Trailer lighting

[Jack P]

Hi All,

I'm after a bit of advice.

I've just bought an Ifor Williams flatbed second hand. All the lights work as they should, however when I turn the lights on the trailer fog light comes on automatically. Is this normal? I can turn it off but as soon as I indicate or brake it comes on again. It only keeps coming on when the lights are on. I've checked the trailer wiring, tracing the fog light wire back and I can't see why it would keep coming on.

I've come to the conclusion that it's the tow bar wiring, which I did just over a year ago using the Toyota wiring kit. My other trailer doesn't have fogs so never noticed an issue.

So the question is have I wired something up wrong or is the fog meant to come on?

Many thanks for your help.

 

[Shayne]

Fog lights are illegal in good visibility so it's unlikely to be normal . As you didn't have a trailer fog light to test with when you wired the socket maybe you left the fog wire loose ?

 

[Jack P]

That's what I thought.

I definitely wired up the fog lights when I did the towbar. When the fog light is on on the trailer the rear fogs on the car don't come on, which I believe is correct , so I've done something right.

More investigating. Maybe I'll see if I can find another trailer with fogs to test it on......

 

[Chris]

Not sure I can answer directly, but correctly wired, the tow socket should be wired so that the fog lights on the vehicle don't work. This is to prevent bounce back off the front of a caravan for example. Usually there is a small pin in the plug (edit, I meant socket) which interrupts a circuit. Essentially, I think, you tap into your fog light circuit and run it through the towbar socket. When there's no trailer all is as it should be, plug the trailer in and the pin lifts a contact in the plug to break the fog light circuit, sending it to the trailer lights instead. Not all 7 pin sockets have the feature. I only know because I bought a new plug recently and it had this feature in it. If this was wired incorrectly or the wrong plug has been fitted, it might be sending current up the wire somehow switching the lights on.

it might help as a starting point for your investigations.

 

[Shayne]

Something is touching where it shouldn't and because every tow socket and plug i have bought for my truck regardless of cost , metal or plastic has proven itself to be made of recycled eggshell in a very short time so my bets are on the socket .

 

[Jack P]

Not sure I can answer directly, but correctly wired, the tow socket should be wired so that the fog lights on the vehicle don't work. This is to prevent bounce back off the front of a caravan for example. Usually there is a small pin in the plug which interrupts a circuit. Essentially, I think, you tap into your fog light circuit and run it through the towbar socket. When there's no trailer all is as it should be, plug the trailer in and the pin lifts a contact in the plug to break the fog light circuit, sending it to the trailer lights instead. Not all 7 pin sockets have the feature. I only know because I bought a new plug recently and it had this feature in it. If this was wired incorrectly or the wrong plug has been fitted, it might be sending current up the wire somehow switching the lights on.

it might help as a starting point for your investigations.

Thanks for the info. I shall have a check of the socket, I had no idea how it disabled the car fogs. Cheers.

 

[Jack P]

Something is touching where it shouldn't and because every tow socket and plug i have bought for my truck regardless of cost , metal or plastic has proven itself to be made of recycled eggshell in a very short time so my bets are on the socket .

I must admit the socket is looking a little past it's prime!

 

[Chris]

Fed up with wiring issues, I bought one of those trailer socket wiring testers that you plug in and it has LEDs in that tell you what's going on.

Now if I can only figure out how it works ...

 

[Shayne]

Fed up with wiring issues, I bought one of those trailer socket wiring testers that you plug in and it has LEDs in that tell you what's going on.

Now if I can only figure out how it works ...

red means go and green means stop its upside down so start at the top !

 

[StarCruiser]

My guess is that the fog light wires in and back out of the socket on the contact and pin that Chris has mentioned, are possibly reversed so worth a check. Other than that it's usually water ingress that starts things off like this.
To test the above, remove the outgoing fog light wire and test with a meter or test lamp between it and earth. If that wire becomes live with the fog lights switched on (and no fog lights show on the truck) then that wire needs connecting to the pin on the socket and the other wire will be the outgoing wire to the truck fog lights, switched by the contact in the socket.
Have a test but you will need to disconnect the wire in the back of the socket rather than test the pins unless you do it with a trailer test plug like Chris mentions.

Another possibility is that the fog light wire is swapped with another in the socket. If all is correct, it's probably water somewhere.

 

[Jack P]

Thanks for the advice guys. I'll check the socket and plug connections next and let you know how I get on.

 

[Jack P]

Right then, so I've done some prodding about with the multi meter and am even more confused

I found that little mechanical switch and that works fine. With no trailer hooked up the car fogs work normally. When I Connect the trailer and turn the fogs on they only come on on the trailer and not the car. Perfect.

I tested to see if a current goes through the fog light connector on the tow bar socket when the little switch in pushed in and I turn the lights on, nothing. So then I tried the brakes and indicators, nothing. So that's all working as is should.

Now plug trailer in and turn the lights on, trailer fogs come on. Turn them off and brake or indicate and the fogs come back on again (only with the headlights on). So now I'm thinking it's definitely the trailer. I had a look (haven't tested with multi meter yet) at the plug, it looked OK, wires in correct places, nothing to short out. I've traced all the wires through, all seem OK and connected correctly. I shall have to get the multi meter on the trailer. Or I could just disconnect the fog wire on the trailer, but I don't like to be beaten (plus it's illegal).

 

[Shayne]

put a dead bulb in the trailer fog and if you live to be 185 you might get asked about it one day .

 

[Chris]

I have three trailers and two towing boards and there ain't a fog light between the lot of them.

But I agree - I don't like to be beaten either.

 

[Jack P]

put a dead bulb in the trailer fog and if you live to be 185 you might get asked about it one day .

Good thinking Batman

 

[Jack P]

I have three trailers and two towing boards and there ain't a fog light between the lot of them.

But I agree - I don't like to be beaten either.

Trailers more than 1.3 meters wide must have at least one fog light. Or so the interweb tells me.

 

[StarCruiser]

Another line of enquiry is possibly a dodgy Earth connection. This could be at the socket and will likely only show up with multiple lights wired in.

To eliminate the trailer, try plugging it into a similar vehicle and see if it still happens. Of course this doesn't eliminate the possibility of the car/trailer combination causing your problem.

Have a look in the plug, remove the wires. Some manufacturers have a rather poor habit of stripping off the insulation then folding the conductor back over the insulated wire before screwing it into the terminal. That can give rise to all sorts of connection problems. If you find it in either plug or socket, re strip the wires and twist before folding copper to copper and refitting.

 

[Chris]

That's why I always have a trailer board handy. Easy to plug in and you can bring it round so that you can see it when sitting in the driver's seat. Makes checking the lights really easy.

 

[flint]

Might be nothing to do with it, but is the trailer cable itself ok ? I had an IW sheep trailer a while back that had problems with the lights, one indicator was working intermittently (not in the usual way !). All the socket connections and bulbs checked out ok so I put new cable on and it cured it.

 

[StarCruiser]

Cables sometimes hang down and wear against the ground. I saw one once sparking as it went over manhole covers.