I plugged in the lights from my boat trailer to the truck for the first time this year. No lights. I used steel wool and cleaned both the male and female connectors the best I could. The running lights then worked. I then stepped on the brake and all the lights on the trailer went out. I let off the brake and the lights went back on. Any suggestions for repair would be greatly appreciated.
CHECK BULBS..
pull bulbs even if they are good they can corode and short across the contacts!~ pull out clean spray with wd 40 put back in if they are good.. check working condition..
check your ground.. are you grounding threw the ball of the truck or is there a ground wire wired to the trailor?
check fuses..
Weird behavior like that is usually the “ground”. Make sure the white wire from the connector is bolted to clean shiny metal on the trailer frame, and that both light assemblies are bolted down to clean metal too. If your trailer frame is made of bolted together sections, tighten all those bolts.
And if you ever decide to pull new wire in that trailer, run a ground wire from the connector back to the lights. That one step will eliminate a lot of strange trailer light problems in the future.
Rootski
Thank you for the responses, especially to TheguN. I replaced both bulbs, again cleaned the connectors with steel wool, and also sprayed the bulb contacts and the wiring connectors with a spray to remove corrosion. Viola! All lights lit up, including the brake lights. Thanks again.
Quote:
CHECK BULBS..
pull bulbs even if they are good they can corode and short across the contacts!~ pull out clean spray with wd 40 put back in if they are good.. check working condition..
check your ground.. are you grounding threw the ball of the truck or is there a ground wire wired to the trailor?
check fuses..
First, check the fuses. I just ran through this whole scenario. 9 times out of 10 it’s the ground. You should not have to rely on the ball hitch for a ground. We had to take the adapter that your round 4 way plug goes into, apart. First, you need to twist it off from under the rear bumper, then take it apart. I’ll bet the ground in there is green & corroded. But do make sure you don’t have any blown fuses. Hope this helps………. 
glad i could help.. thefact your tail lights worked and then went out when you press brake was a dead giveaway to the problem..
hope you have a good season buddy! 
That phenominom is frequent on trailer ball / hitches that have not been used in a while. the light film of corrosion inhibits the ground. usually hooking up and a drive down the road taking a few turns cures the problem.
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