Trailer wiring question

[allanb]

I'm trying to sort out a peculiar fault in the wiring of my trailer.  I've found the ISO codes for a standard 7-pin connection, but there's something I'm not sure about.

The standard diagram shows that the brown wire (no. 5) is for the side lights (feux de position), and all it says about the black wire (no. 7) is that it is "shunté" with no. 5).  I take this to mean that the nos. 5 and 7 pins are connected in the vehicle's own wiring, and that either of them could be used for that particular pair of lights on the trailer.  Is this correct?

[AnOther]

Yes and no.

Although 5 & 7 are for the rear lights it is not a given that they are directly connected together on the towing vehicle. If it has a dashboard warning for blown bulbs for instance they will be separate and if you join them up on the trailer it could disable that and even possibly blow fuses.

If your car does not have such monitoring and the trailer is never going to be used with a car which has then it won't matter of course.

PS: Why complicate a very simple job by working in French and translating [;-)]

[img]http://cr4.globalspec.com/PostImages/200909/Trailer_Lights_8C9B6885-E03D-B3CA-7020644E59A81346.JPG[/img]

[sid]

Very clear from ANO's diagram. That's how I have mine wired, a separate wire for left and right sides, and it means that any voltage drop, over the the length of the run to the rear of the trailer, is less (half) of what it would be if you joined the wires at the towing vehicle end and then have only the one wire feeding both lights. Many cars have a socket or take-off point built into the loom, usually hidden under the boot trim, for the trailer feed and these often have separate left and right wires too.

You often see trailer lights working very sluggishly, the turn indicators for example, and it can be the result of poor connections, but also the result of the long wire feeding an additional lamp.

[dave21478]

As above. Its just the "french way", I suppose. If you look for trailer cable in the accessory shops, many only sell 5-core cable....ie both tail lights on a single wire and no provision for a fog light.

A larger trailer or caravan will also have front marker lights and possibly side marker lights too, all on the same single wire, pulling even more current from the cars wiring.

[sid]

That's very true about the extra caravan lights!

I haven't looked for 7-core cable here in France, I didn't realise it was hard to find. Internet/Ebay maybe?

Another point, the "earth" wire is often the weak link in all this, as it carries the "return" for ALL of the operating circuits together; it is usually the same thickness as the others.

When you're behind a car or trailer and the rear lamp flashes in time with, or alternately to, the indicators, it's usually a sign of a poor earth connection.

[allanb]

Thanks for all the information.

[quote user="AnOther"]Why complicate a very simple job by working in French and translating...[/quote]

Only because the first such diagram I found when googling happened to be a French one.  Believe me, my problem is not ignorance of French, it's ignorance of electricity.

I now know that the fault is not with the tail lights, it's with an indicator.  But I had replaced everything, and although it wasn't difficult to guess the English for shunté, I needed to know whether the shunting was supposed to take place in the trailer or in the car.  My car does have a dashboard warning system, and I certainly didn't want to mess that up because I don't know how it works.

By the way, confirming what Dave said, when I asked for standard trailer cable I was given the 5-wire stuff – without comment – and didn't realize it until I got home.  Rather annoying.

[allanb]

PS, much later : problem sorted.  Thanks for the help.

I now realize that it would have been wrong to shunt 5

and 7 anyway, because my car – like many others, I think – not only has a fault detection

system, but also gives me the option of turning on the side lights on either side only,

for parking in the dark.  So if the system detects two bulbs where it

expected only one, it will see it as a fault.