Brake lights only work when the driving lights are off

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73Camper

73Camper

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Joined
Apr 29, 2017
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37
Hooked up my tt for the first Time. Only one break light works and will not work at all with the driving lights on. I rewired the trailer and have the same issue. Looks like it's the truck I have checked all the wires on the truck. What am I missing? Please tell me this is common. Thanks for the help. Oh I am suing the round 7 pin

I have checked on voltage on the connector and everything looks good. I have even tried it using the 4 flat pic instead of the 7 pin. Has anyone had this issue?
 
Sounds like a ground problem.  Hook a jumper wire from a good ground on your tow vehicle and run it to your TT.  I just use jumper cables for testing.  Usually will give you quick diagnosis if you have a weak ground.
 
Yeah, bad or missing ground is the likely answer. Exactly where can be more difficult.  It could be the system ground through the trailer plug to the tow vehicle, or it could be a bad light socket or a crimped wire shorting back through the trailer frame, essentially nullifying the ground.

Start with the procedure that whokares2 suggested.
 
73Camper said:
Oh I am suing the round 7 pin
Click to expand...
Just curious  ::) Why are you suing the round 7 pin?  8)    (using, I know!)

It clearly sounds like a ground issue. Whokares has the best first step.  If you have a spare 12V socket and bulb, tin the wires to stiffen them up, then test the 7 pin and 4 pin connectors on the truck.  If everything works as expected, that fairly well eliminates the truck.

Using a 12V battery or charger, test the TT side of the harness, one circuit at a time, then in combination.  Hopefully this will eliminate one circuit and help focus the search.  The 3 circuits I refer to are tail / running lights,  Left turn / brake,  Right turn / brake.

I have seen cases where the running lights were fine, bit the left turn / brake had a faulty ground, so when they were BOTH in use, the current would go from turn filament to running filament to ground, and both were dim.

Let us know how it goes!
 
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