A customer contacted us with a common issue on the 2012 Mercedes GL — the right front turn signal remained constantly illuminated. The problem persisted even after the ignition was switched off, indicating a fault in the lighting control circuitry.
Initial diagnostics confirmed that the headlight assembly and external wiring were in good condition. Further interior inspection revealed moisture under the carpet. After removing the front SAM module (Signal Acquisition Module), it became clear that the unit had been exposed to water.
Once the SAM housing was opened, corrosion marks and moisture residue were found on the printed circuit board. The PCB was thoroughly cleaned using a professional electronic cleaner, carefully dried, and inspected under magnification. Following Mercedes factory wiring diagrams, all power and control circuits were tested.
The root cause was identified as a failed microchip responsible for controlling the right front turn signal output. The damaged component was replaced with a new equivalent, using proper soldering and thermal management techniques. After reinstallation, the SAM module passed all functional tests and the issue was fully resolved.
According to Mercedes technical forums, similar front SAM failures are common on GL and ML models (X166 / W166 chassis). The most frequent cause is water intrusion due to clogged drain channels, windshield seal leaks, or body sealing issues. Moisture inside the SAM often damages output drivers and control chips, causing lights or other consumers to remain permanently active.
If you are experiencing similar symptoms, replacing the SAM module is not always necessary. In most cases, professional electronic repair can successfully restore the unit and significantly reduce repair costs.
