How can I test the 12S and 12N Socket?

by David Thorber


To test both caravan connection sockets, you need to arm yourself with a 12 volt bulb with hook-up leads.

One of those 12 volt electrical test screwdrivers is ideal. If the crocodile clip won't reach far enough into the socket to make contact, simply clip a bent paper-clip into the jaws.

On both 12N and 12S sockets, pins 1 to 6 are numbered clockwise from the top (pin 1 = 12 o'clock, pin 2 = 2 o'clock... pin 6 = 10 o'clock.) Pin 7 is the centre pin. (See the following photos)

12n Socket:

For the 12N road lighting (black) socket, you'll either need an assistant or some way of wedging the brake pedal so that you can make the brake lights stay on.

The pins to test are as follows:

1 and 3 - flashes in time with the LH indicator.

2 and 3 - lights only when rear fogs lights are turned on [1].

4 and 3 - flashes in time with RH indicator.

5 and 3 - lights when side lights (or right hand parking lights) are on.

6 and 3 - lights when brake pedal is pressed (with ignition on if needed).

7 and 3 - lights when side lights (or left hand parking lights) are on.

12s Socket:

The 12S (white or grey) socket should behave as follows:

Pins 1 and 3 - lights only when reversing lights are on (reverse) [2].

Pins 2 and 3 - lights only when engine is running (charge) [3] & [4].

Pins 4 and 3 - always lights (permanent live).

Pins 6 and 3 - lights only when engine is running (fridge) [4].

Pins 4 and 7 - always lights (permanent live) [5].


[1] If the vehicle has only ever towed small trailers, then the original owner may not have bothered with the rear fog connection. This is highly unlikely if you also have the 12S socket. If this is the case though, you do need to get this rectified before towing a caravan.

[2] Many people don't bother to wire up the reversing lights, so don't be surprised if these don't work! It's up to you whether you get them fixed, and as far as I'm aware, they're not legally required.

[3] A new method of battery charging wiring means that pin 2 on the 12S may not operate as described (i.e. it's always 'dead.') If this is the case, then you will probably be perfectly fine with a modern (late 1998 onwards) caravan but will need to get this resolved if you want to charge the battery in an older model.

[4] Depending on how the switched supply is sourced, you may or may not get a reading from 12S pins 2-3 and 6-3 with the ignition on but the engine not running.

[5] This tests that there is a ground connection on pin 7 - required for caravans produced after mid 1999. Without this connection, the fridges in these late model caravans will not run off 12 volts. For older caravans, this is not required.