I have a problem with the light board on a trailer - a tow-bar mounted one for carrying bikes. The brake lights aren’t working. The other functions are fine - indicators and side lights both come on. All the lights on the car are working fine (ie brake lights on the car come on, just not on the trailer).
This is a brand new light board, bought today because the problem arose on my old one (which was generally a bit knackered). So it seems likely the fault is with my car wiring. I’ve done simple things like check the connections, reseated the brake wire on the 7 pin plug. Past that, I’m stuck.
I can take it into a specialist, but I thought I’d ask here in case there was something obvious to check that I don’t know about - like a fuse or an earth connection somewhere.
It’s always the ground.
Whenever dodgy trailer wiring is in question, it seems that 98% of the time, it’s a bad ground. The other 2% of the time, it’s a blown light bulb or corroded socket. Just kidding, but the reality isn’t too far away from that.
Normally, there’s no “brake lights” wire. The older 4-pin connector commonly used on small boat trailers or little trailers without electric brakes just lights up the tail lights, left turn and right turn. Brakes are done by lighting up both turn signals. The larger 7-blade plug used on RVs adds 12 volt power and a variable-power signal for electric brakes.
Either way, my guess is the turn signal bulbs are getting their ground through the other side’s turn signal bulb, and that a shared ground is bad. If you remove one of them, I suspect the other will stop working. The question is where is that bad connection? If the trailer lighting is new, the fault is probably with the car, but it’s not impossible for a new device to be faulty.