Does the turn signal on the tow vehicle function properly?
If it does, the problem is in the trailer wring. Check the connection from the turn signal on the tow vehicle to the turn signal wire on the trailer. Make sure that hasn't become loosened. Check the ground from the trailer to the tow vehicle frame. Don't rely on the coupler ball to provide a proper ground,
If the turn signal on the tow vehicle isn't working right, either, replace the flasher unit and see if that helps. If it doesn't, you may have a problem with the turn signal switch.
What kind of vehicle, My Titan came from the factory with the harness in the back and the salesman told me all I had to do was buy the plug to plug into there harness to enable me to hook up a seven pin. Well that was B.S. I had to buy tow relays to get the lights to work properly, my lights were acting wierd like you describe before I installed the relays. All I had to do with the relays was to plug them into the fuse box under the hood in there designated spots ( tow 1 tow 2) and all was well. Or you might need a converter - For use with vehicles that have separate turn and brake light wires. Some vehicles send only one signal per wire, creating what is called a 3-wire system: one wire for the left turn, one wire for the right turn, and one wire for the brake signal (common on vehicles with amber turn signals). A converter will reduce it to a standard 2-wire system needed for wiring a trailer. There are still additional wires for the running lights and for the ground. Any vehicle with amber turn signals will need a converter. However, there are some vehicles with all-red tail lights that can also require a converter. A wiring harness with a converter has a black box built in it. Five wires go into the box, and only 4 come out. The converter transfers the brake signal on the vehicle into the left and right turn signals for the trailer wiring system. check this site, alot a info. Or was everything working at one point and now it don't? If so do as previous posts suggested. http://www.etrailer.com/faq_wiring.aspx
Many directional signal systems do not flash when bulbs are burned out or are the wrong wattage.
If your trailer directionals draw more than a certain amount of current, connecting them to even a "compatible" harness can cause the car's (and the trailer's) directionals to not flash. Some bulb may be steady on and others may stay off.
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