littlethorn

rsktm300xcw you were right. I tried disconnecting 10 running lights and the rest lit up. I plugged in one at a time and they lite until I had 4 left then they all went out, unplugged different lights will work with any 4 unplugged so I ordered some led's to substitute incandescents hope this will work.

my problem is different apparently in new gmc's too little draw from led lights or too much from incandescent causes truck computer to not send power to tail light connection. my trailer has about 20 running lights, I disconnected about 10 connected truck and the rest lit up, so I plugged in one at a time and they lit until I got to 16 then they all went off. tried switching different lights in case it was I short but would light at 16 but no more no matter where they were. ordered some led's to substitute about 10 incandescents, hoping they draw soo little it will work. hope they will work together.

It crossed my mine that I might have too much draw because lights flash on for a sec. like the truck senses the draw and shuts off power. maybe I'll try disconnecting some lights. If that works I'll bring to dealer for module reprogram. thanks for all the help.

It crossed my mind that I may have too much draw on my running lights circuit because of the I sec flash then they go out, like the truck is sensing a large draw and shutting down. I have a lot of lights 19 small lights plus 2 tail lights. most forums talk about too little draw because of led's but maybe I have too much. Maybe I'll try disconnecting some lights if that works I'll go to dealer for reprogram. maybe I can mix led's although I think its not recommended. thanks for all the help.

I have the same problem with my 2019 gmc slt all trailer lights work except running lights. I tried trailer with daughters ford they worked fine. no power at running light pin. went to dealer they said trailer is not compatible with truck said it was a pulse width modulated system in truck and I needed a 55ma draw from trailer lights to make it work and need a load resister on lights. I think load resister is needed if you have led lights since they don't draw enough. bought a curt plug # 57003 adapter which has resisters built in, it didn't work tried truck on friends trailer, running lights worked fine, had truck tested at trailer place he said it was fine. Tried connecting an old headlight to running light pins and it light it up. also have a trouble call on truck trailer screen. here is the strange part when trailer is connected and I turn lights off then on running lights go on for I sec then off, every time I turn off then on same isec blink of lights. Wondering if I have too many lights on that line and the heavy draw kicks lights off since they are not led.