Troubleshooting Trailer Lights Not Working with Wiring on 2005 Ram 3500 with Factory Tow Package

Question:

I have a Dodge Ram 3500 with factory tow package, the lights on my trailer do not work when I plug in the 7 way connector from the trailer to the truck. I have a spare set of magnetic tow lights that I plugged into the truck via a separate 4 way connector and they work. If I understand it correctly my truck has separate bulbs for turn, stop and park and the trailer has common bulbs. Therefore I need a converter for the trailer lights to work. The trailer lights have three wires ground and two feeds the truck has 5 wires ground and individual feeds for park, stop, turn and backup What do I need to get to make this work? Thanks

asked by: Steve B

Helpful Expert Reply:

It sounds like your truck has a combination factory 4-pole/7-way socket such as # 37185. If this is not the case please let me know what type of wiring is on the Ram. If your 4-pole magnetic lights work when connected to a separate aftermarket harness that plugs in at the taillights this would still work even if the tow package fuses were missing.

In both 4-pole and 7-way trailer wiring setups the stop light and turn light signals are combined. The green wire typically carries right-side stop and turn signals and the yellow wire does the same for the left side. Trailer wiring is usually set up this way too. Please refer to the linked article on trailer wiring that shows the functions of various connector types.

There are a few things to check. First, make sure any fuse or relay positions in the power distribution box that are labelled "trailer" or "tow" have the appropriate value fuse or relay in place.
Next you can test the truck's trailer connector with a circuit tester like # PTW2993. Use of the tester is demonstrated in the linked video.

Have a helper activate each lighting function (by applying the brake pedal, turn signals and headlights, one at a time) while you apply the tester to the appropriate pins on the 7-way. Again you can refer to the linked article and photo for help with which pins to test for which function.

If your fuses and relays are all in place but you do not detect the light signals when you test you can inspect both the vehicle socket and the trailer plug for any signs of corrosion or discoloration on the contacts. Sometimes corrosion will build-up inside a connector and be difficult to see from the outside. Also check for bent contacts, trapped dirt or moisture or anything that would interfere with signal transmission.