Hey collection of experts….

I’d love some help. We’re nearly done on our crossflow MK locust, and have an odd wiring issue.

All works well except the LED high level brake light comes on partly (not full brightness) when I put the side or dips on. It has no feed from the side or dips, the only live input is from the brake light switch, and an earth. So it behaves like a stop/tail, but it has no live feed for tail!

The 2 LED stop/tail lights seem to work fine, behave the right way, and all other lights work. All three brake lights are wired with a shared live wire from brake light switch to all 3, so we tried adding a separate wire for the high level only, but the same thing happens.

Either way, what seems to happen is somehow when the taillight parts of the stop/tail lights are on, they somehow feedback via the brake light wire or the earth, and then that little bit of voltage reaches the high level brake light.

So I have 2 questions:
- Does it matter if the high level brake behaves like a stop/tail? Dimly lit on side/dips and brighter with brakes? Or must it be brake only?
- Any ideas why it’s happening or how to stop it? Is it because they’re all LED? Everything else works well so don’t want to rip it up and start again….

Thanks in advance all! Any help appreciated…

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Bad earth on one of the tail lights to the chassis and it's back feeding through the brake light circuit via the shared earth in the lamp.

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Originally posted by adithorp
Bad earth on one of the tail lights to the chassis and it's back feeding through the brake light circuit via the shared earth in the lamp.

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Magic - thanks both, will check that at the weekend, much appreciated!

Anyone who owned an 80's Ford will recognise this activity, although it was usually related to indicators and stop lights...

However, it's interesting that it's affecting an LED lamp. I'd have thought that the backflow would be stopped by an LED. Or is there a bit of electrickery in an LED lamp that allows it to work either way round?

Presumably, the ground of the LED light is good. Its switched 12V will be connected to the switched 12V of both brake lights. If the ground of one of the tail/brake lights is poor, instead of being at 0V, it may rise. If the brake lights are standard bulbs and not LED, the raised voltage will be passed through the brake light filament, to the floating switched side and feed the LED.

Thanks all, it must be something like that, bit of testing by taking one rear light off at a time, makes the 3rd brake light behave itself, but it's the same with each side, weirdly....
But yes I think you've hit it, we just need to go through all wires again.

Or - if i read the IVA manual right it's not needed is it? It calls it 'Optional'
Stop Lamps Min 2 Max 2 Mandatory Red
Stop Lamps (Optional) Min 1 Max 2 Optional Red

So maybe easiest to just remove it for the test!

Thanks
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some automotive led's incorporate a bridge rectifier, so can be used either polarity.

also it may be the bulbs have electricary, to avoid problems for cars with canbus.