Motorcycle Trailer Wiring Codes

Standard 7 pin UK trailer wiring - '12N'

Wiring up a trailer? Here is the wiring diagram. Looking at rear of socket, front of plug:

7 pin trailer socket wiring

1 Yellow Left Indicator
2 Blue Fog light
3 White Earth
4 Green Right Indicator
5 Brown Right Side Light
6 Red Brake Lights
7 Black Left Side light

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Number plate light(s) wired to side light(s).

Indicator circuits should have a buzzer or other warning so that the driver knows the trailer indicators are working. This buzzer should buzz or beep-beep or flash in time with the indicators.

Over 1.3m wide, must have rear fog light. Over 1.6m wide, must have front marker lights. Over 6m length, must have side marker lights.

This is motorcycle.co.uk's site, so this is relevant for bike and car trailers in the UK. Not multi-axle caravans, agricultural tree-haulers, Winnebagos, etc... Broadly similar in Europe, may well be very different elsewhere. And there is now a 13-pin 'Euro-Plug', see below.

Some basic regulations...

  • Motorways & Dual Carriageways, 60 mph. May not use the outside lane of a 3 or more lane motorway.
  • Other roads, 50 mph.
  • You must have a full licence to tow anything. UK licence Category B passed before 1997 - up to 8.25 tonnes car+trailer, after 1997 - up to 3.5 tonnes car+trailer. (The full regs are here at DVLA)
  • Trailer over 750Kg gross weight - or over half the weight of the towing vehicle - trailer must have brakes. That's gross weight, which includes the weight of whatever you are carrying on it.
  • The trailer must show the same registration number as the towing vehicle. (It seems to be allowed to use the plate off the back of the vehicle on the trailer, which might be useful for hiring or borrowing....)
  • All trailers must have suspension. So motorcycle.co.uk's old solid-axle trailer is illegal then, oh dear.

For buying and fitting lights and stuff, it is easiest and cheapest to buy a trailer board from your local parts supplier, Halfords or wherever. It will come with all required legal lights and reflectors, a length of cable and a socket, all ready to bolt on and plug in for somewhere north of a tenner. Or cut the light units and reflectors off the backing board and bolt them direct to the trailer. Probably cheaper than buying them individually. If you want to make your own wiring, we like Vehicle Wiring Products. Construction and Use regulations apply, you can look them up in various places. But as long as things are positioned sensibly and look like a tidy job, you'll probably be OK.

If you need a number plate, your local parts supply again, but take some ID and your registration document or equivalent, they need proof that you are allowed to use the number. You still sometimes see sticky-backed-plastic yellow plates and black numbers at autojumbles, but its now virtually impossible to get them through even specialist suppliers like Indespension or Pinnell & Bax.

If you want to make the whole trailer, you want your local Indespension supplier or similar.

You won't need this on a bike or car trailer, but while we are here, if you are wiring up for a caravan or similar....

7 pin UK Supplementary Socket wiring - '12S' - pre 1999

7 pin supplimentary trialer socket diagram

1 Yellow Reversing light
2 Blue Battery Charging
3 White Earth
4 Green 12v power
5 Brown Spare
6 Red Fridge
7 Black Spare

7 pin UK Supplementary Socket wiring - '12S' - after 1999

7 pin supplementray trailer socket - after 1999

1 Yellow Reversing light
2 Blue Spare
3 White Earth, pin 4 feeds
4 Green 12v power
5 Brown Spare
6 Red Fridge
7 Black Earth, pin 6 feeds

Potential conflict on pin 2 if car wired one way, trailer the other.

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13 pin Euro-Plug

These sockets are not common yet, but if you have to modify your trailer wiring to plug into one....

13 pin europlug trailer socket wiring

1 Yellow Left Indicator
2 Blue Fog light
3 White Earth (35amp)
4 Green Right Indicator
5 Brown Right Side Light
6 Red Brake Lights
7 Black Left Side Light
8 Yellow Reversing Light
9 Green Permanent live
10 Blue Ignition live
11 Red Earth (35amp)
12 Black Coupled Trailers
13 White Earth (35 amp)

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