EPDM Coatings
rvupgradestore.com Composet Products PO Box Zone
Over The Network Custom Yacht Interiors

Author Topic: Brake lights only work when the driving lights are off  (Read 168 times)

73Camper

  • ---
  • Posts: 37
Brake lights only work when the driving lights are off
« on: July 16, 2017, 05:42:33 AM »
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?
1973 Sports Coach Motorhome 28ft - Sold
2002 Fleetwood 22ft TT - Sold
2006 Outback 34ft TT - Sold
1973 Cavalier. TT Work in progress and loving it.
1999 F250 5.4l TV  4x4 286K miles
Awesome Wife and two wonderful girls

whokares2

  • ---
  • Posts: 89
Re: Brake lights only work when the driving lights are off
« Reply #1 on: July 16, 2017, 07:19:02 AM »
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.
1994 Fleetwood Southwind 30,000 mi.
2017 Jeep Patriot toad
Spencer and Lucky pooches.

Gary RV_Wizard

  • Forum Staff
  • ---
  • *
  • Posts: 60367
  • RVer Emeritus
Re: Brake lights only work when the driving lights are off
« Reply #2 on: July 16, 2017, 08:17:42 AM »
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.
Gary
--------------
Gary Brinck
Summers: Black Mountain, NC
Home: Ocala National Forest, FL

grashley

  • ---
  • Posts: 3548
  • Western KY for now.
Re: Brake lights only work when the driving lights are off
« Reply #3 on: July 16, 2017, 08:11:56 PM »
Oh I am suing the round 7 pin

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!
Preacher Gordon
09 Grand Junction 35 TMS - not yet received
2013 F350 Lariat LB SRW Supercab diesel 4X4
Nimrod Series 70 popup (sold)
It's not a dumb question if you do not know the answer.

 

Hosted by Over The Network