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The Complete Guide to Testing Your Car Alternator

Alternators

David Taylor

28th Feb 2018

It’s Monday morning and you’re running late for work. There’s no time for tea and toast so you just grab your car keys and go. You put the key in the ignition- and nothing. Your car won’t start. It’s a common occurrence and it could be your alternator. It happens to most of us at some point, however, there are usually warning signs. So what should you look out for? How do you know if you have a bad alternator?

Here are six signs that point towards a bad alternator. If you spot them early, you could save yourself a costly breakdown fee.

Your lights are dim

Are your car headlights dim? Maybe the lights on the dashboard are not as bright as they used to be? This is a strong signal that your alternator is not running efficiently. An alternator is a key part of a vehicle’s electrical system and provides enough power to operate all electronic accessories, from the car radio to the powered windows.

You can smell burning rubber

Have you smelled a rather unpleasant whiff of burning rubber recently? A car alternator is powered by a series of belts. If one of these belts does not turn freely and slips on the alternator pulley, it can create friction and produce a burning rubber smell.

Your car is making unusual sounds

It’s possible that you may hear a few odd rattling sounds. This is a sign that not all is as it seems. When an alternator fails it can cause bearings to fail in the engine. So, that unusual sound that your vehicle is making could, in fact, be the alternator.

Your warning light is on

Many modern cars make it easy for you spot an alternator issue because they have a dashboard warning light. The warning light in a majority of vehicles will take the shape of a battery, however, some differentiate the route of the problem by displaying ALT for alternator or GEN for the generator.

You spot visual clues

One obvious sign of a bad alternator is any noticeable damage to the serpentine belt, or, one of the connecting belts. Does it look loose? Or maybe too tight? You might even notice that a belt is looking worn or cracked.

Your car battery is flat

While a car battery can fail of its own accord, it can also be caused by the alternator not functioning correctly. This is because the alternator is failing to recharge the battery, meaning it uses up its capacity.

For more information about spotting the signs of a failing alternator, check out our previous guide by clicking here. If you spot the early warning signs of a failing alternator, you could avoid a costly breakdown.

So, you suspect you may have a problem with your alternator but what do you do next? It is possible to run basic checks on your vehicle using a multimeter. A multimeter is a testing tool that measures two or more electrical values. It typically includes a voltmeter, for measuring volts, an ammeter for measuring amps and an ohmmeter for measuring ohms.

Multimeters can be purchased for £20-£30.

Step One: Set your Multimeter

The first step is to open up your bonnet and set your multimeter to DCV (which stands for direct current volts.) Set the multimeter so it points to 20 DCV.

Step Two: Locate Positive and Negative Battery Terminals

Every car battery has a positive and negative battery terminal. Before connecting your multimeter, locate which one is which. Some car batteries may indicate this by using a (+ ) sign for the positive terminal and a (-) sign for the negative terminal. However, if your car battery terminals are not clearly marked, remember that the positive terminal is always the bigger one.

Step Three: Attach the Multimeter to Battery Terminals

Now that your multimeter is set to 20 DCV and you have distinguished which is the positive battery terminal and which is the negative battery terminal, it’s time to connect your multimeter. The red wire of the multimeter should be connected to the positive battery terminal and the black wire of the multimeter should be connected to the negative battery terminal. At this stage, the multimeter should ideally read around 12.6v. This means that the battery is properly charged.

Step Four: Start the Engine

To check the functionality of the alternator, you need to switch on your car engine. Only when the car engine is running can you check the efficiency of your alternator.

Step Five: Check your Voltage Reading

When the multimeter is connected to battery terminals and the engine running, a good alternator will show a reading between 14.2v and 14.7v. A voltage reading within this range means that the alternator is running efficiently enough to charge the car battery. If the voltage reading is over 14.7v, it means that your alternator is likely to be overcharging your car battery. If the voltage reading is below 14.2v, it means that the alternator is not charging the battery enough. While the battery will still charge if the multimeter reading is above 12.6v, it will not generate enough power to charge any electronic accessories. This can cause some of the more noticeable warning signs listed above, such as dim lights or failure of electric windows.

Step Six: Load Test your Alternator

To fully test an alternator’s ability to charge both the car battery and other electronic accessories, a load test must be carried out. This simply requires you to switch on a combination of electrical features. Turn on your car headlights, switch on your fan and your car radio; components that you are likely to run simultaneously during a car journey. If you have a good alternator, the reading should not change much. The voltage reading should not fall below 13v.

Step Seven: Turn off your Engine

Now it’s time to turn off your running engine. When you do this the multimeter should still read 12.6v or higher. This means that the alternator is still charging the battery. If you carry out these checks and find that your alternator is failing, don’t forget that we will buy your old core. Find out how you can contact us via the bottom of the homepage.
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