We recently had a fast blink when using left turn signal--the rear bulb for left turn signal had burned out. The expired bulb was a 2-filament Sylvania 4157k that worked as tail/brake light and turn signal. I replaced the expired bulb and the corresponding bulb on the opposite (right rear) side with this LED bulb. The LED bulbs have polarity (positive and negative connections) and will only work with voltage flowing across their circuits in a specific direction. I didn't know how to identify positive/negative sides on the socket/wiring, so used the "plug and pray" method. On both sides the bulb was first installed with incorrect polarity (wouldn't light up), so I pulled the bulb, turned it 180 and re-plugged. I didn't replace turn signal flasher or hazard relays. The LED bulbs are nice and bright.
What works:

• Left and right turn signals (blinking seems normal; no unusually fast/rapid blinking)
• Brake lights (both side brake lights and cyclops / CHMSL)

What doesn't work or works inconsistently:

• Tail lights (when headlight switch is in on position, only right tail light is lit)

With headlight switch in on position and gear selector in reverse position, both tail lights (not just backup lights) are on.
When turning the headlight switch to the on position, the left tail light comes on briefly (between positions?) but goes off when the switch is in the on position.

The replacement LED bulbs have two pairs of circuit leads, like the replaced 4157k bulbs did. What are the chances the wiring in the LED bulbs doesn't match--the wiring on the left results in a reverse polarity situation with the headlight switch in the on position...? I think I need to do more testing. Suggestions for testing/troubleshooting are welcome!

Thanks in advance.

 

Um, right, so it was late when I did a couple tests. The reason the tail lights worked with gear selector in reverse was because the brake pedal was depressed. The tail (brake) lights go off (completely) when the brake pedal is released, even with the headlight switch/**** in the on position. Ai yai yai!
Still seeing some weird intermittency. Sometimes the right (but never the left) tail light is on with headlight switch/**** in the on position and brake released / in park, etc. More often, neither tail lights work (when not braking or using turn/hazard signal). Both work just fine for braking and turn/hazard.

 

Welcome to the site.

Sounds like an issue with the LED’s, more so since other than the rapid flash due to the spent bulb, everything was working.. Eveything work spec if both bulbs were replaced with OE?

 

Thanks for the response and happy Thanksgiving!
I think you asked if everything works if I install original equipment? Not sure these are considered "original", but I ended up buying a pair of Sylvania 4157ll (long life) bulbs: https://www.walmart.com/ip/Sylvania-...-Pack/16913659
All functions (tail/brake/turn) work normally with those bulbs. I guess I'm not convinced the LED bulbs are at fault. I was looking over Autolumination 3156 3157 4157 Selection, Troubleshooting and Installtion Tips, and wonder if the following isn't the problem: