This is not really a question or request for help, more a tale of what I discovered when we got our caravan recently that might help others with a similar issue. On hooking up at the dealer the car initiated its inbuilt trailer lamp test sequence. We all stood at the rear of the van watching the car cycle through the lights and saw them all light up, so off we went. Upon leaving the dealer a "trailer brake light failure" warning came up on the car's dashboard - I thought nothing of it, thinking a bulb had maybe blown or had a poor connection and I'd sort it later.
After the initial buzz of having a caravan died down, a few days later I reconnected the car and instead of letting the car run its trailer light diagnostics, I asked my wife to do it the old fashioned way. Well, rather than shouting we did it over the phone, with me at the rear and her in the car, pushing pedals and turning switches to get various lights to illuminate.
What I found was that when the dipped beam headlights were put on, the van's brake lights lit up. But also when the brake pedal was pushed the brake lights lit up too. This was all accompanied by the "brake light failure" message in the car, although all brake lights appeared to be working.
Without really thinking it through I soon had the 13-pin plug apart and could see there were some exposed wire cores that could have been shorting to adjacent connectors - I sorted all that out, put it back together and expected it to be fixed. Nope.
So I pulled the rear light lenses off - on the nearside all was well but as I had the lens off I got my wife to do combinations of dipped lights, brakes lights, together and separately and noticed that there was a small difference in appearance between using both together and using only one - just a slight intensity difference of one of the filaments. This suggested to me that the van was getting the right signals from the car but something, somewhere, was cross-wired. The nearside lights all looked OK though so I moved to the offside. Again, lens off and all looked clean and dry inside but on removing the stop/tail bulb I discovered that someone had jammed a single-filament indicator bulb into the holder that is designed to take a twin-filament stop/tail bulb. They are physically similar but the stop/tail bulb has offset mounting pins supposedly to avoid this sort of problem. Also, this van had been PDI prepared for us and had left the dealer workshop like this. Using the car's light-check system hadn't highlighted the error because we had simply been looking for lamps to illuminate, without knowing exactly which lamps the car was testing at which times. Using the old wife-pushes-buttons method revealed the issue.
I pulled the errant bulb out and, having no replacement on me, left it out whilst we re-ran the lighting check and sure enough, now it all lit up as it should (nothwithstanding the missing offside stop/tail bulb). A quick stop into Halfords and I had the correct bulb, plus some spare bulbs and fuses to carry in the van, and all is now well.
If I'd thought about it I'd have started with the bulbs rather than diving into the 13-pin connector, but all's well that ends well, and now at least I have seen and understood the inside of the 13-pin.
After the initial buzz of having a caravan died down, a few days later I reconnected the car and instead of letting the car run its trailer light diagnostics, I asked my wife to do it the old fashioned way. Well, rather than shouting we did it over the phone, with me at the rear and her in the car, pushing pedals and turning switches to get various lights to illuminate.
What I found was that when the dipped beam headlights were put on, the van's brake lights lit up. But also when the brake pedal was pushed the brake lights lit up too. This was all accompanied by the "brake light failure" message in the car, although all brake lights appeared to be working.
Without really thinking it through I soon had the 13-pin plug apart and could see there were some exposed wire cores that could have been shorting to adjacent connectors - I sorted all that out, put it back together and expected it to be fixed. Nope.
So I pulled the rear light lenses off - on the nearside all was well but as I had the lens off I got my wife to do combinations of dipped lights, brakes lights, together and separately and noticed that there was a small difference in appearance between using both together and using only one - just a slight intensity difference of one of the filaments. This suggested to me that the van was getting the right signals from the car but something, somewhere, was cross-wired. The nearside lights all looked OK though so I moved to the offside. Again, lens off and all looked clean and dry inside but on removing the stop/tail bulb I discovered that someone had jammed a single-filament indicator bulb into the holder that is designed to take a twin-filament stop/tail bulb. They are physically similar but the stop/tail bulb has offset mounting pins supposedly to avoid this sort of problem. Also, this van had been PDI prepared for us and had left the dealer workshop like this. Using the car's light-check system hadn't highlighted the error because we had simply been looking for lamps to illuminate, without knowing exactly which lamps the car was testing at which times. Using the old wife-pushes-buttons method revealed the issue.
I pulled the errant bulb out and, having no replacement on me, left it out whilst we re-ran the lighting check and sure enough, now it all lit up as it should (nothwithstanding the missing offside stop/tail bulb). A quick stop into Halfords and I had the correct bulb, plus some spare bulbs and fuses to carry in the van, and all is now well.
If I'd thought about it I'd have started with the bulbs rather than diving into the 13-pin connector, but all's well that ends well, and now at least I have seen and understood the inside of the 13-pin.
Last edited: