A vehicle presented with charging and headlight problems, accompanied by numerous diagnostic trouble codes (DTCs) and incorrect vehicle identification by the automotive scan tool. The Transmission Control Module (TCM) was programmed for a different vehicle. The alternator exhibited erratic charging behavior, fluctuating between high, low, and intermittent output. Replacing the current sensor did not resolve the issue. Turning the steering wheel resulted in decreased battery current sensor readings and dimmed headlights.
Further diagnostics revealed the TCM held a different VIN. Reprogramming the TCM with Techline Connect corrected the vehicle identification issue. The Body Control Module (BCM) stored a B101E:43 DTC, indicating incorrect ECU software EEPROM programming. Following service manual procedures, the BCM was reprogrammed, clearing the fault. However, the original charging and headlight symptoms persisted.
Post-programming, the charging voltage at idle measured around 15.1-15.2V. While a multimeter on the battery, the scan tool reading from the DLC connector, and all other modules displayed consistent voltage readings, the BCM consistently read 1-1.5V lower. The vehicle also exhibited multiple headlight faults. Headlight wiring was thoroughly inspected and tested according to wiring diagrams, revealing no issues. A new BCM was installed and programmed, but the problem remained. Power and ground connections to the BCM were verified, showing correct voltage and no high resistance.
Further testing revealed that disconnecting the alternator’s electrical connector reduced the charging voltage to 13.8V. Using a digital multimeter (DMM), less than 1V was measured on both wires within the alternator connector while the engine was running. The question remains: how to effectively test the alternator’s L and F circuits, understanding that the Engine Control Module (ECM) controls the alternator and communicates charging data to the BCM via the CAN bus. Could the lower voltage reading from the BCM contribute to these issues? Is this potentially an alternator feedback voltage problem? Further investigation is needed to pinpoint the root cause.
