Trailer right and left blinkers and break lights not working. changed 2 bad relays in fuse box and they still do not work. problem is not trailer because we hooked tester to both trailer light plugs on rear of truck. running lights work just no blinkers and break lights. Truck has only 54000 miles and is garaged and all wiring looks good on truck. I did tow old trailer with bad wiring about a month ago. seems like would have maybe another fuse associated circuit but I can't seem to figure this out. Please any suggestions for trouble shooting?

 

the first thing to do is verify you have a good ground from the trailer to the truck... Normally using jumper cable from trailer hitch to truck frame ( CLEAN) is a good test.. Have also used a 25 ft extension cord and connect trailer frame to truck battery with small clamps....... Sometimes the running light wlll work by grounding thru the brake / signal wire back to the truck...

 

Want to explain what "changed 2 bad relays in fuse box and they still do not work." means?

Did you verify that fuse F73 in the BJB was actually passing power?

Do the two trailer turn/stop relays in the BJB "click" when the brakes or 4-ways are activated (or individually when a turn signal is activated?

Do the truck's rear stop/turn signal lamps work properly?

 

Use a strong flashlight to examine the fuses for a blown one, some are hard to see and some are not.