Thread: Discovery 4 - zero lights on any trailer

I don't even need to turn on the headlights with mine (MY12).

Both my trailers have LED lights. I simply plug in (no adapter or resistor added) and all lights on the trailers work as they should.

Trailer indicator light on the dash doesn't work - doesn't bother me, and the reverse parking / proximity alarm goes off when reversing - l ignore it.

I've been meaning to add a resistor to the indicator circuit as described elsewhere on here so that "trailer recognition" works fully as intended, but have never got around to it. Not a high priority for me - I don't pull a trailer very often.

Problem as described definitely sounds like a faulty earth circuit. I'd try running a bridging wire from the back of the earth pin (PIN number 3) on the trailer's plug directly to a good earth point on the car and see if the trailer lights work. If they do, pull the black socket out of the back of the car, and run a new wire from the earth pin (number 3) to a suitable earth point on the vehicle.

Originally Posted by Redback

OK so once you fix the issue with nothing working, this is all you need to do, on the D4 the only issue with LED lights on the trailer will be one indicator won't work and the trailer symbol on the dash won't flash, to fix the indicator, all you need to do is turn your headlights on, and as long as you don't mind that the trailer symbol still won't flash, it's all good and no need for an adaptor box like they use on the D3.

That's all I do.

The D4 pulses all of the light circuits when you close the drivers door - it does this to detect faults.
If you don't have sufficient resistance on the trailer (eg LEDs don't have enough resistance) then the car systems won't know a trailer is hooked on BUT the lights will still work.
Rear parking sensors won't turn off, adaptive learning of transmission will still continue so it will have to "unlearn" after the trailer removed, trailer sway dsc program not activated straight away and there's other stuff I can't recall.

Def sounds like earth problem - do you know someone else with another trailer with known working lights to test?

Also dealers usually have a test box they can plug in to car to check

Originally Posted by Meken

The D4 pulses all of the light circuits when you close the drivers door - it does this to detect faults.
If you don't have sufficient resistance on the trailer (eg LEDs don't have enough resistance) then the car systems won't know a trailer is hooked on BUT the lights will still work.
Rear parking sensors won't turn off, adaptive learning of transmission will still continue so it will have to "unlearn" after the trailer removed, trailer sway dsc program not activated straight away and there's other stuff I can't recall.

Def sounds like earth problem - do you know someone else with another trailer with known working lights to test?

Also dealers usually have a test box they can plug in to car to check

Mine doesn't and neither should your's, only the D3 pulses, the D4 shouldn't, I'd say you have an issue if your's does.

Nope the d3 pulses the circuits continuously the D4 does it once on the driver door close
The continuous pulsing of the D3 is what makes the trailer lights flicker and thus requires a pulse buster or similar. The D4 only needs a "led load resistor " that you can buy from super cheap across one of the indicator circuits on the trailer

Originally Posted by Meken

The D4 pulses all of the light circuits when you close the drivers door - it does this to detect faults.
If you don't have sufficient resistance on the trailer (eg LEDs don't have enough resistance) then the car systems won't know a trailer is hooked on BUT the lights will still work.
Rear parking sensors won't turn off, adaptive learning of transmission will still continue so it will have to "unlearn" after the trailer removed, trailer sway dsc program not activated straight away and there's other stuff I can't recall.

Def sounds like earth problem - do you know someone else with another trailer with known working lights to test?

Also dealers usually have a test box they can plug in to car to check

Just a small clarification, LEDs appear as a high resistance/low current and therefore presents efectively as "open circuit" to the car which is why it does not recognise that the trailer is attached. Adding the resistors lowers the resistance allowing more current flow which the car can then detect.

Originally Posted by Meken

Nope the d3 pulses the circuits continuously the D4 does it once on the driver door close
The continuous pulsing of the D3 is what makes the trailer lights flicker and thus requires a pulse buster or similar. The D4 only needs a "led load resistor " that you can buy from super cheap across one of the indicator circuits on the trailer

I don't see the point of buying something you don't really need and I think you misunderstood what I was saying.

Originally Posted by Redback

I don't see the point of buying something you don't really need and I think you misunderstood what I was saying.

Because if you put in the resistor (1 only) the car knows it has a trailer and modifies the appropriate programs (gearbox, ABS, trailer stability, rear parking sensors, rear camera etc) to compensate.
Mine asks if you have connected a trailer and then asks you to set up a whole lot of parameters such as length from drawbar to axle, number of axles, weight etc. It would appear that it takes these parameters into consideration when modifying the various programs.

Originally Posted by Geedublya

Because if you put in the resistor (1 only) the car knows it has a trailer and modifies the appropriate programs (gearbox, ABS, trailer stability, rear parking sensors, rear camera etc) to compensate.
Mine asks if you have connected a trailer and then asks you to set up a whole lot of parameters such as length from drawbar to axle, number of axles, weight etc. It would appear that it takes these parameters into consideration when modifying the various programs.

That's for the trailer reverse assist isn't it?