Trailers and fog lights - Ian L
My first really foggy morning of the winter, visibility
less than 50m so I use dipped lights and front and rear
fog lights (and turn them off when conditions improve before
you all start flashing lights at me!). 20 minutes
later a car towing a large trailer pulls out from a side lane,
and while I see it because the car briefly displays its rear fog light this is soon masked by the trailer and despite normal rear lights becomes almost invisible again.

I have never seen fog light repeaters on a trailer, caravan or
temporary light board. Is this correct? Is this not a safety issue for towing in bad weather?
Re: Trailers and fog lights - John Slaughter

Yes, rear fog lamp rules do apply to trailers, along with a specific set of lighting regulations.

Based on some rather old info I have from when I built a trailer, I believe any trailer manufactured after 1/10/79, and first used after 1/4/80 is required to be fitted with at least one, and no more than two approved rear fog lamps.

These lamps should be operational with the exception that if the towing vehicle is an old vehicle not wired for rear fog lamps, then the lamps need not be operational.

Seems that any trailer under 1.3 m wide does not require to have fog lamps fitted at all.

Any decent modern trailer board should have them.

Problem is few trailer user ever seem to have checked on the regulations for towing. I was passed by a van towing a sailing dinghy the other day - must have been clocking nearly 80 in the 3rd lane of the M4, so travelling well over the 60 limit for trailers, and illegally using the third lane.


Re: Trailers and fog lights - Darcy Kitchin
These inconsiderate types get us law-abiding towers a bad name.

The foglight issue was confused a few years ago with the introduction of the second socket for caravans. The constant power supply was moved from the black to the grey socket, and that pin on the main (black) socket was henceforth used for foglights. So if you see a car and trailer bowling along in broad daylight with only the trailer foglight on, that would be an oldish car and a newer trailer.

Most trailer sockets fitted today allow for cutting out the car foglights when the trailer is connected. This is to save your inside mirror being full of brightly-lit red trailer illuminated by the car foglights. This is usually done with a pair of contacts that are lifted apart with a plastic pin inside the socket. The contacts, being low down and in a hardly warterproof housing, often corrode. First port of call if your car is rigged for towing and the foglight fails.

Er, you do check your lights occasionally, don't you?
Re: Trailers and fog lights - Stuart B
Yes John, I do accept was guilty of some of this when I had a Laser, at least the >60mph bit in lane 2 of the A74 up and back to Scotland.

My trailer board had no provision for fog lights so it was a rewiring job. Added a rear fog hanging below the board on an ally bracket plus two white caravan marker lights on the front face of the board near the outside edges where it stuck out beyond the transom.

As a Laser is such a long low profile it struck me that it needed forward facing white marker position lights and did not want to wire up the trailer itself. Never sure whether these on the front of the trailer board were totally legal but they were certainly effective, also good for seeing where the corners were when reversing at night.

I checked on the trailer regulations using an excellent free book courtesy of the Daily Mail I believe. It is now well out of date of course but it reduced the trailer lighting regs to something understandable. Not sure where you would find such information today.

Re: Trailers and fog lights - Darcy Kitchin

Try this for legal/lighting stuff


Whoops, didn't realise I was having a pop at an advanced dinghy sailor. Went to sailing school at Tighnabruich near Glasgow and grappled with GP14s, Wayfarers and national 12s. Lasers look just too d*mn fast.
Re: Trailers and fog lights - John Slaughter

I thought it was me that was having a pop at one particular dinghy sailor!

Thanks for the extra info - I'd forgotten the rash of fog lights due to the trailer socket wiring changes.

As for dinghys, I used to crew regularly for a colleague in an Enterprise. Even got some (minor) trophies!


Re: Trailers and fog lights - El Dingo (Martin)
I occasionally see little-used trailers with all sorts of lighting problems - the most frequent is brake light/indicator swap - probably due to bad earth - that is in turn caused by corroded terminals in the connector/lamp assembly/etc.
Re: Trailers and fog lights - Brian
Inadequate earthing on trailers and caravans is a perennial problem.
On my van I have two front lights, two rear lights, two stop lights, a set of indicators (usually only one on at a time), and a fog light, fed through a total of six cables.
The seventh cable is supposed to earth all that lot.
Result: when you brake the side lights dim.
When you indicate all the other lights come up and down in sympathy.
Conclusion: inadequate earth.
It would seem logical for the earth cable to be MUCH heavier duty, but all seven seem to be of equal weight.
And the situation is compounded if the pins are tarnished or the earth cable connection at either the trailer or car end is a little corroded.

Value my car