Dual Filament Bulbs
Original submission by T Mar 21st 2007:
Dual Filament Bulbs:Thread
If the earth comes open the filaments interact. The 21W part of the globe (blinker) has a much lower resistance than the parker's 6W.
Tail Light Earth Failure Animation:
One Bulb Grounds Through the Other:
If the earth disappears from one globe the blinker will ground through the parker and the parker ground through the blinker making voltages appear where they ought not.
Shorted Filaments Inside the Globe:
Also a short inside a dual filament globe can cause odd things to happen, like the engine won't turn off until you take your foot off brake and turn off the headlights (on VW's) because the short in the dual filament bulb powers the ignition from the taillight circuit.