Caravan Road Lights – Basic Fault Finding

Here is a quick guide to basic fault finding on your caravan or trailer road lights.

I’ve written this simply as I can in easy basic steps so that hopefully anyone with a little understanding will be able to ‘help themselves” as much as they can before having to ask for assistance.

I’ll assume you have a digital multimeter and are able to use it to take voltage readings and measure resistance. If you are unsure how to use your multimeter, have a quick read through the instructions supplied with your multimeter.

How do we start?

In many caravans, there is a fuse box located inside the caravan where the cable comes from the 13 Pin plug. These are usually the same type of “blade” fuse found in your car. Check these first and replace any ‘blown’ ones.

This may seem daunting at first, but taken a step at a time, it’s quite easy to test the road light circuits. I’ll assume you have connected up your caravan (or trailer) and found one or more of the road lights, indicators, side lights or brake lights that are not working, and you have checked the fuses, if any. By following this guide, it will take you through all the steps required to undertake a basic fault-finding process.

Before we start, take a look at the drawing below of one of the indicator circuits……

Road Light - Fault Finding 01

Before we jump in, there might be something we can work out to help us. As we are going to be using the multimeter set on “Resistance” on the lowest scale. Your multimeter might have the symbol “Ω” – the Greek letter Omega  used to indicate “Ohms” or resistance. The settings might be 20, 2K, 20K, 200K, 2M. You want to use the lowest, in this case 20.

It would be helpful if we had an idea of what resistance reading we should expect for each circuit. Well with some simple maths we might be able to get a good indication.

Each bulb has a rating, given in Watts (the unit of electrical power) For any bulb, we can calculate the resistance if we know two other things – Voltage(V) and the Power (W). To calculate the resistance we use the formula R = V2/P  (Resistance = Voltage squared divided by wattage)

So calculating for a for a 21 Watt indicator bulb we get :

122/21 or 144/21 = 6.8 Ω

But…. we have a big problem here! If you measure the resistance of your 21 Watt lightbulb using your multimeter, it will probably read 0.2 or 0.3 Ohms. So what’s going on? 

The metal in the filament has a ‘positive thermal resistance coefficient’. That simply means when its cold, the resistance is lower and as filament of the bulb warms up and starts to glow. the resistance increases.

What seemed like a good idea on paper isn’t so good after all ……. or is it?

If we have a reading below 0.8 Ohms, we can assume the circuit is OK, but we should never get exactly 00.00 Ohms, there should always be ‘some’ resistance. If the reading is exactly 0 Ohms then it is likely a ‘dead short’ in the circuit, but more of this later.


The first step is an easy one. Test the bulb! If it is just one light that doesn’t work, it is a simple task to remove the light unit lens and check the bulb. For guidance on removing any of the bulbs on your caravan, have a look through the owners handbook as there is usually a section on changing bulbs. Most handbooks also have a chart of what the correct wattage each bulb should be as well.

The easiest way to do the following tests is on the 13 pin plug itself. To do this without disassembling the plug there are a couple of ‘tricks’ we can use to make it easier.

The first trick is to secure the plug in a location that reduces the requirement to have three hands! I like to use a large spring clamp….. 


If we make sure the plastic location ‘key’ inside the plug is at our 6 o’clock position, it makes identifying the pins so much easier….


In the photo’s I am testing the road lights on my trailer, so clamped the plug to the hitch handle. On your caravan a good place is to clip it to the hand brake and use the ‘A’ frame fairing to rest your multimeter on. In this instance, it was easier to place it on the ground…..


The second trick overcomes the need to be able to hold the points of the multimeter test leads on the end of the pins with the skill of a surgeon. I use a couple of the straight through crimp connectors…… I find the blue size the best.


Simply push them on to the ends of your multimeter test leads and they provide an easy way to hold the test lead probes on the 13 pin plug’s individual pins.



Using the crimp connectors makes the task so much easier, the test leads don’t slip off the pins.

We are now ready to start testing, lets look at the pin connections seen from the face of the plug. Note the orientation of the plug, especially the ‘key’ at the 6 o’clock position.


TOP TIP: Remember, if you want to see any of the drawings or pictures full size, you can simply click on them, or you can down load them as PDF’s from the links at the bottom of the page. Please remember, they are for personal use only.

Logical Order

Even if you know which circuit isn’t working, it’s always best to work in a logical order. Start with pin 1 – Left hand indicator. This is the positive (+ve) cable to the lightbulb, so we also need to connect to the negative (-ve) or “earth” for that circuit. All the “earths” for the road lights are connected to Pin 3. Set your multimeter to the lowest ‘resistance’ range and touch the tips of the test leads together. You should have a reading of 00.00.

Place a crimp sleeve on each test lead and put the Negative (black) lead of the multimeter on Pin 3 and the Positive (red) test lead on Pin 1. You should get a reading between 00.01 and 00.06 Ohms.

Continue to check each pin and note the readings…..


If all goes well, you should have a similar reading for all the road light circuits.

However, if all the circuits show a ‘high resistance’ of 05.00 Ohms or more, then it is probably a bad earth connection in the plug and you will need to open the plug and check for a bad connection. If the ‘high resistance’ reading is for only one set of lights – left indicator and left side lights, it is more likely that the earth connection in the light cluster is the cause.

The most common problems with the electrical connections between the car and caravan is the earth connection. A faulty earth connection can lead to strange things happening… for example, when you put your indicators on your brake lights flash instead.

If you have read 13 Pin Socket – Basic Fault Finding or Understanding the Leisure Battery Charging Circuit you will know that the caravan’s 12 volt system has three Neutral’s or Earth’s – road light circuit, leisure battery charging circuit and fridge circuit and within the caravan these should all be separate… i.e. not electrically connected.

Why are there three ‘Earth’s”?

Each circuit has its own ‘return path’ (Earth) for safety. If any one of the earth’s should become disconnected in the plug (or socket on the vehicle) then the remaining pins would have to ‘share’ the load. So for example, if the fridge earth became disconnected, the earth path would be through one of the other circuits and could lead to excessive current through the remaining cables and plug pins. By keeping the earth’s separate within the caravan the chance of this occurring is zero.

We can do a quick check to confirm the earth’s are indeed separate. It is three quick measurements between the three earth pins….


On your multimeter, when it is set to read ‘resistance’ as long as the test leads do not touch each other the display will probably read ‘OL” (Open Line) or “OC” (Open Circuit) whichever it is, this is what you want to see. In the example above, my multimeter reads “OL” which indicates there is no connection between any of the three earth return circuits.

NOTE: Some smaller multimeters just show a ‘–” when set to the resistance range and the test leads are not touching each other. Please check the instructions that came with your instrument for details.

If you do get a reading showing a connection between any of the earth circuits, you must stop and investigate this further before doing anything else.

Hopefully by now you have been able to perform a basic test on your caravan or trailer’s road lights. If you haven’t already tested the 13 Pin socket on the tow vehicle, then help can be found in 13 Pin Socket – Basic Fault Finding

If you still have problems then more in-depth fault-finding is covered in “Caravan Road Lights –  Tracing a Fault” you might also like “Why do my sidelights flash when I indicate?”


If you want to download the drawing above, these are in PDF format so you can add them to your iPad, Tablet or eReader…

Road Light – Fault Finding 01

Road Lights – Fault Finding 02

Road Lights – Fault Finding 03

Road Lights-Fault Finding 04

Understanding Watts, Amps, Volts and Ohms – A very basic introduction to some simple maths that allow you to work out power, current and resistance.

Copyright © 2011 – 2015 Simon P Barlow – All rights reserved

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31 thoughts on “Caravan Road Lights – Basic Fault Finding”

  1. Martyn Talbot said:

    I have used your site and checked the feeds into the caravan, which are fine (I have no left hand side or tail lights on the caravan), I realise there is only the black lead on pin 7 of the 12N socket and am in the process of chasing the circuit through the caravan but the routing seems odd, I know this is a difficult question but is it normal for caravan road light wiring to go back to the middle of the caravan before it splits off from side to side? I am now back as far as the fridge and haven’t found anywhere that the circuit splits off to the left side as the main harness runs down the right site of the caravan. Any advice you could offer would be much appreciated.
    The caravan in question is a Swift Classic Coronette and the fault started as an intermittent fault which is now permanent, when it first happened the lights came on within a couple of minutes when the caravan went over a bump!

    • Hi Martyn
      The ‘usual’ method is to route the cable in an “L” configuration… down one side then across the back. This way only one earth cable is required to the first set of lights, then across to the second set. If it was a “Y” configuration the earth cable would require splitting at the junction and hence need more cable. This is also true for some of the other lights – reversing lights for example. If you look at the labour cost, its lower too as it takes less time to route an “L”.

      I would try and trace the wiring loom from the failed lights back to its junction with the main loom. I would tend to suspect that the failure point is the earth connection there. it might also be worth trying to see if the working lights have two earth connections… one going to the front 12N plug and the other going across tot he failed lights. sometimes manufacturers use the connection to the light cluster as a junction for the earth cables. Not good practice but quick and easy (cheap) for them.


  2. Martyn Talbot said:

    Thanks Simon, the problem is that the front and rear side lights on the left side of the caravan aren’t working so tracing back from the lights isn’t easy!
    I used an Ohm meter so I know there is a break somewhere in the positive circuit between the lights and the fuse board and am now in the process of chasing it back.
    I will persevere but at least now I know what the configuration is, thank you

    • If all the lights are not working on one side, check the earth link to the lights as that is usually the only link that is broken. If it’s only one or two not working, then it will be the positive feed to the lights. If its that, then have a think if anything could have been moved recently that could have trapped the loom and damaged the cables.

    • Oops, just re read your original post, I read it as no rear side lights… thinking all the lights were off on one side (tail, stop, etc.) not as I think you intended rear and side lights (my bad!). OK, then it must be the +ve feed. Just a thought, have they split out just the front, side and tail lights from the front of the van to run down one side and the rest in a loom to the rear on the other?

      • Martyn Talbot said:

        I have traced the harness from the input to the caravan from the towing vehicle to the fuse boards and am now following the main loom along the right hand side of the caravan but haven’t yet found where it splits off from one side to the other.
        As you have said that the “normal” configuration is an “L” shape I will go to the back of the caravan and check it out from there, but it doesn’t seem to make much sense to take the circuit along one side only to come back down the other just for the front side marker light, i would probably have split from the fuse board and taken one short cable run to the front side marker and run another circuit to the rear, but I have never wired a caravan so this is probably not the way it is done.
        Thanks for all your help.

  3. Bill Bush said:

    I have a 2010 Honda CR-V with factory fitted loom. 2 socket system as been changed to 13 pin socket which has a micro switch. Have found the micro switch cables are not connected. There are 1 x red cable with black stripe. 2x grey cables (joined) and the same with 2 blue cables. Is there a diagram to show where these 3 connections go to the switch?
    Also all 12 main pins have shown to be o.k. But the car rear fog lights are not working whilst not connected to caravan.

    • Hi Bill
      The rear fog lights will have been programmed in the ECU not to come on when a trailer is attached. Since you have had the 12N & 12S sockets changed over to a 13 min socket, the vehicle thinks there is a trailer attached all the time and has turned off the rear fog lights. (unless the bulbs have blown on both lights!)

      You will need to check which module is installed (there are around 4 different ones on the market) between the Honda vehicle electrics and the tow bar electrics to establish how the vehicle detects a trailer, some are done on the canbus by detecting if the trailer lights are present, some are done via a microswitch in the socket detecting the presence of a plug in either the 13 pin or 12N socket.

      There should have been a set of paperwork for the module given to you when the tow bar was installed, one will be the conformity certificate and the other should be the install/user guidelines. Either one should have details of which module was installed.

      Once you know which module, you should be able to find a wiring diagram on line which will tell you what each of the cables relates to.

  4. Bill Bush said:

    Thanks very much.
    I’ll see if there’s any clues in my paperwork.

  5. Here’s a real puzzle. I tow my 2013 Sterling Eccles with a 2010 Ford Galaxy. The Galaxy is fitted with a Witter towbar with dedicated electrics that Ford had to “switch on”. Everything has always worked. I’ve just bought a 2013 Volvo XC70 and had a towbar fitted by Volvo again with dedicated electrics. When I plug the caravan into the XC70 a “indicator bulb failure on trailer” message appears on the XC70 dash, the caravan indicators don’t work nor do the caravan reversing lights. Everything else does work (brake, fog and side lights, constant live, fridge, battery etc). I know someone with a 2010 Bailey so I plugged the XC70 into that and everything works including indicators and reversing lights. I plug the Galaxy into the Sterling and everything works. So there’s some sort of incompatibility between the XC70 and the Sterling. Anyone got any ideas?

    • I thing your caravan has the LED lights fitted. Swift caravans had a bit of an issue with LED’s and would not work properly with a number of vehicles. Sargent Electrical produced an ‘interface’ box that had to be installed in the caravan. Contact Sargent Electrical:

      • I’ve fixed the problem and am posting this in case anyone else experiences the same problem. The only LED light on the van is the high level brake – everything else is tungsten. One day I was fiddling and the caravan offside side lights went off. When I twisted the 13 pin plug in the Volvo socket they came on. Loose connection? Indicators still not working. I bought a trailer light tester from Amazon. It was 7 pin with 13/7 pin adapters both ends. The tester said all circuits working correctly car and caravan. I then unplugged both 13/7 pin adapters from the tester and plugged them in together so 13>7>13. I plugged this and hey presto, indicators working. I couldn’t tow with this in place because there was no constant 12v for the ATC, but it proved the Volvo could power the indicators. So the fault had to be in the plug or socket. I dismantled the socket. The van is only 4 years old but the connectors were badly corroded. All electrical connections were tight. I slackened each one and re-tightened and plugged it in and everything worked. That’s all it was after all those weeks investigating.

  6. Just read your article and find it very helpful. I appreciate being in a minority but here is one area that does not seem to have been covered and that is the electrical connector/adaptor for vans purchased abroad but used with a UK vehicle. I bought a new one (12S) about a year ago for my 2005 Hobby as the original was getting worn and took considerable “working” to get lights to work properly, however the new one produced weird results when tried last week. Most road lights no longer work even though I have cleaned all contact areas in the clusters. Of the two that do, one produces poor light and the other comes on very brightly like a brake but it is an indicator bulb. I am wondering whether the continental colour system of the wiring the same as ours, been changed in adaptors recently or is mine faulty?

    • Hi
      The road lights are connected using 12N, not 12S (S is “Supplemental”) which is the leisure battery charging, fridge and reversing lights.

      From the details you have given though I would suspect a faulty earth.

  7. Hi Simon.

    I have a rather puzzling one.

    On my caravan, my indicators work perfectly fine, but my brake lights are dim (and when braking my fog light also comes on dimly). Also, when I brake it cancels any indicator flashing on the caravan too.

    I’ve tested the 7 pin coming from the car (I just have single electrics) and everything works as it should.

    Scratching my head here but wondered if you had any thoughts?


    • Hi Caleb
      Sounds like an earth fault. Indicators seem to work OK but they will be earthing back through something… like caravan brake lights back to the vehicle and then to earth via the vehicle brake lights. (think of following the path of the electricity.. it always wants to go home so will find a way to earth somewhere) When you break, that path is lost so the other alternative is the fog light.

      Start by just getting a long length of wire from an earth point on the vehicle to a light cluster and see if everything works OK, if it does, you know to trace the earth cables from the lights back to the vehicle.

  8. Hi when I hook up the power to my van, the left hand side tail light bank goes out.It works fine on my trailer so it must be a problem in the van.I have a 2015 Jeep Grand Cherokee.Can you suggest a resolution

  9. There doesn’t seem to be a single earth to the left side taillight,is there only one to the entire van

    • Hi Ken
      There is usually only one earth cable running from the front of the caravan to the rear for the rear road lights. This is to keep down cost and make it easy to install the wiring loom when the caravan is constructed. You should find an earth cable running across the back of the caravan from one light cluster to the other. If only one side of your caravan lights are not working, it is usually a fault with this cable.

  10. Dead Mr. Barlow. This is an excellent review. Issue at hand. Trailer disconnected from tow vehicle, the plug stayed connected to vehicle but all of the trailer side wires pulled out from the plug. How do I figure out which of the 3 ground wires go back into the appropriate trailer side plug pins?

  11. Simon, pursuant to my comment. This is a euro boat trailer with a 13p plug. Thank you.

  12. Robert Alder said:

    Hi simon I have recently changed my tow car from a Nissan X Trail to a Kia Sorento, since doing so my side lights on my caravan light up when the brake lights come on, the only difference I can see is my Sorento has LED rear lights, what do you think?

    • Hi Robert
      I think you might have a dodgy earth for the road lights in the caravan. When you brake and the brake lights come on, it sounds like they are finding an earth return path through the side lights.

      Try taking a length of wire from a good earth point on the vehicle to the earth point in the caravan where the cable from the plug enters the caravan (it is usually a small fuse box that services just the road lights) and see if that makes a difference. If it does… its either the earth connection in the caravan plug or where the tow socket is earthed in the vehicle.

  13. Janet harrrop said:

    Hi have a problem that the lights on my 2009 luna clubman es caravans rear lights have stayed on when we have disconected it from the car.
    Its not the outer lights but the next ones in and they look like a circle of red leds.
    Be grateful for any suggestions.
    All switches sre off on the caravan.
    Many thanks

  14. My triton is hooked up to my jayco journey outback. No lights are working. No brake lights, no tail lights, no white side lights on caravan. Help…..

    • Hi Tara
      I would guess if no lights are working then it is a broken earth (ground) connection between the vehicle and trailer.

      You can easily check if you have a length of wire by making a temporary earth link between the vehicle and trailer and checking the lights.

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