Brake fuse blows when trailer attached

  • 1 POST
  • 4.8L
  • V8
  • 2WD
  • 130,000 MILES

Brake fuse blows (so I have no brake lights and gear selector is locked in park) intermittently (maybe not until 2nd or 3rd frip0) when I have ANY trailer attached so problem is not with trailer wiring. I have replaced the 5 wire to 4 wire module and connector with no positive results. I figure there must be short activated with extra load. How do I find it? Thanks David

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Tuesday, July 21st, 2015 AT 2:49 PM

1 Reply

  • 29,775 POSTS

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

In this case, if you're suspecting there's just too many lights on the circuit, you will see the test bulb get dimmer by just a little each time you remove one trailer bulb. If you remove all the brake light bulbs from the trailer and the fuse still blows, the best suspect is mismatched wiring. If your car uses two separate rear bulbs on each side, one for brake and tail light and the other for turn signal and tail light, you need an adapter, which might be the module you're referring to. If it comes to diagnosing that, you'll have to provide me with more details including wire colors and whether this is a standard Ford part or an aftermarket module.

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Tuesday, July 21st, 2015 AT 6:57 PM

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