BMW alternator operation, voltage output, and battery charging are monitored by the DME module. 

When BMW alternator voltage output is above or below the recommended limit ( typically 13.3-14.5 volts), then the red battery warning light will illuminate on the dashboard and a message will pop up on the iDrive screen.

BMW battery warning

Cars with an iDrive system will display alternator malfunction warning messages such as vehicle battery charge low or vehicle battery discharged while stopped. 

Once the voltage output is within the proper range again, the warning light will turn off.

In most cases, early stages of a failing alternator will cause these warning lights and messages to come on just for a few seconds and then turn off.

Common Problems

BMW Battery Alternator Problems

One of the most common problems that can cause a BMW battery or alternator warning messages is an old battery that is no longer holding a charge. If the battery is older than six years it may need to be replaced. Follow this guide on how to replace and register a BMW battery yourself. 

BMW alternator problems

Another common problem that causes BMW battery and alternator warning is a faulty BSD BMW module which is typically mounted on the alternator.charge bmw battery

If the battery is relatively new try charging the battery with a trickle charger.  Additional problems that can cause this issue include. 

Low voltage output

low battery voltage

Although the car will run with no noticeable issues when the alternator output is between 11 and 13 volts, it will not charge a battery to full capacity.

However, if the alternator is producing less than 11 volts, it will cause important systems, such as ABS, wipers, or climate control, to turn off.

And eventually, the engine will shut down, as there will be no electricity for spark plugs and engine (DME).

High voltage output

A voltage output higher than 14.5 volts can cause expensive damage to sensitive electronic components.

The high voltage output can even damage the battery.

AC ripples

As the name suggests, all alternators generate alternating current. An additional assembly within the alternator turns the AC into direct current used to power car systems.

If the alternator produces too much of alternating current, it will cause a whole range of errors and problems with car electrical systems.

Belt

All accessories including the alternator, AC compressor, power steering pump are driven by the serpentine belt which is rotated by a crankshaft pulley.

The alternator pulley spins two or three times faster than the crankshaft pulley to produce the power.

If the belt is loose or one of the accessories has a bad bearing, the alternator will not rotate fast enough to generate the required electricity.

Also, there may not be power steering and air conditioning. And, as a coolant pump will not work as well, the engine may overheat in a short time.

Bad Alternator

The alternator has several rotating components, such as a rotor, bearings, and a slip ring with brushes assembly.

Just like other moving parts, these will eventually wear out. The spring-loaded brushes are the most likely failure point.

Also, as the BWM mounts alternator below the oil cooler assembly, this can lead to oil contamination caused by leaking oil.

Voltage Regulator

A voltage regulator controls the amount of voltage produced by the alternator. Without this regulation, the voltage would vary depending on the engine speed.

The engine ECU keeps the voltage output in the desired range by adjusting the amount of current that goes through the rotor.

If the regulator fails, it will cause random low or high voltage outputs.

Diodes

A rectifier, or diode assembly, converts alternating to direct current. With time, the diodes can short out electrically and stop working.

Once the diodes go bad, there will be ripples of alternating electricity going through a car™s direct current system.

Battery

The battery itself could be the culprit if you are experiencing voltage problems. An old battery may have faulty cells or no longer hold a charge.

Conclusion

The alternator is a key piece of equipment on a BMW. It powers the engine plus it charges the battery while driving, and it also provides electricity for various systems.

Fuel and ignition systems, climate control, entertainment system, and a variety of other components depend on the alternator operating properly.

In all newer BMWs, it is possible to check the voltage by entering the ˜hidden menu™ features.

However, the best way to verify the problem is to use a digital multimeter. Attach the two probes from the digital multimeter to the positive and negative battery terminals and turn the selector switch to DC voltage.

Next, start the engine and let it idle.

With all electrical connections and battery cables in good shape, the voltage output should be anywhere from 13 to 14 volts. It is best if this value is between 13.8 and 14.2 volts.

If the voltage is in the recommended range, the next step is to read the fault codes with a BMW All System Scanner. A BMW scanner will show a specific fault code.