Alternator Repair & Replacement

The alternator is an essential part of your car’s electrical system. You may think that all the power for your car’s lights, air conditioning, radio etc. come from the battery, well, that’s partly right. The battery is initially needed to power the starter motor which starts the car’s engine, but after that, the alternator takes over and provides a constant supply of power to the battery. If your alternator fails to work properly, your battery will run down very quickly and your motor will either fail to start, or if you’re already out on the road, it will come to a halt.

If you find that your car or van is getting a little difficult to start, it may be that the battery is reaching the end of its useful life and is not holding a charge, or it may be a faulty alternator. Rather than trying to guess what the problem is, why not put your vehicle into the hands of an expert from the AutoMend network? With access to more than 9,000 members across the whole of the UK, help is never far away. We have a wide selection of mechanics based in garages and out on the road ready to diagnose and repair your alternator issues without fuss and at a price you will be happy to pay.


A bit of background

The alternator on your car or van is simply a generator of electricity. In fact, in older cars, what is now called the alternator was referred to as the generator or the dynamo.

Modern cars and vans operate on a 12-volt direct current (DC) system, but the alternator produces an alternating current (AC), hence the name. AC is the same type of power as you use in your home to power your TV etc. DC is the same type of power as that produced by the car battery. In order for the car and the battery to use the power generated by the alternator, the AC needs to be converted to DC.

How the alternator operates

In brief, an alternator is made up of a stator, a rotor and a rectifier. The stator is a series of wire coil windings, which remain stationary. The rotor is a powered electro-magnet which revolves inside the stator when the alternator is in operation.

The rotation of the rotor within the stator generates AC electricity (the flow of the current changes with each rotation). This electricity is rectified within the alternator to DC (the current flows one way only) via a series of diodes.

The voltage output from the alternator is controlled by a voltage regulator. The regulator keeps it between 13.5 and 14.5 volts in order to prevent damage to the various electronic devices and components found throughout the vehicle.

How it is powered

The alternator is driven by a belt which is powered by the rotary motion of the car’s crankshaft. The belt may power other components (including the power steering and water pumps and the compressor for the air conditioning) or it may be an individual belt just for the alternator. These types of multiple belt require a periodic tension adjustment to keep them working efficiently.

A more recent innovation is a single belt which drives all of the rotary components. This is known as a ‘serpentine belt’, because it ‘snakes’ around all the various components. Serpentine belts usually have a spring-loaded tensioner, so don’t need a regular tension check, but they do have a projected life (usually around 30,000 miles) before needing to be replaced. It’s wise to get your serpentine belt checked out and replaced by an AutoMend expert; it can save you a great deal of trouble in the long-run.


All cars have some kind of warning system that indicates if the charging system is not working properly. This most often takes the form of a warning light which could be labelled ‘Alt’, ‘Bat’ or ‘Gen’. If the engine is running and this light is illuminated, it indicates a problem with the charging system. It may mean the alternator is not charging the battery, and the most frequent cause of the lack of charge is the alternator drive belt being loose or broken.

In some cars there is a voltmeter or an ammeter on the dashboard, and this displays the amount of voltage or amperage being produced by the alternator, so you can see if it is working efficiently. See below for a description of these terms.


If you have an ammeter or a voltmeter on your dashboard, it may be useful to know what it is really telling you. You don’t need a degree in electrical engineering to understand the basic principles.

The output from your alternator can be measured in volts (the voltmeter) and amps (the ammeter), but to get to the meaning of these measurements, we also need to examine ohms – this is the measure of resistance. We will use the simple but well-established analogy of the flow of water to represent electricity.

Imagine water running through a pipe – the water will exert a certain pressure. The diameter of the pipe (the size of the bore) will exert resistance to the flow of the water. The smaller the bore, the higher the resistance to the flow; the larger the bore, the lower the resistance.

If you want to get more water to flow through the pipe, you have two choices. Either increase the water pressure going through the same sized pipe bore, or increase the bore allowing you to work at a lower pressure. Increasing the water pressure too much can damage the pipe, so there is a need for some kind of regulator.

In terms of direct current (DC) electricity:

The voltage is represented by the water pressure, the amperage is the volume of the water flow, and the size of the pipe bore equates to the size of a wire with the amount of resistance measure in ohms. Just as too much water pressure can damage a pipe, too much voltage can damage electronic components, and needs to be regulated (by the voltage regulator).


Yes there is – as one of the UK’s premier networks of van and car repair specialists, we have locations covering every region. Our members are all trusted and qualified mechanics, able to repair and maintain a wide range of vehicles. They offer best prices, great customer service, and they all hold appropriate industry approvals. We’ve made it totally easy to find your local AutoMend approved garage – we just need a little information from you. We start with your type of vehicle and the repair or maintenance services you are looking for. Add to that your post code and location and we can narrow down your search to all the local garages and mobile mechanics who can offer the service you need.

With thousands of garages and mechanics to choose from, you will always get the best possible deal from the AutoMend network. We give you the option of getting a series of free no-obligation online quotes, and we also provide you with all the details to enable you to contact your local mechanic directly, to discuss your problem and book an appropriate appointment. Don’t get caught out by a dead battery or a faulty alternator – get a great value alternator repair from one of our trusted experts.