Incorrect bulb fitted - road lighting problems

May 11, 2021
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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.
 
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Jan 19, 2002
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Thank you for your detailed information. I think others will have experienced that the lamp holders on caravans are not always of the best quality and it is much easier to fit the bulb wrongly or the wrong bulb than it would be on a car. Glad your diligence solved it and you now have a well checked 13 pin system too (which is much more reliable than the old twin plug 12N and 12S) although it is worth a couple of quid for the green alignment tool in case of difficulty connecting car and van. The test run system on the car sounds a great addition as a pre-tow checker too!
 
Mar 27, 2011
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Off topic sorry but in response to the mention of the alignment tool, I’ve got a 13 pin now and I’ve plugged it in probably 15 ish times and I may be just lucky and it’s been straightforward but I don’t know what the green tool is for, is it the socket on the car or the plug from the caravan that needs to be realigned? My plug from the caravan seems to only go in one way and then I turn it clockwise I think a quarter turn, are mine just the same as everyone else’s, I’m a bit puzzled.

BP
 
Jul 30, 2007
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Good question Beehpee.
I've also got one of those green "thingymajigs".
I've been lucky so far with the pins always lining up but would like to know how to use it properly if the need arises.
Anyone help us? 🙏
 
May 11, 2021
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.The test run system on the car sounds a great addition as a pre-tow checker too!
Assuming the right bulbs are in place yes, it’s a real bonus. The people at the caravan dealer had never seen or heard of it and a small group gathered to watch it. Both our current and previous Volvos have had this system.
 
Jan 19, 2002
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Two web-site description to help the discussion:
"Handy little tool to help you line the rotating body of caravan 13 pin plug up correctly before connecting to car. Also can be used to protect the plug from the elements when not in use. 3 legs with screw holes enable it to be mounted permanently to your caravan. "
and
"Pin alignment tool for 13 pin caravan plugs.
Sometimes the inner housing which holds the pins on 13 pin towing plug rotates making it difficult to connect the plug to the socket on your vehicle. This simple, handy tool re-aligns the pins so they are in the correct position and the plug can be easily connected to the vehicle.
Can also be used as a cap to fit tot he plug when not in use. Prevents the pin from going out of alignment and keeps dirt water from getting to the pin and causing corrosion."
 
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Sam Vimes

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Sep 7, 2020
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When we bought a new Swift Motorhome a few years back we had numerous problems with it, which after much discussion Swift sorted by sending an engineer to us. The most mysterious problem was that of the Engine Warning Light coming on. This resulted in a discussion between Swift and Fiat as to who was responsible for fixing it. In the end Swift arranged for us to take it to a Fiat truck dealer.

What happened was that one day after a trip I reversed onto our drive way and left the van for a few days. When we ventured out again the warning light came on so it went to the Fiat Dealer. The could only find a problem with a blown fuse which when replaced caused the light to go out and the journey home was ok.

Reversed onto the driveway and then took it out a few days later. Warning light came on. Back to dealer. This time they ran the diagnostics and declared the Mass Aif Flow sensor was at fault along with something else. They replace that and we took the van home and all was well.

Reveresed onto the driveway and then took it out a few days later. Warning light came on. You can see a pattern here. After some research on the internet I came across someone who had a similar problem with a rusted bulb holder on the reversing light. I checked ours but being new it was Ok.

Back to the dealer where I mentioned the problem might be related to the reversing light. They actually listend and spent sometime investigating and eventually found that there was a wire going to the reversing light that was trapped between the body and the chassis. Every time I reversed onto the driveway over a few bumps the wire would short out, blow the fuse which stopped the supply to the bulb, Mass Air Flow sensor and a few other bits.

Another discussion between Swift and Fiat as to who was responsible for fixing it but in the end the Fiat dealer fixed it and said they'd argue with Swift about who pays.
 
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May 15, 2023
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I converted my trailers from 7 to 13 pin , and fit the green tool on the chassis so there is a place to park the plug. This ensures that the plug is not exposed to the elements, and that the pins are correctly aligned to the plug barrel.

Last year I picked a car up with a friends trailer, he advised me that the 13 pin plug had a problem in as much as it wouldn't twist and "lock" he had attached rubber band to keep the plug in place! the look on his face when it took all of 2 seconds to re align the plug with the green tool was priceless!

Its worth keeping one of these green tools in your tool kit.

Hitch lock 2 (2).jpg
 
Jul 18, 2017
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Never bothered with one as we swapped our connection for the Jaeger brand. No more problems.

BTW the thread will probably get locked as it is over 2 years old.
 

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