Duster fusible link wiring problem

Discussion in 'Electrical and Ignition' started by Dakota Smith, Sep 14, 2018.

  1. Dakota Smith

    Dakota Smith 1976 340 Feather Duster

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    867DA27A-23BB-4E95-AA40-6AE0668E24D8.jpeg My duster isn’t starting now for some reason. There’s some pretty shotty wiring under the hood that I plan on replacing in the near future, but for right now there’s one wire that I figured out is causing a problem. It says “fusible link” on the wire and it’s two bare wires that are wrapped around each other. The previous owner had it like this. Whenever I put the positive terminal cable on the battery it makes an arcing noise and the wire that says fusible link will get hot and smoke. Should I solder the two wires together? The car ran with them like that for months, but now it’s acting like they are hooked up wrong. I did have electrical tape over the wires, but it melted off.
     
  2. AJ/FormS

    AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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    Before you do, go look for a short.
    With the key off, and the lights off including the dome, and the brake sw off, that circuit should be dead. And if it's dead....... there would be no sizzling.
     
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    • TF360

      TF360 Well-Known Member

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      Yearone sells a nice new under hood harness- correct colors and everything
       
    • Dakota Smith

      Dakota Smith 1976 340 Feather Duster

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      That would be perfect because that is the only wiring that is bad in the car. Thank you! I have looked for an engine only wiring harness, but was unable to find one. I can install that in one day.
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      You'll still have to look for a short, or risk frying the new harness.
       
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      • Dakota Smith

        Dakota Smith 1976 340 Feather Duster

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        Wouldn't replacing the wiring harness under the hood fix the short? The wiring is only shotty under the hood. Everywhere else is fine.
         
      • AJ/FormS

        AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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        How can you know unless you test it?
        It could have a short in the bulkhead connector, the column connector, the signal sw or the headlight sw, or the brake lights could be stuck on, or the dome light was on, or the ammeter shorted to the frame, or the alternator is shorted, or a million other things, none of which have anything to do with the underhood harness. And that's just the factory wiring.......
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          It will help you understand why AJ posted that once you understand the main circuits are always hot. No current flows in those wires when the switches are off, but the potential is there. So you just need to figure out what is completing the circuit to ground when hooking up the battery. If the key is off, headlights are off, then it really limits the possibilities. Current is flowing, and it shouldn't be. If the fusible link is not compromised (and it may be from your previous posts) then getting hot indicates a lot of current.

          Read this so you understand how a fusible link protects the unfused wires.
          Fusible Links in Charging Systems with Ammeter
          Shown there is a basic circuit with one fusible link. Once you understand how it works, you can make a similar diagram for your car. This will help you find the short.

          If the diagram here is to be believed, your Duster has two fusible links. Knowing which one (or both) are getting hot will narrow the problem. If that wiring diagram is correct, the power is distributed from two points instead of one main splice used in earlier years. The wire to the ignition switch is spliced in on the engine compartment side and gets its own fusible link. Thats one reason why it looks different than what we typically see here.
          (A factory shop manual diagram is better to rely on than the one in that link. But if it matches what you have in the car, that's great.)
           
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          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            Revised schematic which may better represent '76. The gray represents the 2nd fusible link which you may have.
            Should be helpful in seeing which wires are hot.
            You'll have to mark up for details to match your car.
            Main_charging_wires_plus1976-off.png
             
            Last edited: Sep 14, 2018
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            • Mattax

              Mattax Just the facts, ma'am FABO Gold Member

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              The diagram above is probably wrong. Looking at the factory diagram, and the photo above as well as this one, there should be four fusible links. Additionally, on your car, there probably never was a fusible link at the starter relay. Instead of going through the ammeter, the battery connects to the alternator output on the engine side of the firewall. If so, this means you car's ammeter is externally shunted. Earlier cars use internaly shunted ammeters. (Illustration on this page)
               
            • AJ/FormS

              AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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              Speaking of internal shunts
              I had a guy show up with an interesting problem about a year ago with an 80s or 90s Dodge van. Everything was great until he turned the lights on. Then the engine would die. After doing all the checks I figured there was something wrong at the ammeter, so pulled the dash out, and found the shunt was not passing juice. Rather than taking it apart, I just made a big old copper bar shunt that looked like it would do the job as a temporary fix, and told the guy to go find another van with a good ammeter and I'd swap it over. I told him the current gauge might no longer be accurate, and he didn't seem to care as long as he could drive it at night again.. Well said I, keep an eye on the brightness of your lights then, cuz they will be your charge indicator,lol.
              That was about 20 months ago and he hasn't been back.
               
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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                This probably is an external shunted ammeter. The ammeter is not in the picture unless we need it for diagnostics. It does the exact same thing; shows the amount of current going to or from the battery - not including the starter.
                @Dakota Smith Try to read this revised diagram. Its still a work in progress, but I *think* the damaged wires are shown correctly.
                To start, just see if the colors and relative size of the fusible links seem to match up to the connected items. The gray fusible link is 18 AWG, the dark blue fusible link is 16 AWG and both go to a bulkhead connector.
                Main_charging_wires_plus1976rev1-off.png

                The heavy black wire from the alternator is the output wire. It attaches on the alternator with a ring terminal onto stud. The alternator case will be marked Bat or Batt (because that's where the wire eventually connects to.)
                In your pictures, it looks like the 14 gage red fusible link may be the one thats burned away. See if you can verify that.
                3e20e4d7-111f-496a-8d78-63aa01ec1363-jpeg.jpg
                Does the other end of the single wire make a junction with a thin wire, and another heavy black wire?
                If so that is the battery feed.
                 
                Last edited: Oct 20, 2018
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                • 67Dart273

                  67Dart273 Well-Known Member

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                  Random thoughts from the old shit, "how to find" a short:

                  WITH (and MAKE SURE) that EVERYTHING is "off", the key, the dome light, the trunk/ underhood, glove box lamp, the tail lights

                  IF YOU GET a spark and "sizzling" you have something powered on OR A DRAW that should not be, OR A SHORT

                  IN ORDER TO PROTECT against blowing further fuses, fuse links, or burned wiring while you find it, DO THIS

                  Remove the batter ground wire. INSERT a heavy test lamp such as a turn/ stop lamp in series with the battery ground, in other words

                  from battery post...........to test lamp..........from test lamp..........to ground cable.

                  NOW IF YOU have a heavy draw or short, the only thing that will happen is that the test lamp will light and it will limit current so that nothing burns up!!!

                  First thing I'd try is remove the fuses one at a time. RECHECK that everything is off, and then unhook the big black charge wire from the alternator

                  If none of this causes the test lamp to go dark, then you likely have a short, "wiggle" all your wiring see if it flutters/ flickers/ goes away.

                  LOOK and SMELL for melted/ damaged/ bare/ pinched wires, melted plastic wrapping/ insulation.

                  If that doesn't work, you in for further surgery
                   
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                  • Dakota Smith

                    Dakota Smith 1976 340 Feather Duster

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                    All of the fusible link wires are black. After tearing apart all the tape, I found that the long side of the fusible link wires runs to the battery, then comes back and was connected as a third wire back to the rubber piece seen in the picture. The dangling bare wires were connected to this wire.
                     
                  • Mattax

                    Mattax Just the facts, ma'am FABO Gold Member

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                    That's very hard to understand. Your description sounds like a loop. That would make no sense. The only parallel circuit is the ammeter and it uses 20 gage wires. Additionally those ammeter wires should have a stripe of different color on the insulation.
                    I do understand the fusible links are connected to a wire that leads back to the battery's positive terminal.
                    If you found two thin wires along the way, then that matches the schematic.
                    If you also found a heavy red wire along the way, that too matches the schematic. We see that red wire in your photo cf the starter relay; where it terminates with a ring terminal which is attached to the threaded stud.

                    a. Trace each of the surviving links to their respective connecting wire. The color of the link is not too important. If the insulation got dirty or was heated it could look black. A towel dampened with a drop of soapy water will help clean them. Relative size you should be able to eyeball.
                    a1. The color and size of the connecting wire is more important.
                    a2. Figure out where each connecting wire makes its next connection(s). It will either be a spliced junction or a terminal connector. With power off, the connection can be verified by testing for continuity in the wire. Use an resistance scale, or continuity setting on the multimeter or use a test light.

                    b. Go the the alternator and find the output stud. There should be a 10 or 12 gage black wire attached to it with a ring terminal. Find where that black wire attaches to on the other end. The drawing shows it orginally attached to the same junction as the two fusible links in your photo. Further, that it had a 14 gage fusible link protecting the wire. I don't know if either is correct but that's how I read the FSM diagram and everything else matches the photos you've posted.
                     
                    Last edited: Oct 21, 2018
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                    • Mattax

                      Mattax Just the facts, ma'am FABO Gold Member

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                      Here's what I see in your photo.
                      Something, probably a link? could be a hacked wire, is hanging on by a couple of strands at the spliced junction. I suspect this link/wire connects to the alternator output stud.
                      Of the two links, each looks to have a different color than the other.
                      They are both spliced to a heavy black wire, probably 10 gage. We know this wire eventually connects back to the battery positive terminal.

                      The 10 or 12 gage red wire attached to the relay stud must eventually connect back to the battery positive terminal. It may do this by a connection or splice with the heavy black wire. (This red wire carries current for the starter relay to power the starter's solenoid.)

                      1976-Link-Relay-broken1.jpg
                       
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                      • Mattax

                        Mattax Just the facts, ma'am FABO Gold Member

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                        This is a diagram of the bulkhead connectors.
                        If the FSM matches your car...
                        The solid red arrow points to the terminal position for a red wire that connects to one of the fusible links.
                        The solid black arrow points to the terminal position for a heavy black wire that connects to the other fusible link.

                        An ohm meter with a probe on each of those two terminals should show zero resistance. (Battery must be disconnected to do a resistance or continuity test)
                        That would show the two are connected together.
                        You could then check for continuity of from the terminals to the battery positive terminal (disconnected). That would verify they are on the main cable

                        1976-BulkheadJ1R6.jpg
                         
                        Last edited: Oct 21, 2018
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