How to Fault-Find Trailer Electrics
The first thing to do is a visual check to see that the connector pins are clean and not damaged.Next I dismounted the plastic socket and checked that the wires were all connected firmly and that the colour codes followed the diagram below.
Wiring Diagram Trailer Connector
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| Wiring diagram |
All the wires were firmly connected and the colour coding was correct so the next thing to do was check that the pins were connected correctly using a volt meter.
First I checked that the earth pin had continuity with the earth strap of the battery.
Then I checked that there was continuity between each pin of the trailer connector and the live connector of the corresponding bulb holder on the van.
All the connectors checked out OK for continuity, so the next step was to check for shorts.
To do this you have to remove the bulbs, otherwise all the pins will register a short circuit to earth.
The next thing to check is that each pin gets 12V when it should be lit (see wiring diagram above).
When I checked this I found that one of the rear bulbs didn't get 12V, odd seeing how there was continuity and there didn't seem to be any short circuits.
When you find something odd like this, a bulb which doesn't get voltage but which seems to be connected correctly. The chances are that it is because of a bad earth connection. So I decided to add a second wire to connect the earth pin of the connector to the chassis of the van (see photo)
Once this was done all pins checked out correctly with the volt meter, so I took the van back to be retested and "hey presto" it passed the MOT!
