1996 GMC C3500 Uses Headlight Switch for Trailer Marker Lights

Discussion in 'Etrailer.com Trailers and Towing Forum' started by xenon, Mar 9, 2018.

  1. xenon

    xenon New Member

    Mar 9, 2018
    The trailer marker lights on my 28' horse trailer are controlled by the headlight switch. The switch gets hot and over time burns out (there are about 20 lights of various configurations on the trailer). The marker lights are also dim as the feed from the switch is only about 8 volts.
    How do I go about fixing this? I don't want to wire the trailer markers to a separate switch on the dash as I know for certain I will forget to turn them on.
  2. 1380ken

    1380ken Well-Known Member

    Nov 7, 2013
    You could add a relay that is controlled by the headlight switch.
    Fbird likes this.
  3. Snow

    Snow Well-Known Member

    Jul 19, 2007
    Sounds like whom ever wired your vehicle plug, skipped using a relay on the power feed..
  4. f5moab

    f5moab Retired from the Federal Government

    May 7, 2013
    Somewhere in Idaho
    Switch to LEDs in the trailer?
    My 2016 GMC also uses the headlamp switch to turn on the trailer lights. I don't have as many as you do.
    If you don't want to install LEDs, then a relay would be the best alterenative.
  5. xenon

    xenon New Member

    Mar 9, 2018
    Thanks for those responses.
    Would changing to a heavy duty headlight harness such as the one LMC sells solve this problem?
    It has a relay for each headlight and apparently takes power directly from the battery.
  6. SteveP

    SteveP Well-Known Member

    May 21, 2015
    Due to your extreme voltage drop I would guess that the trailer connector on your truck, and possibly the one on your trailer is very corroded. Also possible that you have some corroded wire on one or both sides of the connection. Another place to look is where the grounds from the connectors connect to the frames of the TV and trailer. Take a look at the wiring, replace the connectors with new ones, clean up the ground connections and see if that helps.
    jnc likes this.
  7. mstrbill

    mstrbill Active Member

    Mar 17, 2013
    Austin , Texas
    No, you need to add relay(s) connected to the battery and to the trailer socket. My Suburban has relays for running lights and each turn signal. It doesn't have a relay on the 12 VDC aux line for some reason. My former F150 had one for that.
  8. f5moab

    f5moab Retired from the Federal Government

    May 7, 2013
    Somewhere in Idaho
    Need to check the wiring first to determine if a relay is needed. And per the OP it appears relays are there. On more recent GM vehicles the headlamps switch is just a switch to notify the BCM to turn on the lights via relays on older trucks....not sure. My suggestion would be to go to a GM truck forum, such as SilveradoSierra.com. join and ask the question. My set of wiring diagrams do not go back as far as 1996.

    As for the B+, chances are there is no draw on that circuit for a horse trailer for the running lights so why a relay. I travel with a 12V fridge on all the time and my GMC has a direct 12V circuit from the battery to the trailer through a 40A fuse but the switch is not even in that circuit so why a relay?
  9. jnc

    jnc Welcome from New Hampshire

    I'm with Steve on this one.

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