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……
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.
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 want to download the drawing above, these are in PDF format so you can add them to your iPad, Tablet or eReader…
Understanding Watts, Amps, Volts and Ohms – A very basic introduction to some simple maths that allow you to work out power, current and resistance.