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Rear lights wiring resistance?


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I've made an LED rear light for my roundie. Just one for now as an experiment. When I bench test it on a spare car battery (12.8V) it lights up all 4 sections perfect. When I swapped it into my car I couldn't get them to light up properly. I checked I was still getting 12.8v in the car at the terminals. One light would work but when I put another on both or one of them would die e.g. reverse works fine but when I press the brake it goes out. Or as the blinker flashes with the side light on they flash alternately. Its like I cant get enough juice to them to light them all.

I assume something in the system is limiting me as it works fine on the bench.

I'm not an electrician so have no idea where to start. I'm thinking of replacing lengths of wire but I'm not sure where to start.

Resistance in the old wiring?

Do I need a thicker gauge of wire to them?

A better earth maybe? a thicker gauge?

More amps? more volts?


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It's not the wire resistance.

99% chance it's the connections at the ends, and the taillight housings

are notoriously bad at this.

I'd start there. Clean, clean, clean, and tighten the brass whiskers at the

ends. They're supposed to have 2 layers- a contact and a spring.

It's really common to find one layer broken...

It's possible you have damage somewhere else.... but not nearly as likely.

Especially if the standiard lights worked ok before you changed them.

You do know, of course, that the turn signals need to have a certain resistance

to trip the little green indicator in the dash...


"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Check how the tail light is earthed. It sounds a lot like the problems you used to get with 80's cars where you would get a disco every time you braked and indicated.

This is a rise in 'potential difference' that is effectively limiting the voltage across the lamps. Check for a poor connection from the lamp fitting to body ground. If you put a volt meter from the body of the lamp to a known good ground point you will most likely find some voltage (less than battery voltage but more than nothing).

rtheriaque wrote:

Carbs: They're necessary and barely controlled fuel leaks that sometimes match the air passing through them.

My build blog:http://www.bmw2002faq.com/blog/163-simeons-blog/

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I'll give these a go at the weekend. My lights don't use the original wiring terminals the LED wire with new spade terminals simply stick out the holes where the bulbs would be. It could be the original wiring spade terminals though.

Regards the green dash light, I noticed when my indicator was on it would only flash once then go off all together, something it never did with my original bulbs, it worked fine. I read somewhere that this is due to too much resistance, thats what pointed me in that direction.

I will try running a separate thicker gauge earth wire to see if that helps and clean up as many connections as possible, other than that I don't know what more I can do. I don't understand why the voltage on my reverse light would drop when I indicate when they are surely separate systems.

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