That is one of the most common questions that I am asked about road light problems with caravan’s and trailers. Others are::
“Why do my side lights come on when I brake?”
“When I turn my side lights on why do the indicators glow dimly?”
“When I put my side lights on why do my reversing lights come on?”
“When I brake why do all my rear lights come on?”
The answer is fairly simple, and a lot of people will tell you “It’s an earth fault” So often said simply because they have heard someone else say it but really they don’t understand how it can be an earth fault. Trying to find a fault even if you know what it is can be frustrating if you don’t know a bit of theory behind it, but I’ll try to give you an insight into what’s going on. Lets have a look at a basic wiring diagram for the caravan or trailer road lights and their connections.
The term ground and earth are basically the same thing but the ‘earth’ refers to the cable and the ground is the termination of the earth cable to the car in this case.
The drawing above shows the basic road light wiring. (Click on the diagram to enlarge it) You can follow each circuit through from the tow vehicle through the 13 pin connector to the rear lights of the caravan or trailer and then back through the return path (earth) to the 13 pin connector and back to the vehicle ground as shown in the diagram.
What happens when you operate the right hand indicator? Electricity flows down the green wire and lights up the right hand indicator on the vehicle and continues down the green wire through the 13 pin connector to the caravan or trailer right hand indicator. It then returns through the black earth wire to vehicle ground.
So what happens if we have an earth fault?
Here is the same drawing only this time with an earth fault.
Now the return path for the lighting circuits is broken, but using the indicators now also flashes the rear side lights. So what’s happening?
In the diagram below we have again turned on our right indicator, electricity flows down the green wire (shown a bit thicker this time) to the rear caravan or trailer indicator. It flows through the bulb and should then follow the black line to the vehicle ground. But it can’t as there is a break in the wiring.
Electricity really does like to go home!
As the return path through the black wire is effectively ‘blocked’ because of the break the electric current will try to find another way home. The only path it can find is back through another circuit, in this case the right hand side light circuit all the way back to the tow vehicle and through the bulb in the rear light to earth, as illustrated below.
So as your indicator flashes, it is also powering the side light on the caravan or trailer and the rear side light of the tow vehicle. Electricity will nearly always find a way home!
But it’s not quite that simple. In the diagram above I have shown one return path (the heavy black lines) but in fact the return path will be through all the light bulb circuits that are not powered. As there are multiple paths back to earth and only one path or circuit powering the indicator bulb, the indicator bulb will be at full brightness but the other bulbs may only be dim.
If you now turn on the side lights and indicate right, the current from the side lights and the indicator will follow the path of least resistance through the other lights that aren’t lit back to the tow vehicle earth.
So the original question of “Why do my side lights flash when I indicate” might now be expanded to “Why do my side lights flash when I indicate? It can’t be because there is anything wrong with the side lights because they work normally”
We now know that the side lights only appear to work correctly.
The diagrams all show a common earth from both sets of rear lights on the caravan or trailer. Sometimes for convenience the manufacturer will run a separate earth for each light cluster, so all the wires for the left hand lights run down the left hand side and all the wires for the right down the right hand side. The common point is usually a small junction box at the front of the caravan or trailer.
If we have similar symptoms as before but only affecting one side (the RH indicator is OK, but the LH Indicator also flashes the LH side light and LH brake light) then the break in the earth cable will be only on the left hand side earth wiring.
So now you know the theory behind earth faults on caravan or trailer road lights. If you want to do a bit more reading you might find these two articles interesting:
Caravan Road Lights – Basic Fault Finding – a simple fault finding guide using a multimeter to check your caravan or trailer road lights.
Caravan Road Lights – Tracing A Fault – step by step guide to tracing faults in your caravan or trailer road lights.
Caravan Chronicles 