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.
Need some electrical diagnostics help
Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Thetrashnoob, May 14, 2021.
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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. -
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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. -
lester Thanks this.
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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 -
Edit: if you don't mind me asking, how much did you give for that? -
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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. -
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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