You can jumper Pin 4 across to Pin 3 on your trailer side plug. Your coach battery will turn on the Running lights on your trailer. If this works then you know it is an issue from the truck running light plug. If they do not turn on, then you need to go to the rear of the trailer and check to see if you have running lights at the tail lights.
In some applications, the MFG will run the solid brown wire to the trailer tail lights, the side marker lights will be Tee Tapped off of the brown wire, but daisy chained together in parallel.
If your tail lights are on, but the side markers are off, then it is the connection point at where the side markers attach to the brown wire.
If the tails and Side markers are out then do the following.
Remove the jumper for testing I mentioned above.
Using a Digital Multi Meter, put on DC volt.
Place the red lead into pin 4 (battery voltage) and black lead on pin 3 (tail lights), Do you see voltage across these two pins?
Yes, ground is good and voltage is flowing, check bulbs and grounds at each point.
No,= Ground or power not working, Confirm as follows.
Remove the black lead and touch to Pin 1 (ground) or touch non painted surface on trailer hitch or trailer frame. Do you have power?
Yes, Ok, battery is OK and so is power to plug.
Verify if Tail Light Harness is good or has broken connection.
Remove red and black lead from trailer side of harness.
Place meter on continuity
Put Red lead into Pin 3 of trailer side of harness.
Touch Black lead to either Pin 1 (Ground) or to trailer frame. (must be paint free area).
Does meter BEEP? Or indicate that there is continuity to ground?
Yes, connections from plug through entire harness may be OK, unless shorted to ground. Must remove all bulbs and retest.
With all bulbs removed, does it indicate continuity to ground? (Beep)
Yes, =Bad, means short of power feed to ground. Wiring or socket connectors need inspected.
No= Good, means no short to ground, install bulbs
Re-check, with all bulbs back in, is there continuity to ground?
Yes, ok Wire harness and bulbs are all good,
Issue is on truck side.
On truck side, turn on running lights, does truck lights work?
Yes,= test truck side connector. Using DVM or harness tester, go from pin 3 truck side harness with red lead and black lead on either Pin 1 or touch metal on truck frame, bumper or receiver bare metal spot.
Does it indicate power?
Yes, hmmm, this will get complicated,
Must now connect trailer harness and remove one of the side marker light covers then check for power. If lights are not on then there is no power obviously, but might be loss of ground between trailer Pin 1 and truck Pin 1.
Remove bulb from side marker light or tail light, put red dvm lead on pin at bulb socket that is the power side of the pin, and black lead to chassis ground or trailer frame, you could use a long piece of speaker wire or such, to wrap around the black lead and touch the truck receiver, is 12vdc showing?
If yes, then it is a flaw in the ground between the truck and trailer.
FYI, you could also, test, as long as ONLY the trailer harness is connected, on continuity, go from coach battery Ground, with red lead, and touch the black lead to the trucks receiver hitch and see if you have continuity from the battery, through the harness, to the truck frame. If yes, then ground is OK between truck and trailer.
If you find you don't have power at the trucks side of Pin 3 then you need to look in your owners manual to see if you have a separate tail light fuse, FYI, many newer vehicles do have a separate trailer harness fuse.
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