Here’s the wiring diagrams showing the pin out for the plug and socket for the most common circle and rectangle trailer connections in use in Australia. I put this here because I had a hard time finding and figuring this out for my trailers.
If you have a round connector, commiserations. Get a flat one. Why? Well first of all, which round one? There are at least 3 different 7 pin round connectors in use in Australia right now. Queensland prefers the small 7 pin round, and apparently Victorians prefer a large 7 pin round that has a completely different pinout! There is also a heavy duty 7 pin round, which is different again! Why? Oh Why? And you can chuck in a couple of older 5 and 6 pin round connectors as well just to make it more confusing (I’m not sure if they are still in use, so I haven’t bothered with them here). Plus to top it all off, sometimes the small 7 pin round is rotated so that the yellow pin is at the top!
On the other hand, flat connectors have only one possible pinout in Australia, one possible orientation (they aren’t half rotated), and the extended 12 pin version is even compatible with the 7 pin one. So every rectangular connector in Australia is compatible!
And in my personal experience the flat is more reliable. I used a small 7 pin round for 10 years, and then after replacing the plug multiple times changed to 7 pin flat for the last 12 years, and I have never had a problem with a flat connector. I had one vehicle with a 12 pin socket and a trailer with a 7 pin connector and they played happily for years.
Why is it more reliable? I think part of it is that the flat connector has the female part (the part you need to protect most) covered and attached to the vehicle, whereas the 7 pin round connector has the female socket as the plug on the trailer. I don’t have the luxury of keeping my trailers in a garage – they are out in the elements – which is great if you like providing homes for bugs. I can’t count the number of times I had to remove critters from the female 7 pin round connector on the trailer end. Not so with the male 7 pin flat.
It might just be me, but I also think it makes more sense to have the socket powered rather than the plug (just like household 240v sockets – imagine if they were 240v prongs sticking out of your walls at home that were live). It’s hard to short out a powered socket as you need two conducting prongs, whereas shorting out a powered plug is simply a matter of touching two of the prongs with something conductive (like a screwdriver, finger, etc).
So if you have the choice, go flat! For years in Queensland the small 7 pin round was the most common, but recently I bought a car and immediately had the round socket changed to a flat one, and the towbar place who fitted it said that flat was now more common – a change that had just occurred in the last few years.
Oh, and I’m a programmer, not an auto electrician, so please check first 🙂 Images are from Varta – a manufacturer of plugs, sockets and adapters sold in Australia:
7 Pin Flat
12 Pin Flat
This is an extension of the 7 pin flat. The 7 pin flat plug will fit into a 12 pin flat socket and work perfectly.
Small 7 Pin Round (QLD)
Identifying: Plug size is similar to an Australian 10c coin. There is a noticeably larger gap between 1 and 6 on this plug, though some trailer places rotate this connector that the yellow is at the top. Socket and plug are keyed.
Large 7 Pin Round (VIC)
Identifying: Plug size is similar to an Australian 20c coin. Pins evenly spaced. Socket and plug are keyed.
Heavy Duty 7 Pin Round
Identifying: Plug size is similar to an Australian 50c coin. Earth is bigger pin. No key (other than oversized earth pin) on the socket or plug.