, fully charged one but you’d be buying one almost every hour on the hour, assuming you drove the car constantly in between.
The clue to a car battery’s job lies in three letters: SLI. They stand for starting, lighting and ignition. That’s essentially what a battery does.
It supplies the power to perform these vital functions – nothing more, nothing less.
So what’s this got to do with the alternator? Simple: the alternator is the thing that generates electricity to recharge the battery, after the battery has started the engine. The alternator also powers the car’s electrical systems when the engine is running, so the battery doesn’t have to.
Where is the alternator and what does it look like?
The alternator is about the size of an old-fashioned pint glass and has lots of vents on its aluminium body that allow it to keep cool as it does its job.
On one end of it is a pulley around which is a rubber belt. This same belt also goes around another pulley attached to the engine.
As the engine turns, the belt turns with it and as it does so, it turns the pulley on the alternator. This pulley sits on a rotating shaft, called a rotor, that goes into the alternator.
Look around the lower front and sides of your car’s engine. You’ll spot the alternator because unlike other components, it’s got those vents. You might also see that it’s pivoted at one end.
Slackening off the bolt that secures this pivot allows you to move the alternator and adjust the belt. Too slack and it won’t turn the rotor; too tight, and it risks damaging the alternator’s bearings.
How does an alternator work?
The rotor is actually an electromagnet spinning inside fixed coils of tightly wound copper wire. These coils are called the stator, because they’re stationary.
As the rotor spins, (remember, it’s being turned by the engine) it generates a small amount of electricity from two carbon bushes touching a couple of metal rings. This small charge generates a much bigger charge inside the stator.
The electrical charge the stator generates is in the form of alternating current (AC). The trouble is, a car battery likes direct current (DC).
The current needs to be rectified, so enter the rectifier. Rather like turning water into wine, the rectifier turns AC to DC.
However, the battery likes its DC just so – not too much and not too little. So, to keep the battery happy, the alternator has a regulator that controls the amount of current going to the electromagnetic rotor.
How do you know when the alternator is failing?
Start your car’s engine. Did you notice that little red battery symbol come on briefly? It’s actually a warning symbol which, if it were to stay on, would indicate there’s a problem with the alternator.
If the alternator has still got some life in it, the light may flicker or, if you make it work harder by turning on additional lights, the air con and the wipers, it may stay on. As the alternator really begins to pack up, those same systems may not function properly or stop working altogether.
Since so many things are powered by electricity you’d be surprised what can stop working, including vital instruments. However, it’s likely things will start failing in a pre-determined sequence with comfort features such as heated seats being the first to stop and the headlights, the last.
If you pull over and raise the bonnet with the engine still running you might smell the alternator overheating, hear it making a graunching or whining noise or even, if you haven’t had it checked recently, see the rubber belt slipping or flapping about.
How do I know it’s not the battery at fault?
When the alternator begins to fail or simply fails, the battery takes over but as we’ve seen, it doesn’t last long. The trouble is, it’s slow death gets mixed up with the alternator’s so it can be hard to tell which is at fault.
One easy way is to check the condition of the battery with a voltmeter. Another is to jump start the car.
If the car starts and runs but soon stops, it’s the alternator that’s at fault because it’s not generating any electricity. If the car runs OK, the battery is likely to be the culprit. Time for a new one.