how to fix my BMW 3 Series E90/E91 after getting a B1399 code?

Solution

1. Preparation

• Gather tools and parts required for the repair.
• Disconnect the battery by removing the negative terminal first to prevent any electrical shock or short-circuit.
• Tools needed: Socket set, multimeter, battery terminal cleaner, battery tester, OBD-II scanner.

2. Battery Inspection and Replacement

• Inspect Battery:
  • Clean battery terminals using a terminal cleaner.
  • Check for any physical damage or leaks on the battery casing.
• Replace Battery (if necessary):
  • Remove the positive terminal followed by the negative terminal.
  • Unscrew any retaining brackets and remove the old battery.
  • Place the new battery in the mount and secure it.
  • Connect the positive terminal first, followed by the negative terminal.

3. Alternator Inspection and Replacement

• Inspect Alternator:
  • Locate the alternator and visually inspect it for wear or damage.
  • Use a multimeter to measure the voltage output while the engine is running.
• Replace Alternator (if necessary):
  • Disconnect the battery as previously described.
  • Remove any drive belts connected to the alternator.
  • Unscrew and detach the electrical connectors from the alternator.
  • Remove the mounting bolts and take out the alternator.
  • Install the new alternator and reassemble in the reverse order.
  • Reconnect the battery.

4. Verify Electrical Connections

• Check all electrical connections for tightness, especially around the battery and alternator.
• Reconnect any ground wires that may have become loose during inspection.