Here are the pinouts for UK standard 12N and 12S plug / socket
Viewed from rear of plug
The 12N socket is used by all trailers and caravans in the UK, whilst
the 12S is used only by caravans.
Some cars that are fitted with factory towbars from Continental Europe
come fitted with a single 13 pin socket that combines most of the 12N
and 12S signals into one socket.
The 12N sockets is fairly simple. The pin out is:-
1: Yellow: Left indicator
2: Blue: Fog lamp (permanent live
on very old cars / caravans)
3: White: Chassis earth
4: Green: Right indicator
5: Brown: Right tail and number
plate light
6: Red: Brake lights
7: Black: Left tail
Some sockets have an eighth connection on the back. The idea is
that you break the circuit to the rear fog light(s) on the car, and
feed them from the eighth connection. When a trailer plug is
inserted it switches out this connection thus killing the car's fog
light. Personally I think that these are a bad idea because it
kills the car's fog light whether the trailer has a fog light or not,
and because it adds a possible failure point for the car's fog light if
the socket becomes corroded or dirty. A much better idea is to
feed the car's fog light through a load sensing relay. If the
trailer's fog light circuit is complete and presenting a load then the
relay switches out the car's fog light, but if it isn't present
or the bulb has blown then the car's fog light still illuminates.
This gives a convenient warning that the trailer / caravan's fog light
is not working if when the fog lights are switched on the car's fog
light can
be seen glaring on the front of the trailer / caravan.
Some newer cars have bulb failure detection and others send a voltage
to some of the bulbs all of the time. In these cases a relay box
or an intelligent switch is needed, powering the trailer lights from a
permanent live source through the relays / switch. In any case a
dashboard light or a buzzer is needed to detect whether or not trailer
indicators are functioning, and a heavy duty flasher relay may be
needed.
The 12S socket is more difficult because the functions changed during
1998. Some caravans that are older than 1998 are wired the new
way too. My own 1988 Adria doesn't have enough wires
to wire to the old standard and so the obvious thing to do is to wire
it the new way even though it preceeds the standard by ten years.
The pin out is:-
1: Yellow: Reversing light
2: Blue: Switched live feed for
battery
charge (pre1998)
3: White: Chassis earth
4: Green: Permanent live feed for
interior
lights / battery charge (switched by caravan)
5: Brown: Sensing device /
spare
6: Red: Switched live feed for
fridge
7: Black: Chassis earth for
fridge (not present on some old cars / caravans)
Pin 4 has always been permanent live. On old caravans only a low
current was needed because it was used for interior lights.
However on post 1998 caravans a much thicker cable is needed on the car
that can supply at least 16 amps. Connecting a new caravan to an
old car can overload pin 4 if the installer has used light guage wire
to an inappropriate 12V source. Occasionally a new caravan will
be found that has pins 2 and 3 shorted. This then blows out the
permanent feed to pin 2 if it is present.... If you wire up pin 2
for compatibility with old caravans then please fuse it. In fact
fuse everything.
On pre 1998 caravans pin 2 was a simple 16 amp switched feed that would
charge the caravan battery if the engine is running.
On post 1998 caravans pin 2 is not used. A relay connects the
caravan battery to the live feed
from pin 4, and disconnects the caravan lights. The relay is
triggered by the fridge circuit.
In order to wire a car for both old and new standards, you must use a
16 amp switched feed for pin 2 and a 16 amp permanent feed for pin 4,
and
then another 16 amp switched feed for pin 6 which is the fridge, then a
35 amp earth on pin 3 (earths both fridge and battery on old caravans)
and a 16 amp earth on pin 7 (for earthing the fridge on newer
caravans).
That's quite a lot of thick cables....
The correct way to wire the car is to run new cables all the way to the
battery (through a new separate fuse box). The switched feeds
(pin 2 and 6 for old caravans, pin 6 for new) need to go through a
relay. The relay needs to be triggered only when the engine is
running, sometimes this can be acheived by triggering from the
alternator warning light circuit, or by finding something that only
works when the engine is running (like the blower motor on some
Peugeots). Powering the caravan circuits from power sources in
the car that were not intended for that purpose (such as the boot
light) might seem easier, but can prove costly if the car's wiring is
damaged by a load that it was not intended for.
Remember that 12S uses a thicker cable (2.5mm) than 12N, even though
the
colours are the same.
13 pin Euro plug / socket
Viewed from rear of socket
The pin out is:-
1: Yellow: Left indicator
2: Blue: Fog light
3: White: Chassis earth for lights
4: Green: Right indicator
5: Brown: Right tail and number
plate light
6: Red: Stop lamp
7: Black: Left tail
8: Orange: Reversing light
9: Brown/Blue: Permanent live feed for
interior
lights / battery charge (switched by caravan)
10: Brown/Red: Switched live feed for
fridge
11: White/Blue: Chassis earth for
fridge
12: Spare
13: White/Green: Chassis earth for
charging / lights
Again, newer caravan will charge the battery off pin 9 through a relay
that is triggered from pin 10.
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