Trailer lights wiring issue

[leen...]

Hi All

I have an old trailer and the light board has been giving me some issues. The latest one is that the brake light comes on but very dim despite having over 11v (can't recall the exact amount but not quite 12) across the bulb holder (earth - white and brake - red). I did some troubleshooting last night and discovered that there is electrical connectivity between the brake light and the side/ number plate light. I.e. if I put my multi meter across the 2 bulb terminals they are connected. Now I assume this is because the bulb is in and both thermals of the bulb are connected to the outer bit of the bulb and hence electrically connected.

My question (probably daft) is what stops both bulbs lighting when either one is on?

Secondly, any ideas what might be causing my issue or more troubleshooting to try?

Thanks

Lee.

[The Natural Philosopher]

The fact that there is an earth in the middle of them. Which is where the current is applied.

Almost certainly a bad connection or corrosion in the brake wiring - do both brake lights show the same behaviour?

[leen...]

Yes both show the same behaviour.

[The Natural Philosopher]

Any other lights affected?

[Tim+]

Eathing issues are the commonest cause. If the earth is poor the brake lights will find an earth path through the other bulbs, often making them glow dimly.

Tim

[Chris Hogg]

I had something similar very many years ago, with my first car (an MG TC). Pressing the horn button caused the headlights to glow dimly, and the horn barely sounded. Cleaned up the terminals and the mounting brackets so that good electrical contact was made with the frame and chassis and all was well.

[alan_m]

It may also be how the earth to the trailer connector is made in the car. Maybe the earth connection in the car side is via a electrical tag bolted to the car chassis. It is common for these types of connection to be corroded and a removing the bolt, cleaning off rust and then reassembly with a smear of grease may be the simple fix.

[Brian]

As others have said, almost certainly a poor connection.

My guess is you have a 7 pin connector and the normal single pin / wire for the supply to both brake lights and a single earth pin.

The brake filaments are each 21W, 42 in total. They draw double the current of any other single bulb and 8 times each rear light.

I’m assuming you have the correct wiring in the car - including a interface box if you have CANBUS lights.

[leen...]

I tried to test all the lights with the multi meter to give a full update. I removed the bulbs to remove that variable and the results was that there is indeed an earth issue to the combined brake and side light section. There is a joint between the earth for this bit and the indicators which must be bad because before that point I get 11v with brakes on and after it I get 7v. Going to see if I can fix that. Does that sound like the issue?

[Brian Gaff]

Pay attention if the bulb connection is via a rivet to a case part, odds are its got a small resistance there to corrosion. Have you never encountered the torch that only shines brightly when you bash it? Its amazing how much resistance you can get across apparently firm connections. Brian

[The Natural Philosopher]

7 v will be very dim. Its certainly wrong so fix that anyway.

[leen...]

Yep looks like that was the issue and found that some of the tracks are rusty too. It looks like this is ready for the bin. I have an old trailer I was going to Freecycle for scrap so am going to steal the light clusters off that and see if I can get something functioning.

[leen...]

I ended up removing all the wiring from the scrap trailer too. It had 6 core to a connector and then 4 core to each light all not the normal colours. Took a bit of figuring out but got there in the end and having now used the parts to rewire the old board all seems to be working.

Thanks for all your help all