A frequent question that is e-mailed to Caravan Chronicles is how to fix a fridge circuit that doesn’t seem to be working correctly when towing.
I’ll assume that you have checked the 12 volt heating element in the fridge and all other lights/indicators are showing that the fridge ‘should’ be working OK with no error messages. To check the heating element you will need to know what the rated wattage is and simply work out the resistance of the element and test with your multi-meter. If you don’t know how to work the resistance out from the wattage read: Understanding Watts, Amps, Volts and Ohms.
Most of the other problems are due to issues with the electrical circuit and more often than not the earth (or return) for the circuit. One of the ‘quick fixes’ for earth problems with the 12N & 12S plugs is to link the three earth cables (road lights, leisure battery charging & fridge) together. However this is not recommended, no matter how tempting it is. Doing this might result in a melted tow plug and socket due to a faulty earth somewhere allowing all the return current for the road lights, leisure battery charging and fridge to return through one pin of the plug. The old 12N/12S were not rated to take this current and the connections often overheated. The modern 13 pin plug can take this current, however the damage is more likely to be a melted cable somewhere. It is critical that these three earth circuits are kept separate within the caravan and should only be connected together once inside the tow vehicle at a common earth point.
The fridge circuit looks complicated but in fact it’s quite simple. I’ve drawn out the basic circuit (below) and left out all the wiring that isn’t really involved in running the fridge. The earth connection between the battery and the 13 pin socket might be the vehicle body itself. I would always recommend running a 4mm or 6mm cable from the battery to the common earth point at the rear of the car for the tow electrics as well as using the vehicle chassis for an earth. Copper, after all, has a lower electrical resistance than steel and you are relying on steel sections welded together to provide an electrical path.
To perform fault-finding on the fridge circuit you are going to need a couple of things. The first is a multi-meter set to read DC volts in a suitable range – 0 to 20 Volt range is fine. The second thing you will need is a long test lead – about 5 metres long, with a 4mm Banana plug on one end (to suit your multi-meter) and a crocodile clip on the other. A 5 metre length of 1.0 or 1.5 mm Sq single core flexible cable will be fine. You can usually buy a 5 metre spool of cable for a few pound from Maplin along with a 4mm banana plug and a crocodile clip.
The reason you will need a long test lead is because we will be measuring the voltage all the way to the fridge at various points with one lead of the multi-meter attached to the tow vehicle battery.
The other thing you will need to do is disassemble the 13 Pin plug (or 12S Plug) on your caravan (if you don’t know how to do this read The 13 Pin Plug Puzzle). You will need to plug the disassembled plug into the tow vehicle trailer socket so that you can measure the voltage on the relevant pins while the engine is running.
OK, lets talk safety. The testing outlined here requires that you connect the caravan to your tow car, with the 13 pin (or 12S) plug partly disassembled and take voltage readings with the engine running. While you are taking readings at the rear of the tow vehicle you will be close to the exhaust with the engine running. If you are in any doubt about your abilities using a multi-meter or working on live circuits, please get a qualified auto electrician to do this for you.
You will need to know where all the cable connections are, so there may be some element of dismantling part of the vehicle trim to access the fridge circuit relay in the vehicle and inside the caravan, you will need to locate where the cable(s) from the electrical towing connector (13 pin or 12S) enter the caravan and terminate in the fuse box/junction box. You will also need to be able to access the electrical connections on the fridge. Most of the information you will need on the locations of these items can be found in the relevant handbooks, which should also help you identify which cables and terminals to test.
With access to everything sorted out and the vehicle connected to the caravan you have to use the long test lead and connect one end to the NEGATIVE terminal of the vehicle battery and the other end to the NEGATIVE (-VE) or COMMON terminal on your multi-meter. Double check it is set to read DC Volts at a suitable range (0v-20v or 0v-30v). You can now start the engine. It might take a few minutes for the fridge circuit to become live depending on the vehicle but if you leave it ticking over for ten minutes or so before you start it replaces some of the charge back into the starter battery so the battery voltage reading will not alter much. Make sure the caravan switches are all set to how you would have them if you were towing normally.
Right, the first reading we want to take is the battery voltage – this will be our reference voltage. Using the POSITIVE lead of your multimeter on the battery +VE terminal take a reading. In the example below it is 13.8 volts. Write it down on a notepad. The next reading is the input side of the fridge relay – sometimes the relay is in the engine compartment, but more usually it can be found in the rear of the car. Read the voltage from the input terminal and then check the voltage from the output terminal.
The last reading for the moment on the vehicle is now pin 10 on the 13 pin towing socket (pin 6 in the 12S). Take the reading from the pin on the plug while it is plugged into the socket so we not only check the connection in the socket but the connection between the socket and the plug. That’s why we had to disassemble the plug earlier. Make a note of the reading.
We now have a few readings we can start to work out voltage drops with…..
The next stage is to check the readings in the caravan. There are only two readings to take, the first is at the fuse box/junction box where the cable from the caravan tow plug come in and joins up to the caravan wiring. The second is the +ve terminal for the fridge 12 volt heating element…
Now we can work out the voltage drop for the +ve side of the circuit…. In this case its 1.3 volts.
Next we need to check the voltages on the return (negative -ve) side of the circuit. For this we need to swap the leads round on the multi-meter. We need to connect our long lead from the battery +ve to the +ve connection on our multimeter and use the black test lead connected to the common or -ve on the multimeter as the test lead.
Check the battery voltage again to get a reference, in this case it is still the same 13.8 volts. Now check pin 11 on the 13 Pin plug and make a note of the reading…
Looking at our notes we can start to quickly work out the voltage drop on the return path of the fridge circuit for the vehicle….
We still have two more readings to do. The caravan fuse box/junction box and the -ve side of the fridge 12 volt heating element…
We should now have the last set of readings we need to work out the remainder of the circuit….
We can see that the voltage drop for the return circuit is also 1.3 volts, therefore for the fridge circuit is loosing 2.6 volts, so if the battery voltage is 13.8 volts the fridge is only getting 11.2 volts.
Where do we start?
We have to look for the biggest voltage drops in any one section. In our example nothing stands out too much although we do have a larger drop between the voltage at pin 11 on the return side of the circuit and the fridge which is 1 volt. So I’d check the connections at the fridge end, the fuse/junction box and in the 13 pin plug.
If I get larger voltage drop between the battery and the 13 pin plug connection on either the +ve (live) or -ve (earth or return) then I would suspect that the connections on either the back of the 13 pin socket or in the 13 pin plug. If water gets in to either, then the copper cables oxidises and the resistance can increase. You can tell with a visual inspection – if the copper in the cable under the screw terminals doesn’t look ‘bright and shiny’ but dull and brown then it is worth remaking the connections by cutting off the end and stripping the cable back to bright copper. To protect from water use a proprietary aerosol water dispersant aerosol for electrical connections. DO NOT use WD40 as this can affect the plastic of the plug/socket long-term.
Cleaning terminals is easy. You can use a 1200 grade emery paper to clean terminals so they are ‘bright’ and a switch cleaner aerosol to degrease them. With spade terminals, they can become loose. Careful use of small pliers to squeeze the curled part of the terminal can solve the problem. The earth terminal in the rear of the tow vehicle is a common cause of problems. Usually it’s a short threaded stud welded on the chassis with a nut to hold ring terminals down. As it’s in the boot, it can suffer from corrosion and that’s always worth cleaning up… especially if you think you road lights on the caravan are dim or the side lights flicker when you have your indicators on.
Ideally the voltage drop on the fridge circuit should be as low as possible.
Cable Termination 101 – A look at how to achieve a professional quality cable termination.