On 26/05/2017 21:49, NY wrote:
"Andy Burns" wrote in message
...
bert wrote:

Andy Burns wrote:

the chips start to do snazzy things such as

switch off vehicle rear fog but leave on trailer fog.

I thought the 7 pin sockets had been doing that with an inbuilt switch
for decades?

The 7-pin (as opposed to 13-pin) socket doesn't have a pin that is
dedicated for use by foglights? They have separate pins for left- and
right-hand tail lights (why?) but none for fog lights - unless you
sacrifice the permanent 12V feed for a fridge. When my parents had a
caravan in the early 70s, they had an extra wire for foglights which
they had to remember to plug/unplug when they were hitching/unhitching
the caravan. While some cars may have once had side/tail lights that
could be controlled individually, for use as parking lights, it seems
weird to waste a pin replicating that onto the trailer, when it means
you can't have both permanent live and fog lights.

But all the cars that I have owned (before the current canbus one) have
had separate fuses for left and right tail/sidelights and a single fuse
for brakelights. I have always presumed that this is so in the event of
a single fuse blowing, you are not left on a country road, in pitch
darkness, with no lights at all and liable to be crashed into. Trailer
connections merely continue the existing split circuits.

Mind you, the lack of
a reversing light pin is also odd: how else is the car behind supposed
to know that you intend to reverse?

Not at all odd. Many cars (most) would not have had foglights or even
reversing lights when the 12N socket first appeared and in all
likelihood no trailers or caravans would.

Once the 12S was introduced there was no need for the permanent live in
the 12N, as that could be dealt with in the 12S. Both fog and reversing
lights could have gone in the 12S, but that would have meant that both
sockets would be needed on every towcar, but by putting the foglights
onto the 12N, only one socket would be needed for towing basic trailers.

Reversing lights are not a requirement, hence the 12S is not mandatory.

In fact according to the Road Vehicle Lighting Regulations, reversing
lamps are optional, even on cars, never mind trailers! You cannot be
fined (or fail an MOT) for not having a reversing light or for it not
working.

On the other hand, plug a trailer or caravan with reversing lights into
a car wired to the old (pre-12S), permanent-live standard and they will
be on all the time. And showing a white light to the rear, except when
reversing, *IS* an offence.

SteveW