Alternator failure: symptoms, causes and what to do

Your alternator is the part that generates electricity while the engine runs, powering the ignition, lights, fuel system and everything else, and keeping the battery topped up. When it stops, the car carries on using only what is stored in the battery, and that does not last long.

This guide covers the symptoms that suggest the alternator is failing, other faults that look similar, what you can safely check and what the garage will do to confirm it.

What are the symptoms of a failing alternator?

The most direct sign is the battery-shaped warning light staying on once the engine is running. Other signs build up gradually: headlights that dim or brighten with engine speed, a whining or grinding sound from the front of the engine, and electrical items working weakly.

As the output falls, the battery drains and the car may become hard to start or stall while driving. A burning smell can point to a slipping belt or an overworked alternator.

• Battery or charging warning light on while driving
• Headlights and dashboard lights dimming or flickering
• Whining, growling or grinding from the engine bay
• Slow starting or a battery that keeps going flat
• Stalling, especially after switching on lights or the heater fan

Is it the alternator or the battery?

The two are easy to confuse because both end in a car that will not start. A weak battery typically causes slow cranking from the start, while a charging fault causes the battery to run down over a drive, so the car may start fine, then fail later or the next morning.

The only reliable way to separate them is to test both. A new battery fitted to a car with a faulty alternator will soon go flat as well, so it is worth checking the charging output at the same time.

What else can cause a charging fault?

Not every charging problem is the alternator itself. The drive belt may be worn, loose or slipping, so the alternator is not being turned properly. A belt-tensioner fault, corroded or loose cables, a poor earth connection or a damaged wiring connector can all stop the charge getting through.

The alternator may also have worn bearings, which make the whining or growling sound, or worn internal parts that reduce its output. Which fault it is determines what needs replacing, so testing before parts are ordered keeps the repair accurate.

What will a garage check and how is it repaired?

The technician will look at the drive belt condition and tension, then measure the voltage at the battery with the engine off and running to see whether the alternator is charging within the range the manufacturer expects. They will also check cables and connections, and the battery itself.

If the alternator has failed, it is usually replaced or sometimes exchanged for a reconditioned unit, depending on the vehicle. At Automaster, a garage at Longcombe Cross near Totnes, we confirm the cause first and tell you what we found. Cost depends on the make and model, how easy the alternator is to reach, whether the belt also needs replacing and the type of unit fitted, so ask for a quote and we will not promise a repair before inspecting the car.

Can I keep driving with a bad alternator?

Not for long. With a charging fault the car relies on the battery, which may last only a short distance, and sometimes much less at night with lights and heater on. When it runs out, the engine can cut out, which may remove power steering assistance and brake servo assistance at the same time.

If the warning light comes on, switch off unnecessary electrics, avoid stopping the engine and make for a safe place. If you hear a squealing belt or see the temperature gauge rising, stop immediately, as the belt may also drive the water pump on some engines.