A few weeks ago, my 2013 Volkswagen Passat started exhibiting strange behavior: intermittent failure of the left turn signal, and random trunk openings while driving. After a few days of using a rope to pull the trunk shut from the driver seat, I brought the car to my usual auto repair shop. Despite their best efforts, the repair crew failed to find the root cause.

I started my own investigation and discovered that the wire harness had failed at the trunk hinge, with wire insulation broken open and intermittently shorting to other wires. After repairing the wire damage, I noticed that the left turn signal was no longer receiving voltage. My best guess was that the J519 Electrical Power Supply Control Unit was damaged by all the shorting and no longer delivering voltage to the left turn signal.

Thinking through my options, I noticed that the daylight running lights had a 12V signal that was always powered when the car was powered. The immediate workaround then was to add a pushbutton to the steering column and use that to short the daylight running lights to the turn signal.

Regaining a manual left turn signal solves the urgent safety issue, but it’s a flawed solution. First, the VW uses the same lights for braking and signaling, so whenever I’m braking it’s obvious that the lighting system is broken. Second, it’s distracting to pulse a reasonable turn signal while also driving. Third, I would have no idea if the wiring hack stopped working because there is no feedback to the driver.

The best solution here has to be Arduino! We’ve got 12V to power us, we have a pushbutton to indicate turning intent, and we can pull signal from the center brake light to detect braking. Stay tuned…