Using a 2-Wire Trailer Tail Light on a Trailer with Separate Brake and Turn Signals

Question:

You state that I can use the STL90RB for tail, stop and turn.. I bought two of these and am having problems.. My first request for help states the problems Im having.. Basically, the turn indicator function makes ALL my lights flash: both tail light assemblies, clearance lights on the trailer, both tail lights on my truck and the front lights on the truck.. I have four wires at each tail light position on my trailer= white for ground, black for tail light, red for stop light and green for turn indicator.. Since the STL90RB has only three wires I connected: white to white, black to black, and the two remaining red and green to your red wire.. The lights worked fine on the truck equipted with a 7pin round connector and the Scamp fiberglass trailer.. I installed your led clearance lights recently and am quite pleased with the way they illuminate but Im completely frustrated with the tail lights Ive purchased.. HELP PLEASE!!

asked by: Kenneth M

Expert Reply:

Since you have indicated that you do have separate turn signal and brake light wires on your Scamp trailer, connecting both of those separate wires to the same circuit on the light # STL90RB is probably what is causing all of the problems including back feeding to the truck.

What you will need to do is use a special converter on the trailer side, # RM-732. The separate brake and turn signals will go to the input side of this converter which will then combine them for the output side. Then you can connect the green wire for combined right turn and right brake light, and the yellow for combined left turn and left brake light. The running light wire will bypass this converter.

You will need to install it behind the trailer side connector. The left turn only wire will connect to the yellow wire on the converter. Right turn only goes to green. Brake light only goes to red. White is grounded to the trailer frame. The running light wire is not affected.

It is possible that somewhere on the trailer it as converted the signals from the seven to separate out the turn and brake signals since the trailer seems to have been designed this way. You can determine this by using a circuit tester like # PTW2979 directly behind the trailer side connector. If the wiring at the trailer connector has combined turn and brake signals then you know that somewhere back from there the signals are being converted out.

If this is the case all you would need to do is bypass the converter, where ever it may be, by routing new wires from the 7-Way to the trailer lights.

You want to also make sure there is no corrosion in either 7-Way trailer connector pins or in the backs where the wiring connects. You also want to make sure EVERYTHING is grounded to clean and corrosion free bare metal surfaces.