What does an Alternator do?

The alternator in your car is the device that converts the mechanical energy from your engine into electricity, that powers the electrical systems for everything from the car’s computer to your stereo. 

It works alongside your battery to make sure that your car can start up and run the way it should, but when it goes wrong you can be left by the roadside next to a dead car.

Alternator infographic

Signs the Alternator is not Working Properly

There are many ways a faulty or worn alternator can affect the performance of your car. We’re going to go through some of the most common ways to tell when your alternator is on its way out. 

Weak or Dead Battery

This is probably the most common sign of a faulty alternator. Seeing as the alternator’s job is to charge your car’s battery, a weak battery that runs flat all the time is a red flag that your alternator is on its way out. 

Although this might not be the only reason your battery is running flat, it’s often a problem with the alternator or alternator belt. 

Also, if the battery symbol lights up on your dashboard when you switch on the ignition, but doesn’t turn off soon after, it’s another sign that you need to take a good look at your alternator.

discharged battery warning light

Strange Noises

You get to know the noises and sounds that your car makes when you turn on the ignition, but if you start to hear strange sounds it could be your alternator. We know that cars make an awful lot of strange noises at one time or another, but if you think what the alternator does, it might give you a better idea.

The auxiliary belt on your car takes power from the main driveshaft and runs other functions like the fan or air conditioning unit. The alternator is made to run at a relatively fast speed to generate enough electricity, so if it’s damaged or about to give out it can make a grinding or whining sound before completely giving out.

Dim Lights

One of the most visible symptoms of a failing alternator will be lights that dim when you’ve not got your foot on the accelerator. If the lights on the dashboard as well as your headlights dim when your car’s just idling, it’s probably a bad alternator. 

Other causes of this problem can be a slipping auxiliary belt or corroded battery terminals, but it makes sense that your alternator isn’t keeping up with your engine’s idle speed. 

car with dim headlights

Slow Start-Up

You put in your key and try to start your engine as normal, but your starter motor struggles to kick your engine into life. This is another tell-tale sign of a failing alternator. If it wont start at all, it could be a sign of total failure of your alternator. 

Poor Electrical Performance

Our final red flag that might be somewhat easy to spot, or might occur over a longer period, is a loss of performance in your car’s electrical systems. If the windows wind down slowly, or your windscreen wipers struggle to keep up with the rain, it’s quite likely an alternator on its last legs. 

Testing Your Alternator

If you suspect a problem with your alternator, the next steps to take are testing it thoroughly to see if it’s faulty or worn out. 

Tools Needed

  • A multimeter or voltmeter

This common piece of electrical testing equipment is an invaluable tool when it comes to DIY troubleshooting car problems. You can pick them up for next to nothing online or at your local car parts shop, and use it to test your alternator, battery or anything electric in and around your vehicle.

Alternator Testing, Step-by-Step

Before you jump right in to testing the voltage on your alternator, you should check your battery health. A weak, old or damaged battery could cause many of the symptoms detailed above, so grab your multimeter and set it to read voltage at 20V:

  1. Turn on your headlights to get rid of any “surface charge” for about two minutes, then turn them off
  2. Open the bonnet of your car and locate your battery. You might need the manual for this in some modern vehicles as it can be hidden from view
  3. Locate the battery terminals. The positive (+) and negative (-) terminals should be clearly marked
  4. Check your battery cables are properly attached and not damaged or corroded
  5. Using your multimeter, connect the red probe to the positive terminal and the black probe to the negative terminal
  6. The readout on your multimeter should be around 12.6 Volts. Anything lower than this and you have battery problems
  7. Leaving the probes in place, start your engine and watch the readout. The Volts will drop as the alternator kicks in, but if it drops below 10 Volts, it’s a sure sign of a bad battery. Turn off your engine

Now you’ve checked out your battery, it’s time to do the same with your alternator. Leave your multimeter set to 20 Volts DC for this test, and leave the probes connected to your battery terminals

  1. With your engine still running, check the readout on your multimeter, it should read around 14.2 Volts. A low reading means your alternator isn’t keeping up with demand, and a high reading means it’s not regulating the flow of electricity properly
  2. Now’s time to see how your car systems perform with a load on the battery. Turn on your headlights, fans, stereo and other car electrical systems
  3. Now take another look at your multimeter, the readout from the battery should dip slightly to around 14 Volts with all the load on it, but not much more than that. Any lower than 13 Volts and you’re in trouble
  4. Now turn off your car systems that were putting a load on your battery and watch the Volts fall back down
  5. Your reading should be slightly higher than the original one you got when you first tested your battery. This means your alternator is charging it properly. If it’s the same, or less than 12.6 Volts, your alternator isn’t doing its job properly.
  6. Remove the probes from your battery, replace any covers and switch off your engine

testing alternator with multimeter

What to Do Now?

If you followed our steps to test your battery and alternator, and you think it’s not performing well enough, it’s time to talk to your mechanic. They will be able to test your battery thoroughly and check the tension on your auxiliary belt, as well as replace your alternator for a new one. 

It’s possible to change an alternator yourself, but unless you’re confident enough to remove your serpentine belt and work on electrical components, it’s best left to the experts. 

How useful was this post?

Click on a star to rate it!

Scroll to Top