Test trailer lights

Discussion in 'Electricians' Talk' started by Jay459, Feb 23, 2018.

  1. Jay459

    Jay459 Member

    is there something I can use to test my trailer lights. I presume I need some sort of 12v source.

    I take it I can run long cables from car battery to lights looking something a bit simpler.

    The trailer lights have cables for brake, tail, reverse and indicator.
  2. unphased

    unphased Screwfix Select

    Trailer for sale or rent, rooms to let, fifty cents.
    No phone, no pool, no pets, I ain't got no cigarettes
    Ah, but, two hours of pushin' broom
    Buys an eight by twelve four-bit room
    I'm a man of means by no means, king of the road.
  3. robertpstubbs

    robertpstubbs Screwfix Select

    Does it have a plug with 7 pins? From memory these would be:

    Connect the earth to the neg (black) battery terminal and then try connecting the others to the pos (red) battery terminal one by one.
  4. robertpstubbs

    robertpstubbs Screwfix Select

    And there should be one or more number plate lights on the same circuit as the tail lights.
  5. joinerjohn1

    joinerjohn1 Screwfix Select

    Test the trailer by all means, but there could be a fault with the wiring from the car to the trailer socket. ;)
  6. JP.

    JP. Screwfix Select

    Lol Un..:)
  7. Bob Rathbone

    Bob Rathbone Screwfix Select

    Fix a mirror to your garage door at rear window height, back the car and trailer up towards the garage door, look in the mirror and operate the lights, tests car, socket, plug and lights. Simples.
  8. KIAB

    KIAB Super Member

  9. MGW

    MGW Screwfix Select

    Over the years trailer wiring has changed specially number 2 wire, it was A721.55.2.6 in original 17th edition. To begin with 2 was vac or air warning, then when caravans started to use the 7 pin plug it became live supply, then the electric reverse came in and it became reversing light, then it became the fog light.
    Pins 5 and 7 have always been side lights why two pins not a clue, however many have used the side light supply for other uses.
    When the second S plug came out the idea was to remove the problem of what pin 2 is used for, however the caravan user yet again messed it up, they want three live supplies, fridge, ignition (charging), and always live. However only two pins allocated, 4 (9) Continuous power supply and 6 (10) Power supply controlled by ignition switch so yet again caravan users started to use "Coding for coupled trailer" 2 (12) which is really for air or vac warning.
    As a result even with a tester one can't be 100% sure. Years ago Lucas made a trailer relay box so all the trailer lights used their own fuse, not seen one for years, there were also blocking diodes and relays for split charging, at that time 2 to 5 amp charge was enough not that it ever worked that well, the whole idea was to ensure caravan battery not connected to car battery when cranking car, some cars there was a terminal on the ignition switch. Often it used the alternator warning light to switch, but latter used the voltage.
    However today we use a lot more power in the caravan so a 12 volt to 12 volt inverter is used, these can be stage chargers so anything from 10 to 25 amp charging current. Seen some at 100 amp but clearly not with 7 pin socket.
    So in the main wired to suit trailer, so with a horse box one may have vac brakes, although how one would get vac today with fuel injection I don't know, I know early Ford had a vac pump built into petrol pump to keep windscreen wipers working.
  10. JP.

    JP. Screwfix Select

    To test trailer/caravan lights all you do is get someone to sit in car with ignition on and you go at the back of trailer and shout out various commands such as ''Brake On/LH Indicator On/RH Indicator On/Side Lights On/Fog Lights On'' and this and that on. Make sure you plug the trailer into the car socket though, this is usually found near towball. Failure to do this will result in nowt happening.
    unphased likes this.
  11. Bob Rathbone

    Bob Rathbone Screwfix Select

    This works well until you try the indicators when you ask "Is it working?" and the watcher answers "Yes...No...Yes...No". :) sorry, could not resist the old joke.
  12. Comlec

    Comlec Screwfix Select

  13. robertpstubbs

    robertpstubbs Screwfix Select

    Around the early 70's cars started to have their L/H and R/H tail lights fused separately. This was a safety measure to ensure at least one tail light was still on even if the other failed. Therefore the L/H and R/H tail/plate/marker lights on a trailer are also separated.

    (My "from memory" post above was slighly wrong, but I am remembering a bit better now.)

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