Tow Bar Electrics 7 or 13 pin plug

Mar 06, 2017 119 Replies

Yes - we retrofitted a big-ass leisure battery to our last caravan. It lived in the wardrobe, in a plastic bread bin (to deal with spill of acid if that were to happen).

We tweaked the car - ran a 20A rated big fat wire to the alternator main bolt (with a 20A local fuse there). In the boot, we put that through a Schottky diode then onto the 12N socket, plus a 3pin metal socket in the boot[1].

The diode prevent the caravan battery trying to back feed into the car (ditto point [1] below).

In the caravan, we took the local battery and the car supply from the trailer plug to a changeover switch on a plastic box on the wardrobe that I made. This had a voltage sensor (741 op amp) and a relay and if the car voltage dropped to about 11.5V it cut power so the car had enough to start. Then we flicked the switch to the local battery. That way, the local battery did us for 2 weeks with the car supplying most of the power and the battery just making up any shortfall.

That was running several fluorescent lights, water pump, and a tv.

I did try to make a voltage booster so the car could pump some charge into the caravan battery whilst parked, but I couldn't make it work.

[1] My mum was disabled (stroke left her weak and unable to walk long distances) so she had an electric scooter. That went in the boot and plugged into the 3 pin socket and charged in the same way as the caravan, except it got charged on every day out - which surprisingly, seemed sufficient to keep it charged. So the diode again prevented the gel batteries from blowing up trying to start the car.

At 0.6V forward drop, it did not seem to impeded the ability of the

14.4V on the alternator from charging stuff.

That and you are supposed to turn the gas off at the bottle when towing :)

It wasn't usually. But people could work around it - simple switch you had to remember to operate when stopping for any length of time, voltage sensor to engage only when the voltage went high - all sorts of weird and wonderful workarounds.

and I wonder if the 12N had an unofficial

We always understood that the fridge should be levelled - we stuck a spirit level in the ice box (flat bottom) and tweaked the legs after finding vaguely flat ground.

Ah yes - I remember...

I also remember we were lucky if we got to 50 with a 1300 Fiat puling a caravan at the upper end of its permitted loading! Went to the Forest of Dean once (or was it Wales) and stayed in 3rd gear most of the way.

Did yours have that stabiliser (anti snake) arm thingy?

Really?

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doesn't mention permanently live. Have some of the pins been "re-purposed" over the years? If you plugged a modern caravan into an old car or an old caravan into a modern car (all with 7-pin) would you get some strange results like fog lights permanently on or fridge only coming on if car's fog lights are on?

Yes - there was a practical standard for a 12V live pin - pin 2, instead of the fog light which not many people had in any shape or form in the 70s.

All the cars that I have owned have the left and right side tail/side-lights separately fused and a single fuse for the brake-lights. Trailer wiring matches that.

Steve W

NY presented the following explanation :

There were two specs for the 12N. The later one has no provision for a

12v supply, I suspect what might have been a 12v supply on the early version, became the fog light terminal - hence the need to introduce a 12S additional socket. The change came around 1998-ish.

The original 12v supply on the 12N wasn't intended to supply such a heavy load as a fridge, but back then the standard wasn't really much of a standard. You would tend to have to test how car and caravan had been wired and make adjustments. Often a relay would be used, operated by the charge warning light, to switch that 12v power.

Modern fridges are able to operate from gas, 240v, or 12v when on the move. The 12v is not nearly as effective as the gas or 240v - usually they can just about hold the temperature down enough to cope.

Tim Watts used his keyboard to write :

They were very effective and more effective I think, than the modern coupling with built in friction.

Back in the late 70s/early 80s there was a plug and socket available (possibly 13-pin, but I can't remember) that had the same round pins as the normal 7-pin 12N, with additional flat pins around the outside edge. This was a far better system than the current one, as it gave all the extra connections required, while still allowing a 7-pin, 12N trailer plug to be plugged in, ignoring all the extra connections where they were no required - no adapters required.

SteveW

As I said earlier it's a requirement in some countries - or was. Namely Germany. Hence German made cars having it - even those made in RHD. All my BMWs had, and so does my present Porsche. The large Fords at one time were only made in Germany.

Eh? Piss easy with switches and relays (fixed pitch font required):

Ind SW o--- Left +---------+

+V ---+---| Flasher |---o-- Rest | +---------+ | o--- Right ---+---------- RHS Front Ind | | | Brake SW +---+-----+ | o--- NC --- NO ---o Relay | | / | | +-----------o | o---------- RHS Brake Light | / | o--- NO --- NC ---o | +---------+

LHS ommited for clarity but just has the relay with its NO and NC contacts paralled with the RHS relay and common to the LHS brake light. All switching is SPDT, (center off for the indicator switch...).

Rest: All open circuit, no lights on.

Brake: Circuit via NO brake switch and NC relays to brake lights. Brake lights on until brake switch is no longer made.

Indicate: Current flows through flasher and relevant side front bulbs they light and the relay energises causing the same side brake light to come on via the brake light switch NC and relay NO. The thermo electric flasher unit regularly interupts the +V causing all the relevant sides lights to flash together.

Indicate & Brake: As above except the relay energising switches the brake light off. The relevant sides front and back lights flash in anti phase. The other side brake light stays on via brake switch NO and that sides NC relay contact.

Simples.

I found it rather difficult to be sure. The requirements in Germany read much the same as in the UK: Vehicles and trailers parked on the road at night must be lit, with an exemption for vehicles (but not trailers) parked in a marked parking bay or on the road and adequately lit by street lighting. However, it is unclear from the sites I could find whether the German regulations permit the use of a single light, while the UK regulations definitely do not.

There are enough stupid people out there to ensure that it will happen on a regular basis and it does!

Ignition off, all lights set to off. Hold indicator stalk in appropriate position for side wanted for a second or two. An illuminated P with light "Rays" appears in the multifunction display in the rev counter to confirm.

That's how it is on an R56 registered in Dec 11.

G.Harman

It's interesting that cars are still being designed with this feature if it's no longer to park with only one front and back light showing in places where you need to use parking lights. I bet a lot of people think "if the feature is there, it must be legal to use it".

Strange that I've never been aware of it in any of the cars I've owned (Renault 5 Mark 1 and 2, VW Golf Mark 2 and 3, Peugeot 306 and 308). Maybe I've just not read the instruction manuals in sufficient detail, because it's never been something that I've thought to look for :-)

As I said, all my German made cars have this. Switch off the engine and lights. Move the indicator to the right, and only the right hand side light and tail light come on. Similarly with the indictor to the left. Which says to me this is or was a requirement in Germany. Of course at one time in the UK you needed parking lights too.

I really can't see any car maker designing it in if no longer needed anywhere? I've used the feature in the UK too. May not be needed, but may just make the car more visible.

Perhaps the really large quantities of non German spec VWs made meant it made sense to save the few extra pennies.

Not in a 30mph or less limit if you are parked in the direction of traffic flow. Way back that wasn't the case but an often used accessory was a clip on light that showed white to the front and red to the rear that was clipped to the rain gutter of the car on the offside and roughly in the middle as required. This meant only the load of one lamp being taken from the battery, and even if the battery went a bit too flat to start most cars of that era had a starting handle. As cars progressed the manufacturers built the ability to leave the off side lamps on for parking, two lamps now but batteries and charging systems had progressed. Meanwhile our law changed so you don't actually have to use any lamps in 30 limits, but I occasionally use the parking lamp facility as a belt and braces , 30 mph limit ,no pavements, no street lighting narrow single track lane with field hedge on other side and slight bend. 99.9% of the time our vehicles are off road completely but occasionally we have to shuttle to get a visitor in and park on the edge for a bit. Dark Winter evenings are the most dangerous from about 15.00 , School run time

G.Harman

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