Another “How To:” guide. It’s not intended as a ‘verbatim’ method of solving this little problem, but more of a starting point.
I had an email from someone who has a tow vehicle that when he turned on his rear fog lights, the vehicle rear fog lights came on at the same time as his caravan fog lights. He wanted to be able to turn off the vehicle fog lights and leave just the caravan rear fog lights turned on so he wouldn’t be dazzled at night by the front of the caravan being lit up bright red by the vehicles fog lights.
On the face of it, it would be quite simple to install a switch to turn off the vehicle fog lights, but we have a few things we have to take into account. If we simply installed a switch to turn off the vehicle fog lights, we could in theory with out a trailer attached have the fog light switch turned on and the light to tell us they are on lit, but with our other switch have them turned off. Also for the MOT test if we have left our installed switch off, the vehicle would fail as the rear fog lights would not work.
Under current law, rear fog lights must be fitted to a vehicle, and there must be a visible indicator on the dashboard to show the lights are turned on. Fog light must only operate if the vehicles side lights are turned on. If fog lights are fitted to a caravan (or trailer) they must be serviceable and there must be an indication in the vehicle that the light(s) are operating (turned on). Simply wiring the caravan (or trailer) fog lights into the same circuit as the tow vehicle fog lights therefore complies with the law. So can we make it so that we can turn off the vehicles fog lights?
Now if you are reading this and have a tow vehicle that has a CANBUS system, the vehicle manufacturer will have a software update that will turn off the vehicle fog lights automatically if a trailer is detected. You will need to speak to your dealer about this.
OK, so lets have a look at the typical wiring for the rear fog lights on non CANBUS vehicles. This drawing is generic, so don’t assume your vehicle is exactly like this.
If we look at the drawing above, we can see that turning on the fog light switch energises the relay and that allows current to flow to the rear fog lights on the vehicle. At the same time, the dashboard warning light for the fog lights is illuminated. The feed for the relay switch will usually come from a circuit that is only powered when the side lights are turned on.
Let’s now look at how we can modify this circuit:-
If we install a two-way switch across the contacts of the relay and connect this to Pin 2 of the towing socket (disconnecting the existing wire between Pin 2 and the rear fog lights) we can now control the electrical feed to the caravan’s (or trailer’s) fog light circuit. We also need to install a second warning indicator light next to the switch.
With the switch in position ‘1’, the circuit works exactly as it did before, both the vehicle and the caravan rear fog lights illuminate when you turn on the vehicles rear fog light switch. However additionally, the second warning light now also comes on to tell you the electrical feed to the caravan’s (or trailer’s) fog lights is live.
If we just want to turn on the caravan’s (or trailer’s) fog lights, we turn the new switch to position ‘2’. It now takes the power feed directly from the common side of the relay to the caravan’s rear fog lights and illuminates the new caravan fog light warning indicator.
So the vehicle still works as required by law… no matter which switch we use, we have a warning indicator to tell us the vehicle fog lights are activated and/or the caravan fog lights are activated. For and MOT test, the vehicle lights work as intended, and for the part of the MOT test that includes the 13 pin socket, it still provides the correct functionality.
Switch choice and installation
The rear fog lights on a caravan or trailer are usually two 21 watt bulbs, therefore draw a current of about 3.5 Amps. If you choose a switch that is rated at 4 AMps or above DC, then there will be no need to install an additional relay. For an indicator, I’d suggest a 12 volt Red LED type.
You need to install the switch and indicator close to each other, in a convenient location so that you can see the indicator but not be distracted by it. If you do this modification, it will be worth while adding a short note in the vehicle handbook on how it works, just incase you sell the vehicle and don’t tell the new owner. There is nothing worse than finding a switch in a vehicle and not having a clue to what it actually does!
The circuit I have drawn above is generic and you will have to work out the exact connection points on your vehicle’s wiring and install a new cable from the switch to the connection inside the vehicle where the 13 pin towing socket is wired into the existing vehicle loom. Don’t be tempted to use “scotch-loc’s’ on any part of the wiring, use the correct crimp connectors and heat shrink sleeve where required.
My original question was from a fellow Landrover Freelander owner, so I could give him a drawing specifically for his vehicle. I have links to all the Landrover Freelander electrical manuals and drawings in the “Document Library” If you need electrical drawings for other vehicles, a search on-line will usually bring results.
As usual, if you have any doubts about carrying out any electrical work on your vehicle contact your local auto electrician or tow bar centre, I’m sure they can help.
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