Quote:
Originally Posted by greg737
Instead the bulb gets its power through the side of the turn signals that are being used and its grounding path is through whichever turn signals are not being used, and this is a very long, indirect path-to-ground. The new LED bulb I put in the instrument panel didn't like this situation at all.
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This is a trick to get a single indicator bulb to work with both L and R signals with a minimum number of components. The indicator light gets power to one of its terminals when a turn signal light is powered. Its other terminal, being connected to the other turn signal bulb, is effectively grounded through that bulb filament. The indicator filament is small, so it gets most of the voltage drop, and lights. The unpowered signal bulb gets such a small current through it that it doesn't light.
The reason an LED indicator doesn't like that system is that the current through it reverses depending on which turn signal you're using. This means the LED indicator will light only one way (L or R) but not the other. On top of that, if you're using LED signals, in one direction the non-powered LED will probably light partially from the current going through it.
One cure is documented in various places, and involves grounding one side of the indicator bulb, and powering the other side with two small, cheap diodes, one from the L bulb's power and one from the R bulb's power. The two diodes isolate the L and R power and cure the odd behavior. This is the method I used on my 250.