how to check the alternator in my BMW 3 Series E90/E91 (2005-2012)?

Solution

1. Preparation

• Tools Required: OBD-II scanner, multimeter, load tester, socket set, torque wrench.
• Safety Warnings: Disconnect the negative battery terminal before starting work to prevent electrical shock.

2. Visual Inspection

• Sub-steps:
  1. Open the hood and locate the alternator.
  2. Check the wiring harness for any signs of wear or damage.
  3. Inspect the alternator casing for cracks or signs of leakage.

3. Battery Voltage Check

• Sub-steps:
  1. Connect a multimeter to the battery terminals (positive to positive, negative to negative).
  2. Record the voltage with the engine off (should be around 12.6 volts).
  3. Start the engine and check the voltage again; it should increase to between 13.5 and 14.5 volts.

4. Alternator Output Testing

• Sub-steps:
  1. With the engine running, use the multimeter to measure the voltage at the alternator output terminal.
  2. Ensure the reading is within the specified range (13.5 to 14.5 volts).
  3. If low, the alternator may need to be replaced.

5. Load Testing

• Sub-steps:
  1. Connect the load tester to the battery and follow the manufacturer’s instructions for testing.
  2. Load the alternator to its rated capacity and observe the voltage; it should remain stable above 13 volts.
  3. If the voltage drops significantly, the alternator may be failing.

6. Belt Check and Replacement (if necessary)

• Sub-steps:
  1. Inspect the serpentine belt for signs of wear (cracks, fraying).
  2. If the belt is worn, loosen the tensioner and remove the belt from the alternator.
  3. Install a new belt, ensuring it is properly routed and tensioned according to specifications.