instrument lighting wiring and fuse

Electrical and Ignition

  1. str12-340

    str12-340 FABO Gold Member FABO Gold Member

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    I'm working on a 1970 Dart with relatively new M&H wiring (which was a dream to work with). On the "instrument" fuse terminal I piggybacked an extra wire to feed the added instruments in the car so they would work with those in the stock instrument panel. There are three gauges and a tach and each has its own bulb. Every once in a while the "instr" fuse blows. It is a 3 amp fuse. All the wires feeding various lights are 18 ga like the wires fed by most of the 20 amp fuses in the system.

    What am I risking by replacing the fuse with a 5 or 10 amp fuse to cover the additional lights and perhaps solve the blown fuse issue?
     
  2. Kendog 170

    Kendog 170 Let the boy go !

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    • Kendog 170

      Kendog 170 Let the boy go !

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      Led's draw far less amperage than the older bulbs .
       
    • Mattax

      Mattax Just the facts, ma'am FABO Gold Member

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      That is probably as good of a quick rule of thumb as any.
      The better versions of that are a table or calculator where one also puts in the wire length and how much voltage drop is considered acceptable.
      In other words the first consideration is the loss of performance, not the melt dowm of the wire.

      One caveat when using tables based on AWG is that automotive wire is not made to AWG. Automotive wire gage is the same outer diameter but made up with larger strands. It has slightly less density and carrying capacity.

      My guess as to the weakest connection may be the printed circuit board.

      Here's the thing. There were factory options that added lamps to the standard instrument lighting circuit. For example console and tachometer options. On my '67 there was an unused connector for the console option. I connected to it for lighting the tach and oil temperature gages I've added.

      Do you know what bulbs are in the circuit? That might be the first thing to check. Sometimes the parts stores have a similar bulb in stock or listed in their books but draws more current.

      Instrument lamps probably should be 158. That's rated 2 MSCD and .24 amps
      That's the same mini wedge base as 194. But 194 is rated 2 MSCD and .27 amps
      Just an example of an easy swap that could pull more noticebley more current when there are 5 or 6 of them.
       
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      • Bugman

        Bugman Well-Known Member

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        3 amp fuse is almost for sure to protect the printed circuit board. Looking at diagram, light switch supplies power to circuit E1-18 0 to hot side of 3 amp fuse. There are two branches off that 3 amp fuse. E2A-18 0 goes to printed circuit board instrument lamps ,E2-18 0 to console . leave those two circuits alone. Instead with a separate inline fuse Tap into circuit E1-18 O, hot side of the fuse from light switch. In my opinion a safer way to do this
         
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        • Bugman

          Bugman Well-Known Member

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          1970 DAER-2.jpeg
           
        • str12-340

          str12-340 FABO Gold Member FABO Gold Member

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          I have LEDs in the instrument panel, like Mattax says above, I'm a little puzzled by the situation as one of my other Darts has a light package with two additional wires in the same INST circuit.
           
        • Bugman

          Bugman Well-Known Member

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          blow up

          BLOW UP.jpeg
           
        • str12-340

          str12-340 FABO Gold Member FABO Gold Member

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          Bugman - what excellent advice! I wanted to use that circuit to keep the gauge lights working with the dash lights. This would protect the original circuits and also provide separate protection for the gauge light whie being on the same circuit. THANK YOU!

          Thanks to everybody for the great information and advice!!!
           
        • Bugman

          Bugman Well-Known Member

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          Keep the 3 amp fuse in place and make your connection somewhere where that red arrow is pointing.
           
        • Bugman

          Bugman Well-Known Member

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          The wiring diagram for 1970 has standard dart page 8-99 and page 8-95 instrument panel wiring diagram {rallye cluster} . They are slightly different. The diagrams are for the standard cluster. Upon further investigation circuit E2-18 0 goes to a splice with branches to radio and steering column shift indicator

          splice -1.jpeg
           
        • Bugman

          Bugman Well-Known Member

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          one more place ash reciever tray see red lines

          ashtray.jpeg
           
        • str12-340

          str12-340 FABO Gold Member FABO Gold Member

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          Also in the manual is accessory wiring diagram which provide other potential unused feeds but without the dimmer control and tie to the other dash lights. I ended up using the fuse tap piece on the INST fuse that I found in the parts store - fits around the side of a fuse metal cap and provides a male connection that sticks out of the face of the fuse box (it's shaped like a "P" without the straight bar at the back of the round part). I put it on the hot feed end of the fuse, plugged an inline fuse set up with a 3 amp fuse in it and plugged the wires that feed all 4 extra gauges into it. Looks like it works great. Now the original bulbs are protected by the 3 amp fuse in the box and the extras are protected by the inline fuse separately. All seem to work fine. I'm headed on a three hour drive, each way, to a weekend show this Friday so I'll let you know how it works!
           
        • str12-340

          str12-340 FABO Gold Member FABO Gold Member

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          Bugman - thanks for the great info and suggestions.

          Are you a pest control guy, an entomologist, or a Volkswagon dude?
           
        • Bugman

          Bugman Well-Known Member

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        • Bugman

          Bugman Well-Known Member

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          Back in the 1970s spent a few years in the pest control industry got stuck with the nickname Bug Man
           
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          • str12-340

            str12-340 FABO Gold Member FABO Gold Member

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            Well I made a 300mile trip last weekend to a car show at the beach. It rained all the way down, so lights on all the time and no blown fuse!!!!!!
            Thanks to all for information and suggestions.

            I am curious about one thing. Several of the other fuses are 20 amp (marked right on the fuse box). If you look at the wiring diagram in post #8, they have 14 ga wires on the "hot" side and 18 ga wires on the other (protected) side of the fuse. According to the chart in another post this shouldn't be right. I'm not an electrical expert but am trying to learn - can anybody explain this to me? I get that the 3 amp fuse is to protect the circuit board on the back of the cluster. I'm just trying to be better informed for the next project.
             
          • Kendog 170

            Kendog 170 Let the boy go !

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            I too am no expert but it sounds odd to me also. You said it's marked 20 ? cause I was thinking maybe someone stuffed a 20 in a 10 amp circuit or something. Maybe check the FSM just in case.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            I'll give this a shot. It depends on what you are trying to do with the fuse, whether the loads are constant or intermittent and the air temperature of the wire. Remember I wrote "
            The better versions of that are a table or calculator where one also puts in the wire length and how much voltage drop is considered acceptable.
            In other words the first consideration is the loss of performance, not the melt dowm of the wire."

            Whoever put together that table was clearly most interested in preventing loss of performance - presumable for the LED lampos they are selling.

            On our cars, presumably the fuses are there to prevent or at least minimize catastrophic failure.
            It's also probably generally more important to keep things running.

            Lets look at examples. According to the linked chart a 12 awg wire is only good for 18.6 amps. For good or bad on our cars 12 ga wires often carry more than 18 amps. Heck that 16 ga fusible link routinely carries more than 18 amps. Granted its not usually for long stretches of time, but my point is that it can carry over 40 amps without burning up or even melting its insulation right away.
             
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            • Bugman

              Bugman Well-Known Member

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            • Bugman

              Bugman Well-Known Member

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              Not an expert by any means, but for future projects it can be helpful to know the amperage draw of a circuit. Working on a headlight circuit today measured amperage flow. Low beam 7.24 high beam 8.76 turn on surge 9.7, engine not running. test On 1980 Aspen. 1970 dart headlight switch internal circuit breaker15 amps.

              beest.jpg

              probe.jpg
               
              Last edited: May 4, 2021 at 7:39 PM
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              • Bugman

                Bugman Well-Known Member

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              • Bugman

                Bugman Well-Known Member

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                Now for safe testing with a 3 amp fuse an extra jumper is needed. Pull fuse out 1st jumper goes to power source then to 3 amp fuse, other end of fuse goes to meter probe. The other meter probe goes back to the fuse block. The meter in the picture has an internal 10 amp fuse which would not be safe in a low amp circuit
                 
              • str12-340

                str12-340 FABO Gold Member FABO Gold Member

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                Sorry, but I gotta know this - There's a circuit breaker in my headlight switch???? how does it get reset???
                 
              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                Yes. When it cools down it resets.
                I've had this happen to me at least twice while driving.
                Once was a '75 Valiant coming home from school at night.
                The other time I recall was with my current '67 Barracuda some time after I had installed relays for the H4 Hellas.
                That was a long drive back to Phila from North jersey, thinking about what the problem might be.
                Turned out the problem was some of the wire insulation had chafed on the back of the headlight buckets. Not the new relay wiring, but some of the original wiring which now was used to trigger the relays. Every time it was shorted, the breaker would open. If I recall correctly the high beams rarely shorted on the trip but hte low beam wires touched alot. After looked at the lamps at a service area - it got a little better. I was pretty much home when the dim bulb in my head finally turned on and figured out what was probably going on.
                 
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