GENERATOR (ALTERNATOR) OVERCHARGING/OVERVOLTAGE
TECHNICAL SERVICE BULLETIN
Reference Number(s): 12 04 93 (3848)
GENERATOR (ALTERNATOR) OVERCHARGING/OVERVOLTAGE

Model(s): All BMW Models

Group: 12 - Engine Electrical

Bulletin No.: 12 04 93 (3848)

Date: September, 1993
SITUATION

We are receiving some reports of generators on BMW vehicles putting out excessive voltage, or with severely fluctuating voltage. Customer reported symptoms may include a "Check Engine" warning lamp and "Trans Program" transmission mode. Complaints might also mention slight fluctuations in the intensity of the headlamps and instrument cluster lighting. the DME control module sets diagnostic trouble (fault) code 54 (Voltage too high) into its memory, while the EGS control module sets code 37 (Battery voltage supply) into its memory. These codes are what causes the "Check Engine" warning and the "Trans Program" mode, respectively.
CAUSE

The voltage regulator assembly in the generator has failed, and must be replaced.
CORRECTION

Whenever a vehicle with such a complaint is encountered, the operation of the generator should be checked. A typical properly functioning generator should output between 13.5 to 14.4 volts at 1500 rpm with all electrical accessories switched OFF. Higher or lower voltage indicates a problem.

NOTE: If the voltage regulator is defective, it should be replaced.
PARTS INFORMATION

Voltage regulators are available from Parts for many generators. Consult the Parts Microfiche for the correct replacement voltage regulator compatible with the vehicle generator. The manufacturer, type, and ampere rating of the generator can be found on the outside of the housing. Use this information to select the proper replacement from the Parts Microfiche for the affected vehicle.