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Thread: Wiring up a pair of ballast resistors for towing a trailer with LED lights

  1. #1
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    Wiring up a pair of ballast resistors for towing a trailer with LED lights

    I have just finished wiring up a pair of ballast resistors to my towing plug harness so I can just flick a switch and it will think it has a trailer with incandescent globes...

    It turned out easier than I had feared. If you put a multi meter across the right or left pins and earth you will find that, with the ignition on, there is actually around 10.4 Volts there. This makes checking to see if you have the right wires easy. I assume this is checking to see what the resistance of the globes is.

    It turns out, on my 2019 D5 5 seater Sd6, the wires to the trailer plug are actually the Australian Standard colours, which is handy as there is no way I can get the plug off to open it up and see.

    I removed the tray under the false floor to get to the wiring - lucky me, no sub-woofer to take out - and unwound the black tape from the wiring bundle. There were two wires in the bundle - one plain yellow, one plain green as per the standard - which looked right, and, after stripping off a small area of insulation they proved to be the ones I needed. I soldered one wire from the resistors straight to each one. I then mounted the resistors on a section of aluminium angle as a heat sink and used a convenient bolt to attach the angle.

    The other end of the resistor wires I soldered each one to one half of a piece of twin core wire and then I ran both twin core wires across the back of the car to the jack storage compartment and put a double pole / single throw switch on the end. Back at the resistors I joined the other half of the twin wires together and crimped on an eye which I attached to a convenient earthing point.

    Job done.

    When I flick the switch, I get the green trailer lights flashing with the indicators. Flick it again, no green trailers. Unfortunately, I can't test it with our camper attached as we are in lockdown...

    Cheers
    Numb Thumbs

    PS Thanks to Graeme and Hugh Jars for their help!


  2. #2
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    Some photos.


    Overview:



    Closer in:


    The wiring:



    The Resistors on the angle aluminium heat sink:



    The switch in the jack storage area:



  3. #3
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    My new 2019 model D5 recognises my caravan with no mods and all the smart stuff just works as it should. My 2013 L405 would not recognise the same caravan without adding resistors but luckily I never got around to adding them to the caravan. It seems that LR have finally got the sensing of LED lights on caravans correct.

  4. #4
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    Not on all of the D5s...

    Cheers
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  5. #5
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    Quote Originally Posted by Numb Thumbs View Post
    Not on all of the D5s...

    Cheers
    Numb Thumbs
    What is your build date? Mine is 05/19.

    This model is obviously better at detecting a caravan with LED lights than the 2013 RRV but it probably also depends on the draw of the trailer lights. My caravan might draw more current than yours. The minimum load is 1.75A according to the 2019 manual, so I assume that my caravan exceeds this even with LED lights, although I haven't measured it.

  6. #6
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    It may also be the NUMBER of LED lights. The trailers I tow have one bank of LED lights (Narvas, from memory with one panel each of indicator, reversing and tail lights) on each side. If they had two, that might be enough to trigger the pick up.

    Cheers
    Numb Thumbs

  7. #7
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    Quote Originally Posted by Numb Thumbs View Post
    It may also be the NUMBER of LED lights. The trailers I tow have one bank of LED lights (Narvas, from memory with one panel each of indicator, reversing and tail lights) on each side. If they had two, that might be enough to trigger the pick up.

    Cheers
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    My main point was that the same caravan was not recognised with my D4 or my L405 RRV, so the 2019 D5 is a significant improvement over the previous models. I also read that it has been improved on 2019 model D5's, which is what mine has shown I think.

  8. #8
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    My compliance plate says 06/19.

    Cheers
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  9. #9
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    Mine is 05/19. You just need a tiny bit extra current draw from your lights I suppose. My caravan only has a single LED indicator assembly on each side but it must draw more amps than your trailer.

  10. #10
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    I have now tidied up some wiring and moved the switch to the dash.



    That is, from right to left, ballast resistors, Tow Pro Elite, compressor master switch and compressor on/off.

    Cheers
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