Check alternator: Defects, symptoms and costs

Every car has a generator that is responsible for generating electricity: the alternator, or LiMa for short. The term comes from a time when an alternator was only responsible for the light on the vehicle.

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A modern alternator supplies the numerous power consumers in the car with the necessary energy. Although alternators are not typical wearing parts, defects are not uncommon. But the alternator itself is not always the cause.

Charge control lamp – first indication of defects

An important “tool” to check the operation of the alternator itself is installed right in front of the driver’s eyes. This is the charge indicator lamp in the instrument cluster. The generator or battery symbol lights up whenever the ignition is switched on. If it is still lit when the engine is running and the engine is idling, this could be a sign that the consumers that are switched on are not being supplied with the necessary power and the battery is currently supporting the system.

Repair alternator - New energy for the power plant

If the charging indicator light in the instrument cluster lights up when the engine is running, there is too little or no charging current from the LiMa to the car battery.

A quick push on the gas pedal is enough to clarify this. Most of the time the light goes out suddenly and everything is fine. If the light stays on, it could be a sign that the alternator is defective.

Alternator defective

In principle, three-phase generators are maintenance-free and low-wear and often last the life of the car. However, if the alternator is broken, the battery is gradually drained. In the worst case, the engine stops while driving because the engine control unit and fuel pump still have to be supplied with power.

Things went better when the driver became skeptical because the car had difficulties starting in the morning, even though a long distance had been covered the day before. In this case, the starter battery should actually be fully charged.

Overview: What can break on the alternator


The carbon brushes of the alternator regulator are worn



The alternator regulator is defective



The bearings of the belt drive or the freewheel are defective



Corrosion or defects in cable connections



V-belt has too little tension



Short circuit or open circuit of the diodes



The alternator winding is defective



Alternator rectifier is defective


Charge indicator light is on, battery is not being charged

The battery gets hot (overcharge), the battery is weak (no charge), the charge indicator light gets brighter as the engine speed increases, the lamps burn out

Noise from the belt drive (whistling, grinding), the battery is not being charged, the charging indicator light is on

Charging voltage is not reached, differential voltage between alternator and battery

Charge control lamp flickers, V-belt slips and squeaks

Discharging the battery when stationary (if the positive diode is defective)

Charge indicator light is on, battery is not being charged

Battery not charging, indicator light on

Replace brushes or regulator

Replace alternator regulator

Check belt drive and freewheel and replace if necessary

Check/replace lines and connections

Tension or replace V-belt

Disconnect battery, replace or repair alternator

Replace or repair alternator

Replace or repair alternator

Check alternator with multimeter

Should a problem arise, an alternator is relatively easy to check. The charging voltage of the alternator is checked with a multimeter (available from about 20 euros):

1. Set the voltage range on the multimeter (up to 15 or 20 V)
Connect the black clamp to the ground pole and the red clamp to the positive pole
When the engine is switched off, the voltage of the car battery should be between 12.2 and 13.6 volts (if the voltage is significantly lower, i.e. below 12 volts, charging with a charger is recommended)
Start the engine, the charging voltage should be around 13.4 to 14.8 volts at idle speed
Then repeat the measurement at a speed of 4000 rpm, the voltage should remain constant
Repair alternator - New energy for the power plant

The alternator voltage is between 13.4 and 14.8 volts. Measurements are taken at idle speed and at 4000 rpm. The tension must not change.

If the voltage does not rise to the specified values ​​(after a few seconds) when the engine is switched on, this indicates a defect in the alternator. To rule out damage to the wiring, the measurement at the alternator contacts can be checked. If there is a clear difference, the wiring must be checked!

If the voltage rises noticeably at 4000 rpm above the value measured when idling, this indicates a defect in the alternator regulator. In this case, the battery can overheat and damage the on-board electronics. In the workshop, the mechanic can also check the alternator with an oscilloscope and interpret the measured values ​​accordingly.

Alternator regulator defective

The alternator regulator, also known as the voltage regulator, is integrated into the alternator. One of its tasks is to keep the generator voltage more or less constant at all loads and speeds. This is the only way to ensure that there are no voltage fluctuations that could damage electronic components in the vehicle.

The regulator is designed in such a way that it allows the alternator voltage to level off at around 14 volts (alternator charging voltage) so that the voltage is below the gassing voltage of the starter battery.

Repair starter and alternator

The two carbon brushes sit on the alternator regulator. Here they are already heavily used. New brushes are about four times as long.

The two small carbon brushes of the alternator regulator are often worn out, recognizable by the fact that the brushes are significantly shortened. In older vehicles, only the carbon brushes can be replaced at a cost of a few euros. If the entire alternator regulator has to be replaced, this will cost between 20 and 80 euros. Depending on accessibility, the installation time is between ten minutes and one hour.

But it can also be time-consuming: If the engine compartment is difficult to access, other units may have to be dismantled to replace the alternator regulator, which means that the matter becomes a case for the workshop.

Check V-belt and belt drive

Also to be checked quickly: the tension of the V-belt and the free running of the belt pulley. It decouples the belt drive of the alternator from the crankshaft, which works with vibrations and drives the belt.

Normally, the generator runs at about twice the speed of the crankshaft, which can mean between 1,400 and 12,000 revolutions per minute – i.e. maximum performance. It is therefore hardly surprising that the bearings integrated in the pulley cannot always withstand such a load over the long term.

Bearing damage is indicated by a grinding, squeaking noise when the engine is switched on, which comes directly from the freewheel on the alternator. If this is defective, the belt pulley can sometimes even be dismantled with the generator installed and the bearing in it replaced (fastened with Torx or multi-tooth). Depending on the model and quality, a replacement costs between 20 and 60 euros.

Alternator: costs

A new alternator costs between 150 and 400 euros, depending on the vehicle, alternator output and alternator manufacturer. Added to this are the costs of installation, which usually cost between 50 and 250 euros. In extreme cases, the prices of powerful and water-cooled alternators can be over 700 euros.

Used replacement alternators are recommended for repairing older cars in line with their current market value. The prices of such overhauled alternators start at a good 50 euros. In addition, there is a deposit for old parts, which will be refunded when the defective alternator is sent or handed in.

When buying, you should make sure that all the necessary parts (possibly including a new alternator regulator) are included or must be purchased separately. Authorized dealers also offer replacement alternators in original quality for around 60 to 70 percent of the new part price.

Tasks of the alternator

Today, the alternator is usually a three-phase generator. While driving, it produces electricity for all electrical consumers in the vehicle – ventilation and rear window heating are the major consumers.

But even the engine control unit needs current pulses in order to be able to function. The alternator is also responsible for charging the starter battery while driving or keeping it constantly full. Because the battery is in turn the energy store in the car and forms the back-up of the generator, so to speak, sometimes even while driving.


A rotating magnet generates voltage in the armature winding (picture) of the alternator.

If the combustion engine is switched off, the battery discharges, for example if consumers such as the parking lights remain switched on or if the parking heater is always used for short journeys. It works fine for a while, but soon there is not enough juice in the battery to start the engine.

function of the alternator

To put it simply, the generator converts the mechanical energy of the crankshaft into electrical energy through internal rotary movements. The excitation field is brought about by the fist-sized rotor and usually induces three-phase AC voltage, which is fed into the on-board network after rectification.

In most cases, the generator is driven by the V-belt or V-ribbed belt, which is connected to the alternator via a pulley. In addition to producing electricity, the alternator charges the starter battery while driving.

The alternator takes engine power to produce electricity, around five hp are possible. That costs fuel. Therefore, modern alternators are demand-controlled to make energy conversion more efficient. In this way, the generator power is reduced during strong acceleration and the resistance is reduced.

In overrun phases, on the other hand, in which the engine does not consume any fuel due to the overrun cut-off, more power is drawn from the alternator and the resistance increases.

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