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I have a tow bar and a seven pin plug to operate the lights, fitted on my car. I recently took my boat for repair and when they brought it out of the workshop they dropped it from a forklift truck several inches onto the tow bar. When I plugged the trailer lights in nothing worked. I checked the socket on the back of the car and it was completely dead. On the way home I called on a friend and plugged the boat trailer into his car and everything worked fine. So I knew it was a problem with the car.

A few days later I fitted a new socket on the back of the car, tested it with a multimeter and everything was showing 12 volts when switched on. When I plugged my caravan in none of the lights worked on the caravan but everything worked on the car. I unplugged the caravan from the car and tested the socket on the car and it was completely dead. After a few minutes without doing anything but scratch my head I tested it again with the multimeter and found power had been restored. So I plugged the caravan back in and none of the lights on it worked. So unplugged it tested the socket on the car... dead. A few minutes later power was restored.

Can anybody help me with this situation?

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    Usually, when I see issues like you are describing, it comes from the ground (earth). I'd start there to ensure you have a good connection throughout. – Pᴀᴜʟsᴛᴇʀ2 Jul 16 '16 at 16:08
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The first thing to check (and think about when using a meter) is where the ground is coming from:

  • Is there a ground wire in the trailer wiring?
  • Is there a good solid connection of the ground wire to the frame of the trailer (or to the ground side of the circuits on the trailer)?
  • Is there a good solid connection between the ground conductor on the vehicle side and the vehicle ground (often the body of the vehicle)?

From what you've described so far, it seems like the problem:

  • Is on the vehicle side (the problem follows the vehicle, not the trailer).
  • The trailer plug and/or wiring on the car side sounds like it may have been damaged by the way the boat trailer was handled.
  • The problem may be intermittent.

Ground problems can be intermittent when there are loose connections or multiple return paths. As you troubleshoot keep that in mind. So you should:

  • Test both the "official" ground (the circuit through the plug) and also any alternative circuits you can think of. One will exist through the ball of the hitch. That path is notoriously unreliable due to rust, grease, and the movement of hitch on the ball.
  • Keep in mind that you may have added a test case (caravan vs. boat trailer) and there may be some new unknowns.
  • If need be you could run a wire (for testing and maybe permanently) back to the battery from the trailer connector. That will eliminate possible ground issues on the car.
  • Dropping the boat trailer onto the hitch suggests mechanical damage – maybe pinched wires. Look for any evidence of that.

One other possibility here is the mysterious loss and then return of the power on the new connector. Some vehicles use self-resetting circuit breakers instead of fuses on critical circuits (the ones that I know of are on the headlights, but it seems like the running lights and signal lights would also be candidates for circuit breakers. So keep an open mind for the possibility of a short as well.

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