13 pin socket stops rear vehicle fog lights

thanks, went there last week an dthey believe it is the socket. Now booked in to be sorted as the securing bolt/screws will not free up at home.

fitted by Ssanyong

tried getting bolts undone this morning to gain access to the extra wire but too corroded and likely to break the unit. A lesson to be learnt - grease these bolts well regularly or rplace with stainless steel ones! (wish i had  a garage to work in out of the rain!)

Hope the trailer company it is booked in with will be able to sort out further up the wiring as the circuit can be changed. I understand that when fitted the power supply to the vehicle fog lights is cut and then routed via the extra 14th wire and the wire for the trailer fog light.

WARNING:  EVEN A WORKING 13-pin SOCKET MAY CAUSE MOT FAILURE

My MOT Test for my tow car failed due to a “Major Defect”, specifically:-

“Trailer electrical socket (13pin) not operating the trailer lamps as intended (Rear fog lamp not working) [4.10 © (i)]”

This fault was determined by the garage using a small testing panel with LED lights. The LED indicator for the rear fog light pin did not light up when the rear fog lights were turned on on the car, although the rear fog lights did come on on the car itself.

The car is a 2016 4WD Diesel Renault Kadjar (probably not relevant). The towbar and electrics were an after-market fit from Witter Towbars. Both the towbar and the “vehicle specific” wiring loom were the Westfalia brand. The whole system was installed by their recommended fitter in 2020.

I used a multimeter to check the voltage between pin 2 (rear fog) and pin 3 (ground) and indeed it was zero volts (cf the side light pins showed 14.5V with engine on).

There appeared to be a problem and so I contacted Witter Towlbars, who in turn contacted the original fitter.  He, explained where the Westfalia control box was located and proposed some checks. Nothing was apparently wrong except the pin 2 feed was always at zero volts.

The fitter visited and did not find a voltage on any part of the output wiring for pin 2 from the Westfalia control box. However, the input side of the foglight circuit went live when the rear foglight was switched on and the feed on the output side to the two rear foglights on the car did go live. It then appeared that there may be a fault with the Westfalia control box.

However, I then drove over to my caravan (which had only just been given its annual service, including a check on the external lights).

I started the car and turned on the side and then the rear fog lights. These came on as expected on the car. I then plugged in the 13-pin connecting lead from the caravan and the foglights on the car immediately went out. However, the side lights AND THE REAR FOG LIGHTS had come on on the caravan.

The conclusion is that the design of the Westfalia wiring system only allows EITHER the rear foglight(s) on the car, or the rear foglight(s) on the trailer to come on, but NOT BOTH at the same time. Westfalia use a control box, not a switch on the socket to achieve this.  This prevents the current in the car's rear foglight wiring from being doubled and thus from possibly becoming overloaded and, by switching off the car’s lights when a trailer is attached, prevents any glare from the car’s rear foglights reflected forward from the trailer (e.g. a white caravan front) and impacting on the driver’s vision via the mirrors, or that of other road users, particularly at night.

The problem was the method used by the MOT station to test the trailer circuits and I believe many other testing stations are now using a LED test device. The tester used was clearly not being “seen” as a connected trailer by the Westfalia control box (which, incidentally is CE marked). Indeed, the rear foglight on the car stayed on, when the tester was plugged into the car’s 13-pin socket. This may be because the indicators are LEDs, which draw very little current, or some other connections may be required for the system to operate as intended. Either way, the system therefore IS OPERATING AS INTENDED and should not have failed the MOT test.

Having had all this explained (his current test box would never show power on pin2) and also shown that my caravan’s rear foglights were indeed working, the manager at the testing station then passed my car for its MOT.  Had he not agreed to this and insisted on using his "approved" LED-based testing device, I could have never have been able to get my car through an MOT retest with my tow bar and electrics on the vehicle.

I have sent an email to the DVSA about this, as it is likely to affect other tow cars with the same or similar control boxes and testing stations that use LED based circuit testers. I hope they can issue an advisory on this before I have to go through all this again next year!

'My' MOT tester never ever used his trailer tester when doing the MOT, however one particular time I plugged it in to my car, and as above it didn't really register as it should have. I knew the car powered my caravan OK so didn't worry too much.

Re my long piece above giving a warning that:-

EVEN A WORKING 13-pin SOCKET MAY CAUSE MOT FAILURE

I have just received the following response from the DVSA:-

Thanks for the information provided.

We have been aware of this situation for some time and have issued instructions for Testing Stations to contact their equipment provider to get this issue resolved.

Best regards

Shaun Martin | Policy Specialist (MOT Testing Service)
Driver and Vehicle Standards Agency | Berkeley House, Croydon Street, Bristol BS5 0DA

That is good, but clearly my MoT station (and nobody else I discussed this issue with) was aware that it is a recognised problem with the testing device.  It is not clear if these LED testers are no longer "approved" devices.  I fear this still has the potential to cause MoT failures and quite a bit of hassle for some owners to overcome.  The email I received came from MOT@dvsa.gov.uk, but I don't have a phone number.

I suggest members remain "alive" to this issue and if you have a control box similar to the one I have, take some evidence that your rear foglights do switch over to the trailer to your MoT Station, if that is at all practicable.

Re my warning that:-

EVEN A WORKING 13-pin SOCKET MAY CAUSE MOT FAILURE

I now appeared to have got the DVSA to resolve this issue.

I went back and asked the DVSA for details of their instructions to MoT Testing Stations, so that we could quote those to them in the event that they were minded to fail our cars due to the rear trailer foglight issue.

Today I have received a longer response from the DVSA which includes the following:-

The essence of the notices was that ‘we are aware there are issues surrounding the operation of some 13 Pin socket testers and is working with the Garage Equipment Association to resolve them. If you are experiencing difficulties you should, in the first instance, contact your equipment supplier/manufacturer giving as much detail as possible (e.g. vehicle make/model/year etc). You should continue to pass and advise’.

However, following your email I found that this particular aspect may not have been a known issue at the time of the Special Notices.

As a consequence of further discussions, we have now started the process to remove the check of the trailer rear fog lamp from the MOT Inspection Manual. This is a far simpler solution than trying to get the issue resolved with further equipment modification or replacement and should only take a week or so.

Once this is complete, the check using the 13 pin trailer socket test tool will only include the operation of position lamps, stop lamps and direction indicators.

Not the solution I was expecting, but it will certainly solve this particular problem !

Bear in mind that this could also affect the dashboard indicator or the bleeper that confirms the trailer indicators are working. Exactly the same LED low current problem.

I understand the rear fog light test on 13-pin trailer sockets has now been removed from MoT tests.