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Leds on trailer still on even if the car is off Answered

So, I changed tail light on my trailer for new led ones the problem is the car outputs 12v at all time (which is normal) 12v then splits to 6v. But when 6v is comming to led light its slightly light up (the lights are 12v 0,5w). when i switch on lights, 12v is comming to led lights and they work how they should. So I need a way to light up the leds only at 12v any ideas ?


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2 years ago

I don't think I have a problem with wiring because I tried 4 cars. On 1st car the only blinkers are still on, on the 2nd car only brake lights are still on, on the 3rd car only red lights are still on and on the 4th car it works fine.
This is happening because the cars 1-3 have some voltage in the plug from controller (its checking for something) the 4th car doesn't have any voltage in plug so its fine.

So if I have on trailer bulbs its doesn't matter because that voltage can't light them up
but if there are leds which require much lower voltage they are slightly light up

And yes i upgraded the wiring with the plugs but it was all installed, and tested with professional plug tester by the guy who sells trailers and installs tow hooks (He is a car electrician) in our village, He said that trailers with led tail lights don't even exist, Its probably true in our country, but I think there is definitely a way to do this.

Other weird think is when i put a alternator tester in parallel with + on the light a - on the main ground it suddenly just works how it should

I was thinking about connecting 9v zener diode in reverse so when there is 6v no current will flow but when there more then 9v it will open up and current will go thru. But that's not possible because if the diode is in series there will by 9v on diode and rest on led which means there will be not enough voltage on led, because I need all the voltage to be on leds. Other option would be probably some sort of relay but I'm not sure about that.

Thanks for your help and time btw


Reply 2 years ago

Well, your test partly confirms what I said about the wiring - I had the same issue with a hired trailer.
Wouldn't work on my car but did fine on the cars the hire company had.
Turned out they used one wiring standard (outdated) while on modern and up to date installations 3 pins were swapped around.
Again: What you need first is a clear and clean path from the car plug to the corresponding trailer light.
You can't trust the plug on a car unless at least a test plug was used to show it is wired correctly.

Similar story for the electronics in a modern car.
They should not care about the extra load of some lamps on a trailer, but some do, especially for the indicators.
Although here you can cheat a bit.
Apart from the indicators there should be no problem.
For the indicators you can get resistors to go in parallel with the LED's.
These resistors bring the power draw from the LED up to the same level as a standard light bulb.

Jack A Lopez
Jack A Lopez

2 years ago

After re-reading your post, and your reply to it, I think I better understand what you are saying.

You are describing some kind of problem with the signal levels, produced by your car's trailer light driver. I think what you are saying, is that the car's "light on" signal level is 12 volts, while the "light off" signal level, on the same wire, is 6 volts, rather than the expected 0 volts, or a high impedance state.

Or maybe "light off" is a high impedance state, but it is just not very high compared to the impedance of the new LED bulbs.

Or sort of another way to say that is, this problem with signal levels does not show itself with the old, incandescent filament, style bulbs. But it does show itself with the new style bulbs, which have LEDs and resistors in them.

So you are contemplating building some kind of, "glue logic", to make the signals from your old car display correctly with the new LED bulbs.

The most straightforward way I can think to do that would be to that would be to use a bunch of comparators (or op-amps wired without feedback, as comparators),


with the outputs from those comparators switching on some transistors, to switch current to the LED bulbs. Here I am guessing the comparators own outputs would not have enough current to drive the bulbs directly.

Another possibility is maybe give the LED bulbs a lower impedance, by wiring an appropriate sized resistor, in parallel with each LED bulb, and intentionally waste some power, same as the old style bulb would do.

That last one is kind of a wild-ass guess. I do not know if that would work at all. I mean it is not going to be easy to change the I-V curve


of a new LED bulb, to make it look exactly like the I-V curve of an old hot-filament bulb.

Jack A Lopez
Jack A Lopez

2 years ago

People write to this forum, from many different English-speaking regions of this fair planet, and I often use the English language Wikipedia, the one prefixed with "en.", to try to make sense of the topics people ask about here.

In your case, I am wondering, if the English Wikipedia article for, "Trailer connector", here,


leads to something that looks like the trailer connector for your car, and its trailer?

I am kind of hoping these links themselves will possibly lead you to the path to enlightenment.

That might sound like a pun, since we are talking about lights and stuff, but I did not really intend it as a pun.

I mean, I am happiest when people figure things out for themselves.

By the way, if you have not done this already, it might to help to draw yourself a diagram, of your wires, for to ponder which wire goes where and which current does what.


2 years ago

You got something wrong here...
All lights on your trailer work with 12V and require 12V reach them when required.
I simply assume you also upgraded the wiring while at it or even the plugs?
What you have is a wiring problem for starters.
Check on the coupling of the car where exactly you get 12V for the breaks, reverse, indicators (both sides).
Same for the standard light when you switch your headlights on.
Note down where you get what and make sure you have the correct pin for common ground.
With everything turned off one pin should show a short if measured again a blank part of your tow ball or such.
Start with a voltage check first and only check the pins with nothing for the short to ground.

Now confirm that each pin of your trailer plug actually confirms that it is connected right light at the end of the trailer ;)
If the actual LED's are wired up correctly on the harness then you only need to worry about matching the plug.
If you got the common ground wrong you might have to rewire both ends.

If you have 12V to ground on a pin on your car even with all off and key removed than ignore this pin for your trailer unless it has an electric brake regulator installed.
You only need the pins for the signals to the lights and the pin for common ground.