Alternator and charging system

Alternator and charging system

The alternator is the car's small power station. Driven by the drive belt from the engine, it produces alternating current that is rectified to 12V and charges the 12V battery while supplying all the car's electrical loads while the engine runs. If the alternator stops working, the car runs on the battery first — then stops.

Symptoms of a failing alternator

The first sign is often that the charging warning light (a small battery icon) comes on in the dashboard, either continuously or intermittently. Other symptoms are that the battery doesn't hold charge between trips, that the lights dim at idle and brighten when you rev the engine, that infotainment restarts or shows fault messages at idle, and — in serious cases — a grinding or whining sound from the engine bay (alternator bearings).

On modern cars, several unexpected faults often appear at the same time: hesitant windows, ESP errors, ABS warnings — all because the voltage is unstable and control modules react to it.

Testing with a voltmeter and load test

The simplest check is a voltmeter on the battery terminals. At idle the voltage should be between 13.8 and 14.4 V if the alternator is charging normally. Below 13.5 V indicates a charging problem; above 14.8 V indicates a faulty regulator that is overcharging (and boiling the battery to death).

We also do a load test where we switch on all major consumers (lights, heater fan, rear window defroster) and measure whether the alternator can maintain voltage. Many alternator faults only show up under load, not at idle without consumers.

Common sources of failure

The drive belt is the first thing we check — if it's slack, worn or cracked, that's a cheap fix before we touch the alternator at all. Inside the alternator the typical faults are: voltage regulator (the brushes that transfer current to the rotor wear out), diodes in the rectifier (a failed diode cuts a third of the capacity) and bearings (grinding, whining, eventually failure).

Repair vs. replacement

On older cars it is often worthwhile to replace the regulator and brushes rather than the whole alternator — a job of less than an hour. On newer cars the alternator is often integrated with an electronic regulator, and it's then simplest and cheapest to replace the whole unit — often a rebuilt alternator that matches new quality at half the price.

Book an electrical repair online or call us on 41 17 32 24.