Travelling around Australia

The New World of Trailer Plugs

If you are like me, you probably grew up in a time where if it was to be done you would do it yourself (or have a crack at it), and use materials you had on hand or could easily source. If you needed help, you would ask a mate or perhaps grab a book or magazine.

Well… just over 12 months ago, I found that the trailer lights on my new second-hand pride and joy 6’x4′ box trailer were not working.

To put some perspective on this situation, I had just pulled gone to an out of town pub for a club dinner at around sunset and, as I do, I checked the trailer lights as I locked the car.

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The Lights Aren’t On and I’m a Long Way From Home – Trailer Plugs…

The lights on the trailer were dim and the indicators / tail lights failed to synchronise with the car lights.

My first thought, being the father of a twenty-something son, was that he had re-wired my precious trailer to suit his car’s wiring. It was a long-shot but it had to be exhausted. In short the answer was, “No”.

So, with my recently purchased new multimeter in hand, I checked the voltages at the trailer plug from the car. Unfortunately, the readings I was getting were low and erratic. I knew this was a bigger job. I gave my apologies for the dinner and headed back to a nearby town were I knew there was a petrol station and some much needed light to shine on the subject.

Jury-Rig to get the Trailer Home

An hour and a half later, I had exhausted my level of technical expertise on the problem and drove the hour home with only a bicycle strobing red light strapped to the back of the trailer – as they say, everything you carry should have more than one use.

After a lot of thought during a restless sleep, and knowing that I had not fiddled with the car wiring, I checked the trailer for any shorts or faults and, did what all blokes do in such times of crisis, checked the car manual. Yes, there was a fault in the trailer wiring that was caused by low hanging wires and being caught on a gutter or something.

In the manual, it identified trailer fuses under the bonnet but all seemed to be in order. Wait… what?… ‘trailer fuses’? What is this black art of which they speak.

Trailer Wiring Isn’t What it Used to Be in Trailer Plugs

Now, this is where it all starts to come together. In the old days, trailer plugs used to be wired from the wires that went directly to the back of the respective lights on the car – often using a crimp type connector and no fuses. The result was that you knew exactly where each wire came from, the connection was wrapped in masking tape that slowly fell away and it was all somewhat traceable. My wires disappeared into some sort of loom.

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I discussed my problems with my neighbour (an ex-RAAFie), and he introduced me to a new world of a black boxes and trailer harnesses that was seemingly introduced into cars some years back. However, with the fuses checking as ok and the wiring unchanged, I needed to find this black box and have a look myself. After removing my spare tyre from under the tub on the Mazda BT50, there it was! The ‘Waterproof Trailer Lamp Electronic Control Unit’ (ECU) and the associated wiring harness.

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See below for Photo description.

Photo 1 – The ECU in place up and under the spare tyre

Photo 2 – The ECU on the bench top

Photo 3 – The ECU itself unveiled. Nothing serviceable here.

The ECU, as I understand it, is intended to isolate the trailer from the car – ie faults in the trailer wont affect the fine electronic componentry of the tug vehicle and send false errors to the management system. Apparently the Isuzu Dmax does not have them and the VW Amarok handles trailer faults as part of the management system (please do you own checks here).

Of Course… The Old ECU Unit (apologies to Agent 86)

With everything looking ok and the mystery of the black box solved (but still no trailer lights), I waited until Monday and a chat with the experts at Ipswich Mazda and Arrow Towbars at Geebung, Brisbane. Both told me to check the in-line fuse between the battery and the ECU box – of course, I showed no surprise whatsoever when they mentioned the ECU.

I popped the bonnet and there it was, gone. The 20A fuse had indeed blown! Huzzah!!! I replaced the fuse and checked the trailer lights…. D’ho!! Still no lights.

Thursday morning was another day and an early trip up to the great people at Ipswich Mazda. Mathew took the car in and promised his undivided attention.

After a quick breakfast up the road, I got the call that they had found the problem.

Those Damn Do-It-Yourselfers

Now, I’m not pointing fingers or laying blame here but… after this lengthy trouble shooting activity, process of elimination and learning experience, it was found that someone had not put the plug into the ECU properly after they had pulled it apart – now, if I find that person….. oh, wait…

All is fixed and the car and trailer are now as one.

Know Your Rig

My reason for this Blog and the lengthy article is to drive the point home that trailer wiring from the car is no longer the land of do it ‘yourselfers’, connectors and masking tape. Understand what makes your trailer lights work and, more importantly, how your trailer brakes are powered. How is 12v power fed to your van/camper (via an Anderson Plug or the trailer plug)? Will a fault in the trailer affect your tug vehicle? Did you do your own wiring for your trailer plug? Do you know where your ECU fuse is and do you have a spare.

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