12S Plug and Socket Overheating

Do you have 1999 model year or later caravan? If so you may experience problems with the centre pin of the 12S plug and possibly also the socket overheating. Why? Well from the 1999 model year the wiring of the 12S plug and socket were changed. One effect of this was that the centre pin (pin 7), which was formerly unused, became the return pin for the fridge. In this capacity it has to cope with a current of around 10 amps. Fine if everything is shipshape but likely to lead to overheating if the connection is less than perfect. There are two possible scenarios that can cause problems.

Firstly, if the towcar was originally wired to the pre 1999 standard and then subsequently modified it is possible that the modification was incorrectly done. It is even possible that some new cars could be incorrectly wired. Basically there should a dedicated earth wire from pin 7 on the socket to a good earth point on the tow vehicle. However it is not unknown for pin 7 simply to be connected to pin 3, which is already earthed. This arrangement can put an overload on the wire to pin 3 as newer fridges tend to take more current from the car than pre 1999 models. This can cause the wire, and possibly pins 3 and 7, to overheat.

12S Plug

 
An undamaged 12S plug - pin 7 (middle pin) can clearly be seen to be set forward from the others. Wiggling the plug in and out of the socket closes this pin leading to poor contact and overheating, even melting the plug.
 

Secondly, assuming pin 7 on the socket is correctly wired back to a good earth on the tow vehicle, it can still overheat. Being the centre pin, pin 7 is set forward from the other pins such that acts as a guide when the plug is inserted.

The problem with this is that as the plug is joggled on insertion and removal pin 7, being the lead pin, gets all the hammer. The result of this is that the leaves of the pin (2 or 4 leaves as the case may be) get squashed together such that they no longer make good contact with the female pin in the socket and overheating results.

This overheating can be serious enough to damage both the plug and socket but usually it is the plug that takes the brunt of the damage. The area around the pin becomes softened and darkened and the pin may be pushed back or even pulled out from its normal position. Once the plug is damaged in this way it should be replaced but how can a recurrence of the problem be avoided?

Fortunately there is normally at least one unused pin that can be brought into play to share the fridge return current with pin 7. Pins 2 and 5 are nominally unallocated but pin 5 is sometimes used by caravan manufacturers as part of any alarm system. My recommendation would therefore be to use pin 2, assuming that it is free. All you need to do is to connect pin 2 to pin 7 both at the plug and at the socket. If pin 2 is used it’s important not to plug the caravan into a car with pre 1999 wiring as pin 2 was used to charge the battery and will be live, resulting in blown fuses.

This arrangement shares the current and greatly reduces the chances of overheating problems. It also has the advantage that the towcar and caravan are still compatible with other 1999 or later caravans and towcars, albeit without the extra protection.

If you don't fancy messing around with the wiring then you can just carefully prise apart the squashed leaves of pin 7 using a penknife or very thin screwdriver. Be very careful when you do this not to open the leaves up too far as they are normally very brittle and will easily snap off, requiring a new plug to be fitted. If successful this should restore normal contact for a while but the leaves will eventually get squashed together again so this procedure cannot be regarded as a permanent solution.

If you think you may have either of the problems outlined in this article and are not competent to check the wiring to your 12S socket then, either see me on the rally field, or contact a qualified auto electrician.