DIY Caravan Brake Light Wiring Problem

[davidbann]

The brake lights on my caravan (Sprite Swing) are not working, but all other lights work.

I have tested the globes and both elements are working in both globes.

I've tested the vehicle trailer electric socket, with only brake lights on, and the car is delivering the required 12v to the relevant pins.

I've tested the cables at the back of the caravan at the junction before it splits to the lights, and there is still 12v on the relevant wires.

With both globes in the light sockets, neither come on.

If both globes are out the right side gets 12v for brake light and the left gets 6v.

If I put the bulb in the right, it does not light up, and the left stays at 6v.

If I take out the right bulb, and put in the left bulb, then the right side loses power and gets nothing and the left bulb does not light up.

If I take out the left bulb then the right side gets 12v again.

Mind. Blown.

Anything obvious from the above test results?

Thanks David

 

[DPG]

I would suspect a poor earth connection. Try a length of wire from battery negative to prove it.

 

[brianmoooore]

The fundamental mistake you are making is to use a voltmeter to test an unloaded circuit. The results are close to meaningless, because the current it takes to provide a reading on your meter is a microscopic fraction of that required to light a lamp, so a poor ( but not completely open connection ) at some point will kill a lamp completely, but have negligible effect on a meter reading. If a connection is poor enough, it will affect the meter, which is where your 6 volts is coming from.
Put the meter back in its box, find a 12 volt lamp of around 5W with a couple of pieces of wire attached to it, and use this to probe where there should be 12 volts. Find a 12 volt power source, preferably one that is limited to a current of 2A or so, and use that to inject 12 volts into places where there should be 12 volts, but isn't. These two test techniques will soon led you to the problem.
If the only 12 volt power source you have is a 12 volt car battery, make sure you include a 5A fuse maximum in one lead.