trailer lighting fault

[madkayaker]

i know there is a few auto electrician type on here so here goes. I got led light units and fitted them to a trailer board after checking they all work off a battery. Now the lights all work except the break light will not light if the side lights are on. have checked with a multimeter there both getting lots of voltage and I'm fairly sure it not an earth fault as that was my first thought so ran a new earth in from a good bolt on the body work. have even tried the lights direct to the socket (NATO as i keep killing normal ones) but I'm stuck!

There was some info on a website that sayed that LED lights might need a smart relay on newer vehicle's but how can they not light if there both getting voltage?

 

[shenion]

Does sound like a ground issue.

Try a meter on the vehicle ground then measure the lamps.

 

[madkayaker]

getting 12.63V on both lights. the units are 4 wire with one being the common, then tail, stop and indicator

 

[rtbcomp]

Put your multimeter between the earthy end of the new brake lights and the erth point on the car to see if the lamp's earth is rising when the sidelights are on.

Are all the lights on the trailer board LEDS?

When you tested them with a battery, did you only test them individaully or in all the possible combinations?

 

[madkayaker]

one other thing the van will run a normal light board

 

[madkayaker]

Put your multimeter between the earthy end of the new brake lights and the erth point on the car to see if the lamp's earth is rising when the sidelights are on.

Are all the lights on the trailer board LEDS?

When you tested them with a battery, did you only test them individually or in all the possible combinations?

the only light on the trailer board that is not LED is the fog light and have tryed with fog on and off makes no difference. I tryed all the combinations when i had it on a battery. the lights are designed for 10-30V

 

[excossack]

Newer cars than use CanBus technology detect resistance changes in the wiring.

Maybe this link will help
http://www.towingandtrailers.co.uk/shop/14/31/60/index.htm

 

[madkayaker]

that was what i had seen. will try the lights on a bench power supply at work and see what happens with a lower current supply

 

[FranV8]

LED's are polarity sensitive, so try putting it back in the socket the other way round or rewire the socket the other way round.

 

[madkayaker]

they all work on there own its only when the side lights and brakes are on there is problems

 

[drivewasher]

Are u shure you got the sidelights and the brakelights the right way round?
If you inadvertantly got the brighter leds wired as the side lights, then when side lights are on the brake lights wont show, as they are now the dimmer led's that should be the sidelights.

I hope you get what I mean! I think i'm confused now lol...

 

[madkayaker]

good idea but have checked that

 

[Justme]

Sounds like you have a high resistance & when both lights are on the earth is rising to too high a V for the other light to work. As LEDs need so little power even a small problem can stop them working.

 

[madkayaker]

well i'll be (insert expletives of your choice) if i can work out what's going on but have it working. a relay is fired by the break lights and switches a fused feed to the socket all works grand now. the only idea that we could come up with at work was that the stop and tail will use the same LED's with the tail running through a resistor to lower the intensity and the van is seeing the two together as a fault and is not having it. where as the van at work has a smart relay and bosses car is so old that it has no electronics on it

 

[pidgeon chit welder]

Look up reticulated high resistance joint...

If it is still a prob clean all connections and solder wires on whereever possible.
Clean all terminals on your plug and socket as well using a foam contact cleaner from the likes of RS

 

[madkayaker]

it wasn't a high resistance joint as the light works fine when the side light were not on the problem was only when the side light were on and the N is common for the three function. the relay has not changed the earthing for the light or the supply

 

[pidgeon chit welder]

My thought is that there is indeed a high resistance joint good enough for one set of lights but not enough return for several .
It may well be at a major earthing point .
Sometimes you get joint like that and they turn into a diode effect once several earth returns are needed ( it heats up slightly ) thus effectively blocking off the return path .

I'd try running an independentdant earth wire right back to the battery and see if it cures the fault.
If it does start working your way back along the loom earths using the independent earth and locate the prob that way.

A slight resistance in one earth wire can and usually will cause all sorts of eddy currents / split paths & insufficient current draw in the wiring system , this can affect very low voltage /current devices quite dramatically .

Hope this helps.