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Thread: Ground to Tail Lights

  1. #1
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    Ground to Tail Lights

    Hi all.. Ok, how many grounds are there to make the tail lights work... and where does the ground plug in at. My tail lights are working but are very dim at times, turns work good until I push on the brakes, parking lights don't work, back up lights work until I push the brakes.. then dim... Plate lights work good all the time... am I missing something here?? some people are saying it's a bad ground... I lifted the bed off the truck, replaced the fuel pump.. then placed the bed back on the truck.. yesterday I took the bed back off and double checked my connections and they were good.. no wires pinched... put the bed back on and same problem... I took the gound wire off that is connected to the frame to the gas neck.. sanded the frame and replaced... same problem... Not sure what else it could be at this point... anyone have any suggestions??? Thanks
    David

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    Senior Member chengny's Avatar
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    Did you put rubber shims between the bed and frame. If so you may be losing continuity back to the battery (via the engine ground strap). There are several places where the ground wires leave the harness and are bolted to the frame. The frame is in turn connected back to the negative post on the battery by flexible straps, the mounting fasteners and finally through the black cable that attaches to the battery.

    If only the rear end is acting up (and you just removed the bed recently) remove any rubber and paint between the bed sheet metal and the frame. Or run an independent ground strap from the bed to the frame.

    The ground wire that is led to the gas fill pipe is not the same animal as the wiring on the negative side of your electrical system. It is only there to dissapate the build up of static electricity during fill-up. It bleeds that charge off into the metal mass of the frame so no sparks occur at the gas nozzle/fill neck interface and blow you up.
    1986 Sierra Classic
    Lost it in a fire

    Replaced with 1986 Sierra K3500 single axle

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    Ok.. one question, when I fastened the bed of the truck back on with the 8 bolts, wouldn't that ground the bed to the frame? When I removed the bed, I did see rubber cushions on the frame... But I left those in place.. also, when I turn my parking lights on, and I probe with the test light the sockets at the tail lights, I get power through all my sockets, both sides of the socket. Meaning that in each socket, I get power on each prong so there is no ground there to complete the circuit.. I will try to ground the bed to the frame and see what happens there.. It's got to be a ground problem.. There's only so much it can be...

    David

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    I was thinking that when I check the plug connected to the wiring harness running through the frame, I turn my parking lights on and I get power ONLY through the brown wire.. but when I check it through at the light sockets, I get power through all the sockets at the tail lights. Does that make sense to you? Maybe the plug is bad that connects the lights to the wiring harness?

  5. #5
    Senior Member chengny's Avatar
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    What year and model is your truck?
    1986 Sierra Classic
    Lost it in a fire

    Replaced with 1986 Sierra K3500 single axle

  6. #6
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    It's a "94 1500 chevy" from, I believe, Tail Light troubles...

    If I understood correctly, you get the correct result when harness unplugged.

    Turning on park lights gives hot from brown on front harness to frame.
    When harness connected, hot shows up at all bulb contacts, to frame.

    This would mean ground is "open" - no good to frame.
    It would also mean the the "open" is AFTER the common splice of grounds.

    In other words the grounds are good at bulbs, and good to splice,
    but open from splice to frame. that's a very small target area.
    Uncover/unwrap that splice and you may have the frame side wire fall off.
    If it doesn't, that's the first place to check voltage in this situation.

    My opinion comes from this: any time a circuit is not completed,
    voltage will appear to pass through what IS connected as if only wire.
    (It's something of a head fake, trying to use it will fail to have useable power).
    The correct brown wire voltage is seen correctly at "hot" for park bulbs.
    Since there is an open in the ground wire, it is also seen at "ground" side of bulb.
    Since that "ground" side of bulb is spliced to all the others (like brake bulbs)
    the "hot" then shows up at brake bulb "ground" contact. Snowball continues...
    Since the brake bulb is not in use with it's own hot, the "hot" side of brake bulb
    is also "hanging" or "open" and will show the original brown hot voltage.
    If you were to then ground the hot side of brake bulb with a jumper,
    then the brown wire would try to light the park bulb in series with the brake bulb,
    and the result would be dim bulbs at best, possibly blown fuse park fuse.

    You may wish to test that, just be sure that you don't activate the switches
    for brake or turn or hazards. That would cause the combo switch and flashers
    to get a super load from short and may cause expensive replacements.
    A safer way to test would be to unplug the harness and jump just the brown wire
    from front to rear plug, leaving the other three unconnected and harmless.
    Then ground the rear plug wire for one side brake (use yellow OR dkGrn).
    This will give you dim from brake bulb and park bulb on that side.

    Course all that testing is unneeded, I just like to give a way to "see" what's happening.
    Best bet is open the splice and go from there.
    Likely going to have to sooner or later.

    Hope all that made some sort of sense.

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    So what you're saying is because I'm getting power to all my contacts in each light socket, the problem will be in the harness that connects the tail lights.. that runs under the rear bumper.. ?? Is this where I search for the splice?? From the connection to the bulbs? I will also check the parking light fuse again..

    If I understood correctly, you get the correct result when harness unplugged. yes

    Turning on park lights gives hot from brown on front harness to frame. Yes
    When harness connected, hot shows up at all bulb contacts, to frame. Yes all bulb contacts..
    Last edited by Monte CarloSS; 03-10-2010 at 05:41 AM.

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    yes, it is a 1994 Chevy 1500..

  9. #9
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    "Harness" may be confusing this.

    By harness I was referring to the 4-wire that comes from front of truck.
    Brown, yellow, LtGreen, DkGreen.

    Looked at from the tail lights end, you have the black wire for ground also.
    All the grounds for bulbs are tying together making a splice.
    That splice should then be connected to frame.

    I'm saying that the blacks from each bulb are all getting to the splice
    and therefore showing correctly they are connected to each other.
    However, the connection from splice to frame is not good,
    although it may appear to be from just looking.
    Could be corrosion or broke inside insulation.
    This is all about black wires, not the 4 from the front.

    Follow the black wire from any brake or tail lamp to find the splice.
    Pay particular attention to any trailer ground wire that may be "tapped" in.

    You do not need to check park fuse while hot still on brown,
    that was about if during the testing idea, the load from "extra bulbs"
    was too much for it. So far it seems like it is fine.

  10. #10
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    Ok... I'll try that tonight.. I understand what you are saying... I'll look through the wiring from tail light to tail light and check all the ground wires to frame there... That's the place I was really wondering about... I'll let you know what I find out...

    Thanks.. you've been a big help here... David

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