Was working on a local guy's truck with my DRB3 to make some side cash [some questionable mechanical/PCM issues due to a hack-job previous owner]. The left turn signal was fine, but the right turn signal would make the entire interior flash. My first thought was ground loop, so I pulled up the FSM and chased grounds...everything was good. Pulled the chassis/body connectors at the back of the truck, both sides...no change. Pulled the taillight housings out and looked at the bulbs...they were LED, and the right rear wasn't flashing. On a whim, I flipped the LED tail light around....and it started flashing and the interior lights stopped flashing.

What i've determined is that some LED tails have internal rectifier bridges to allow any polarity input, while some do not. For those that don't, putting them in backwards makes a reverse biased junction, creating a ground loop, sending voltage back on the ground path, creating not only a short but also putting voltage to said ground, making it's voltage potential the same as the power side, making all the interior lights flash off while on (no voltage potential = no voltage flow = no current flow) inverse to the turn signal, or while lights are off, backflows the circuit such that the interior flashes at same time as turn signal.

Synopsis:

When interior lights are on,
Turn signal on = interior off
Turn signal off = interior on

When interior lights are off,
Turn signal on = interior on
turn signal off = interior off

 

Great find...and description!!! hate them damned LED's, they were such a PITA on them Hurleys i used to fuss with....