- Check the tension and condition of the drive belt (see chapter Routine car maintenance). Replace a worn or deteriorated belt.
- Make sure the mounting and adjusting bolts are tight.
- Inspect the cable and connectors of the generator and voltage regulator. They should be in good condition and tight.
- Check the fuses.
- Start the engine and check for abnormal noise from the generator (rattling and squeaking indicate bearing wear, however, they may be due to belt slippage - see point a above).
- Check the specific gravity of the battery electrolyte. If it is low, charge the battery (does not apply to maintenance-free batteries).
- Make sure the battery is fully charged (one faulty battery cell can cause excessive charging current).
- Disconnect the battery cables (first negative, then positive). Inspect the battery terminals and cable ends for corrosion. Clean them thoroughly if necessary (see chapter Routine car maintenance).
If your vehicle's radio has an anti-theft system, make sure you know the correct activation code before disconnecting the battery. Before disconnecting the wire, refer to Section Anti-theft audio system and instrument cluster language.
If a message in another language appears on the instrument cluster display after connecting the battery, refer to Section 1 for the language setting procedure Anti-theft audio system and instrument cluster language.
2. With the ignition off, connect a 12 V test lamp between the negative battery terminal and the tip of the disconnected negative cable. If the test lamp does not light, reconnect the wire and follow the instructions in step 4. If the test lamp lights, there is a short circuit (leak) in the vehicle's electrical system. The short circuit must be repaired before checking the charging system.
Before testing, you should disconnect regular consumers (such as a clock or radio memory).
3. Disconnect the alternator cable. If the indicator lamp does not go out, the alternator is faulty. If the lamp remains on, remove the fuses one by one until the lamp goes out (this indicates a component with a short circuit).
4. Check the battery voltage with a voltmeter when the engine is off. It should be about 12 V.
5. Start the engine and check the battery voltage again. It should now be approximately 14 to 15 V.
6. Turn on the headlights. The voltage should drop and then rise again if the charging system is working properly.
7. If the voltage reading is higher than required by the charging system, replace the voltage regulator (refer to Section Replacing the voltage regulator). If the voltage is lower than required, the generator diode, starter or rotor may be faulty, or the voltage regulator may not be working properly.
8. If there is no short circuit that caused the battery charge to leak, but the battery is constantly discharging, then either the battery itself is faulty. Or the alternator belt is loose (see chapter Routine car maintenance), the generator brushes are worn, have lost contact or are dirty (see section Checking and replacing generator brushes), the voltage regulator does not work properly (see section Replacing the voltage regulator) or diodes, the stator winding or rotor winding has failed.
9. Repair or replacement of diodes, stator windings, or rotor windings is beyond the capabilities of a home workshop. Either replace the entire generator or send it to an auto electrician who can perform the repair.
On models before 1986, if the charge/ignition control lamp burns out, the battery stops charging. After 1987, a resistor is connected in parallel with the control lamp so that current flows around the lamp if the circuit is broken (the control lamp burned out).
Information taken from this resource: BMWMan.ru