Need some electrical diagnostics help

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Thetrashnoob, May 14, 2021.

  1. Thetrashnoob

    Thetrashnoob Light Load Member

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    I could use some help as I’m stumped. I’m slowly going through the truck ( 03 Mack cl713 tri axle dump truck) to get it on the road hopefully within the next week and start hauling with it. So on my box, I’ve got 4 amber marker lights that are out, but 2 red one towards the rear that are working. I’m assuming they are all on the same marker light switch, but am not sure how to trace them to find the issue?
    Second is truck didn’t have a working backup alarm, so I get a new one connect it up and it’s not working, this may sound like a dumb question hut does the truck need to be on or key on power enough?
    The trucks wiring in the back is a mess tons of capped wires, I’m not really sure even how or where to start. I’m looking for a good way to attack this.
     
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  3. lester

    lester Road Train Member

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    If you want to do it yourself and learn more about your truck you just need to get a test light and start testing. For you clearance lights you need to figure out where the power is coming from and then find the break. Just work down the line. For the backup alarm would think key on would be sufficient. Again just have to start following wires. Avoid poking through insulation with test light. It leaves a hole and allows water and then corrosion. Although I've done it plenty
     
    skallagrime Thanks this.
  4. Thetrashnoob

    Thetrashnoob Light Load Member

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    I was thinking a test light. Only issue is the battery box is on the front of the truck. The box is 17.5ft long so I would need a very long tester. Not sure how else I could do it, do they make cordless testers?
     
  5. Mike250rs

    Mike250rs Medium Load Member

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    Regarding the backup alarm, I'm assuming you had ignition on and trans in reverse?

    The marker lights can be a combo of brake, turn and normal marker ones.

    If the wiring is truly crap, work your way up the harness from the rear to a connection that has not been botched and see if you can get the correct harness for everything downstream.
     
  6. Mike250rs

    Mike250rs Medium Load Member

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    the frame is ground, clip on to that or good metal connected to it
     
    lester Thanks this.
  7. Thetrashnoob

    Thetrashnoob Light Load Member

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    2C5F92D5-50AC-45C5-BA82-3F9F74102F75.jpeg
    yes key on and in revere, there’s 4 wires ( 2 sets) that are coming out of the junction box at the rear that I’m assuming is for the body, and couldn’t get the alarm working.

    The 2 marker lights on the rear side of the box come on when I turn on the headlights, I am guessing the others would come on as well, unless they are on the marker light switch, but I believe that’s just for the marker lights on the cab. The 2 round marker lights next to the strobe and the one near the door are the ones I cannot get working ( replaced with new ones) but the red one in the back circled comes on. If that makes sense
     
  8. shooter19802003

    shooter19802003 Road Train Member

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    Looks like a nice rig.

    Edit: if you don't mind me asking, how much did you give for that?
     
  9. lester

    lester Road Train Member

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    Test light doesn't need connected to battery. Clip goes to a ground, anything metal that isn't isolated by rubber or plastic. Screenshot_20210514-114043_Chrome.jpg
     
  10. Mike250rs

    Mike250rs Medium Load Member

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    It sounds like you have wires grounding out somewhere.

    This is where being methodical and patient comes in. Do no be in a hurry or you can miss 1 wonky wire and never figure it out.

    The last truck I rewired, I started with a wire cluster from a nonfunctional light (no power from test light) and unwrapped wire looms, years worth of zip ties and tape. Check for rubbed areas with other wires and frame, corroded spots in wires or connectors. Don't forget to check ground wires during this process. Bad grounds are the majority of wiring failures in vehicles.

    Once you have inspected a section, mark it as checked with something. I used blue painters tape because it was easier to see while schlepping around under the truck. Take your time. Doing it yourself will save thousands at a dealership.

    A straight up marker light will only have power & ground wire. Turn/brake lights will have 3 wires. Knowing what circuit is bad can save you time in chasing wires. Blown fuses/relays/flashers will result in dead circuits.
     
  11. Thetrashnoob

    Thetrashnoob Light Load Member

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    I don’t know much about electrical, but I trying to learn, but at the same time I could spend 40 hours looking at it and chasing wires and never get anywhere where as someone who knows what they are doing mah be able to do it a whole lot faster. anyways.. this is what I’ve figured out so far,
    When I turn the lights on in the truck the headlights, brake lights come on. As well as the Lights on the box closest to the rear ( the check mark in the photo) the rest that have the x do not come on. I’m assuming that a wire or something runs from the light switch all the way back to the junction box and then the brake lights, and then signals as well as the marker lights for the box come from that. The rear most lights are working so I’d say it’s getting power but something between the lights and the front Lights have an issue, I’m off to get a test light and probe around it tomorrow and see what I can find
     
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