In my '99, all the wiring was continuous runs from one fixture to the next. All the connections were at the pigtails for the various lights, and were done with wire nuts that were then dabbed with silicone caulk.
I did some rewiring in '10 and '11, and all the connections were OK, but darkened with oxidation. If lights are not working, checking these connections after verifying that bulbs and sockets are good may find the problem.
The front marker lights were wired separately from the rears on my '99. The wiring went from the end of the 7-way pigtail on the floor straight up the front and daisy-chained from the street side low marker up and over the front and down to the curb side marker.
Bad ground is a major cause of problems with trailer lighting in general. Start with the bulb and work backward checking to see if you have solved the problem as you complete each step:
- Test the bulb first! Or swap out the bulb for a known good one.
- Clean all contacts and coat lightly with dielectric grease.
- In the tail and marker light fixtures, make double sure that all contacts are contacting properly.
- Then pull the fixture and check the pigtail connections to the trailer wiring. If they are suspect, remove the wire nuts, undo the wires, sand them lightly to brighten contact points. Coat with dielectric grease and reassemble using new wire nuts, preferably those that have threaded metal inserts in them. When tight, apply a dab of silicone to keep moisture out.
- Find the ground points. There should be one or more under the trailer, generally along the street side. They usually use standard grounding lugs screwed to the frame, similar to this:
- Grounding points are subject to corrosion and vibration. They are inexpensive and readily available in the electric aisle of any hardware store or big box home improvement store. If it looks bad, it probably is. Replace using a new screw. Sand the frame where the lug will go, first. Make sure the exposed wire that is going into the lug is bright and shiny (sandpaper is great for this) and then coat all wiring and the lug and frame point with dielectric grease. Tighten everything securely.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
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