Checking the alternator

You will find useful information and important tips on the subject of vehicle alternators here.

The alternator supplies all electrical components in the vehicle with electricity. Should the alternator lose its full functionality, the electronics will fail after a certain period – the battery will no longer be charged, and the vehicle will no longer be roadworthy. In order that this does not happen, any faults should be detected in good time. We therefore provide you with various problem descriptions and detailed solutions in the following.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.


The task of the alternator is to supply all electrical consumers within the vehicle with energy while charging the battery at the same time. Three-phase alternators are generally installed in modern vehicles.


The alternator power, battery capacity and the total power requirements of the vehicle's electrical system are matched to each other.


The alternator is usually composed of the following components:

  • Housing
  • Stator
  • Alternator rotor
  • Alternator regulator


The stator with three-phase winding is mounted in the alternator housing. Claw poles, excitation winding, fan and slip rings are mounted on the shaft of the alternator rotor. The pulley is mounted on the front of the external part of the shaft. The electronic control unit with carbon brush brackets is attached in the rear area of the alternator.


Electricity is generated in the three-phase alternator on the induction principle. An electrical voltage is generated in the stator winding when the magnetic field within this winding changes. This change in the magnetic field is generate by the rotating alternating rotor. A sinusoidal alternating voltage is generated by the changing magnetic field with alternating north and south poles. This alternating voltage, which is unsuitable for the electrical system in the vehicle, is converted to direct-current voltage by the rectifier. The control unit adjusts the alternator voltage to the respective operating state of the motor and to the voltage requirements of all consumers in the system.


The following symptoms may indicate a fault in the alternator:

  • Charging indicator lamp lights up
  • Starting difficulties due to insufficiently charged vehicle battery. 
  • Vehicle battery becomes hot due to overload
  • The illuminance of the headlamp deviates in relation to the engine RPM
  • Bulbs burn out more quickly than normal


An alternator malfunction can have different causes. The cause is not always due to an internal alternator fault, such as a faulty winding, rotor, rectifier or control unit. Before replacing the alternator, additional components should be considered as a cause of failure and checked. 

  • Prematurely aged or faulty vehicle battery
  • Electrical connections on the alternator loose or faulty  
  • V-belt or V-ribbed belt loose or faulty
  • Belt tensioner or free-running roller damaged


When performing troubleshooting on the alternator, the following fundamental rules are to be observed:

  • Do not disconnect, short circuit or mount battery or connection terminals when the engine is running or the alternator is in operation (voltage peaks can lead to damage)
  • Never perform voltage or current measurements via short circuit (voltage peaks) - use a voltmeter or ammeter


A multimeter and a clip-on ammeter are required for troubleshooting. Please also refer to the technical info: "Ground (31)".


When performing welding work on the vehicle and when removing or installing the alternator, the battery must be disconnected.

Charging indicator lamp flickers.

V-belt too loose Tighten V-belt
Setting of the contacts incorrect or the control unit resistor burnt out
(Only with contact control units)
Adjust contacts, replace resistor or control unit

Charging indicator lamp lights with unchanged brightness at an increased RPM.

Cable D+/61 has a short circuit to frame
  • Remedy the short circuit to frame
  • Replace cable
Control unit faulty Replace control unit
  • Rectifier damaged
  • Collector contaminated
  • Short circuit in cable DF or in the rotor winding
Check alternator and repair or replace if necessary

Charging indicator lamp lights up brightly with the ignition switched on, but dims or flickers when the engine is running.

Contact resistance in the charging current circuit or in the cable for the indicator lamp Check cable and connections, and replace if necessary
Control unit faulty Replace control unit
Alternator faulty Check alternator, repair or replace if necessary
Übergangswiderstände im Ladestromkreis oder in der Leitung zur Kontrolleuchte Leitung und Anschlüsse überprüfen ggf. erneuern
Regler defekt Regler austauschen
Generator defekt Generator überprüfen, instandsetzen, ggf. erneuern

Charging indicator lamp does not light up with the ignition switched on.

Battery discharged or faulty Charge battery, check, replace if necessary
Cables or connections damaged, loose or oxidized Check cables and connections, attach, replace if necessary
  • Carbon brushes worn
  • Control unit faulty
  • Carbon brushes
  • Replace control unit
Short circuit of a positive diode Immediately disconnect battery or B+ (otherwise discharge in situ) and repair/replace alternator
Oxide coating on the slip rings, break in the rotor winding Repair/replace alternator
Indicator lamp faulty Replace indicator lamp

Battery is not charged or charged insufficiently.

V-belt too loose Tighten V-belt
Cables or connections loose, damaged, or oxidized Check cables and connections between battery and alternator and the respective ground connection, replace if necessary
Battery faulty Charge battery, check, replace if necessary
Control unit faulty Replace control unit
  • Collector contaminated
  • Rectifier faulty
Check alternator, repair or replace if necessary