Heres a quick guide to basic fault finding on the towing vehicles 13 pin socket.
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. If you are unsure how to use your multimeter, have a quick read through the instructions supplied with your multimeter. As meters vary on the switch positions, you need to check that it is set for reading DC (Direct Current) VOLTS and can read voltages up to 20 volts.
One of the first things is safety. As you are working close to the rear of the vehicle, make sure the hand brake is on.
As the 13 Pin socket has a flap covering it, it is easier if you can find some way for holding the flap open. I use a length of string looped over the rear wiper arm and down to the socket flap. You could use a bungee cord in a similar manner.
You need to find a suitable earth point on the vehicle to connect the negative (black) lead of your multimeter to. Sometimes the tow ball itself can be used, although this is not always reliable and it’s best if you can find an earthing point in the rear of the vehicle that wiring in the rear of the vehicle is connected to.
The first test is carried out with the engine off and the multi-meter set to read “volts DC” to obtain a ‘reference’ voltage. The ‘reference’ voltage is obtained by connecting the multi-meter directly across the battery terminals. In our example below, we have a reading of 12.82 volts.
With the engine-turned off, and the negative test lead (black) of our multimeter connected to the earth point in the vehicle. We now check all the sockets connections individually using the positive (red) test lead. Only one – Pin 9 (leisure battery charging circuit) should have a voltage present on it…..
If all that checks out OK we are ready to move on to the next test.
SAFETY: We are now going to test the socket again, this time with the engine running. As you are working at the back of the vehicle you will be close to the engine exhaust. Don’t do this test in a garage. Always be aware of exhaust fumes. Don’t work at the rear of the vehicle with the engine running for more than a couple of minutes at a time. Again, check that the hand brake is applied.
With the engine running we read the ‘reference’ voltage directly across the battery again. In our example now it is 13.75 volts with the engine running.
Returning to the socket once again, connect the negative (black) test lead up to the earth point on the vehicle and go through each pin again. This time Pin 9 should show a voltage as well as should Pin 10 (Fridge circuit) We have now checked that with the engine running, no other circuits should be operating except pins 9 and 10.
NOTE: On modern cars, especially ones fitted with factory towing electrics, the vehicles Electronic Control Unit (ECU) might delay the turning on of the fridge circuit until it detects that enough charge has been put back into the vehicle battery after starting the engine. If you first test Pin 10 and don’t get a voltage reading, wait a few minutes with the engine idling and re-test.
OK… turn the engine off and get some fresh air!
That’s main two caravan supply feed’s checked. Now we need to check the neutral (earth) return path for each of the circuits. The three low voltage (12 volt) circuits are “road lights’, “fridge” and “leisure battery charging”. Within the caravan these three circuits are separate, including the neutral (earth) return path, the only time these three neutrals (earths) come together is at an earth point in the vehicle.
So with the engine off, set your multimeter to read “Resistance” or “Ohms”. To check it is working, touch the two test leads of the multi-meter together, you should get a reading of 00.00 Ohms.
With your multimeter negative test lead connected to the earth point on the car, test each of the three earth connection in the socket… (below). What we are looking for here is the lowest reading. In our example we have a reading of 0.11 Ohms. It is possible to get a reading of 0, but it is more likely to be just above. If it reads 0.5 or upwards, it is probably a bad earth connection. Check your meter’s connection in the car and re check. If the reading is still high, you need to find the earth terminal in the vehicle for the cable that you are testing and give it a clean. It might mean undoing a nut and using some fine emery cloth to remove any dirt from each terminal and possibly rust around the base of the terminal post.
With the test above completed, we are now confident we have good earth connections and the leisure battery charging circuit and fridge circuit are operating correctly.
The next test is for the road light connections.
For this it is really helpful to have an assistant to turn on and off all the lights when required.
SAFETY: make sure they understand that you are close to the rear of the vehicle, as this test requires the engine to be running.
Now we check our ‘reference’ voltage again. Remember to reset your multimeter to read Volts DC. In the example below we have 13.75 volts again.
Now we check each connection in turn. Make sure your meters negative (black) test lead is connected to your earth point on the vehicle. Working logically, start with pin one. There should be no voltage present, now ask your assistant to put the left indicator on. As this is an intermittent voltage, you might not read the same as the battery ‘reference’ voltage as the meter might not be able to register the full changing voltage, so it might only rise to 8 or 10 volts before going back to 0 and repeating. As long as you get a constant on off reading it’s usually acceptable. Keep going round the pins, connect the meter and ask your assistant to turn on that circuit. Remember, the rear fog light will need the headlights on.
When you get to check the reversing light, this MUST for Safety be done with the ENGINE OFF but the ignition turned on. DO NOT ATTEMPT TO CHECK THE REVERSING LIGHT CONNECTION WITH THE ENGINE RUNNING! – I don’t want you to get run over!
Put the test lead of the meter into Pin 8 and ask your assistant to select reverse. You should be able to read the voltage.
After the above tests have been completed, connect up the caravan. If any faults exist, its likely that a fault is with the caravan’s wiring.
If you have a fault on your 13 pin socket, by now hopefully you will know exactly which circuit it is on. If it is the road lights, then the next step is to check the fuses within the vehicle.
If a fault is found with the leisure battery charging or fridge circuits, then more investigation is required. Checking the fuses in the vehicle for these is a starting point, but it may involve tracing the wiring back to find the relays that control these circuits and I would recommend you consult a qualified professional.
A Quick Note about Pin 12…
Although on most of the drawings I show Pin 12 as not being used, this is actually incorrect. Pin 12 is used for trailer detection on some vehicles. How it works varies from vehicle to vehicle and obviously not all vehicles use it. Generally it is connected to the vehicles ECU system. The 13 pin plug on the trailer has a link between Pin 12 and Pin 3 (road light earth) so when you plug-in the trailer or caravan, it ‘shorts’ pin 12 on the towing vehicles socket to earth and the vehicle’s ECU detects this and activates any Trailer Stability Program on the vehicle. The full ISO standard is ISO 1446 for 13 pin connections.
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