Thread: The rear fog lights come on when I plug the trailer in

Originally Posted by DDdisco

Gday all,

I've done a search and couldn't really find a similar problem When I plug the trailer (a horse float) into my Disco 3, the rear fog lights (including the dash indicator) come on. This happens regardless of the light switch or ignition switch position. Everything else works as it should.

Does anyone have any ideas?

Cheers

If I didn't know better I would say your vehicle runs a CAN/BUS system for the rear lights, the wiring for the trailer may be overloading the circuits. The float also may have bridged connectors for running on different sockets. Once you have identified which plug contacts on your D3 are for what lights, you might have to check there are no bridged wires in the float's plug. I have been told the only way to run trailer lights on new cars is to run relays off the relevant rear lamps, and power them from the battery feed up the front.

Thanks for the info. The trailer is wired standard with no bridged connectors, but it does have a number of clearance lights etc. Might have to get someone to look at it.

I would assume you have trailer brakes towing a horse float!
What type of brakes do you run, electric or a brakeaway style?
Can i ask who wired in the trailer brakes? if you have them, Was it a dealer? or a recommended auto electrician?
You need to be careful who wires in the trailer brakes as the wiring needs to be modified not to cause problems.
I think you may have a wiring problem on possibly the vehicle & maybe even the float??

Good luck.

Cheers

I've only just bought the vehicle and haven't had the brake controller installed yet. There was no problem on the previous vehicle (a Patrol), and all the lights on the trailer work properly (including the tell-tale on the dash with the indicators).

I'm going to get the controller wired in by the local dealer, but wanted to get to the bottom of this before I add to the complexity of the diagnosis!

Does the no.2 pin on the trailer need a supply for the battery for the horse float, if it does then this will be back feed to the vehicle suppling power to the fog lights.
HTH

Originally Posted by DDdisco

I've only just bought the vehicle and haven't had the brake controller installed yet. There was no problem on the previous vehicle (a Patrol), and all the lights on the trailer work properly (including the tell-tale on the dash with the indicators).

I'm going to get the controller wired in by the local dealer, but wanted to get to the bottom of this before I add to the complexity of the diagnosis!

I suggest you disconnect each load circuit at the trailer plug one at a time until the fog lamp goes out. Fit a relay to the offending circuit (probably the tail lights) with a battery power feed. Your power feed can be a rear outlet for fridge etc.

Thanks for the ideas. I'll check the number 2 pin. The float has got cameras in it that I have never used. Maybe that's a power feed to them. There's no other power feed (other than the lights). Otherwise I guess I'll just check one circuit at a time.

Originally Posted by DDdisco

Thanks for the ideas. I'll check the number 2 pin. The float has got cameras in it that I have never used. Maybe that's a power feed to them. There's no other power feed (other than the lights). Otherwise I guess I'll just check one circuit at a time.

Sorry I meant a power feed to a new relay inside the car, to prevent the trailer load coming directly off the lamp circuit.

It means the relay puts only an extra 0.2 amp load to the car's light circuit, and all the extra lamps' amps are handled by the relay using power from the car battery, via a new or existing power feed wire to the back.