Troubleshooting 4-Pole Round Trailer Connector Running Light Circuit not Working on Smart For Two

Updated 06/10/2011 | Published 06/09/2011

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Question:

We have the 4 pole system with a round plug on each end wired as shown on many schematics and it all works except the running parking or headlights. This is the brown wire. The RV is a 35 ft Winnebago and the toad is a Smart For Two made by Diamler AG and built in France. Do we need an isolation diode? Thank you for your kind help,

asked by: Walter &

Expert Reply:

The problem with the 4-Way round connector is that there is no standard for how they are wired. The first thing I would check is to make sure you are getting a running light signal on the RV side connector using a circuit tester like # PTW2993. If you are, make sure the matching pin on the vehicle side connector is for the running lights and re-pin if needed.

Depending on how your Smart For Two is wired you may need additional diodes or specific wiring in order to get all of the functions to work properly. I have included a link to our series of articles on flat towing, several of which cover wiring a vehicle for lighting functions.

expert reply by: Michael H