I'm having a problem with my trailer light wiring on my 86 truck. I've installed a new connector 4-way and a new converter but still not fixed. Ok all the lights on the truck work but with a trailer hooked up only the running lights , pass side brake and pass turn signal work.I tested the voltage at the connector with the trailer unhooked and I have 10-11v at the pass side wire but I only have 6-7v at the driver side wire but when I unhook the driver turn signal wire before the trailer converter I have 10-11v at the pass and driver side wire and both brake lights work on the trailer. I think I have a problem in the driver turn signal circuit. Is this caused by high resistance? So far I've unhooked both rear taillight housings to rule them out but it did not help.

 

I would double check your ground to your trailer connector...trailer will ground through the hitch as well...but sometimes will be weak due to grease, rust, etc.

 

I have never used a 4 pin and this converter so maybe the problem is in your converter.

Just how did you make the connections when putting in the new parts I have seen plugs corroded will cause problems

I just use a 7 pin plug or what the customer wants and never have many problems on the truck side.

getting trailer lights to work that have been sitting can be fun.

Double check your ground and make sure it is clean. If needed replace it with all new wire and lug

 

I have already ruled out the trailer and my trailer ground is good. Everything is connected with heatshrink terminals.

 

i re-read your description...I think your converter box is bad. if you have good voltage before the box, but bad afterwards, this only leaves one thing...