85 359 Fuel gauge question

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by 85pete359, Jan 20, 2022.

  1. 85pete359

    85pete359 Light Load Member

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    Hey guys I’m back again, just wrapping up a few loose ends and ready to put this beuty on the road, trying to figure out how does the fuel gauge work on this thing I got a hot wire #248 coming from the breaker going to a pole on the gauge and another wire #14 on the other pole of the gauge going to to the switch and from the switch 2 sending wires to the sending unit #1, #2 I have my constant power at 248# but how and when does #14 receive power to send to the switch ? Becsuse 14 is getting nothing and it’s also not showing itself to be a ground switch is working o tested it gauge isn’t working but before I buy a new one water to see how it works so I could diagnose things better also the sending units don’t work either my real question is how does 14 transfer power to the switch
     
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  3. 201

    201 Road Train Member

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    Sounds like the most important item was saved until last. No fuel, no fun. The owners manual for my 359 was thicker than a NYC phone book from the 80's. Sounds like you may be having trouble with the R-L tank switch, if any. On mine, rather than trace down the wire, of which there's like 50 miles of, I just ran a new wire.
     
  4. 85pete359

    85pete359 Light Load Member

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    Hey good morning yea i know I have the manual also and I’m from NY so I know what you mean lol but the switch is good I tested it with a multimeter and also checked all my wires this is a project truck everything is apart so I was just curious how the thing functions so I could diagnose it does the gauge have to be working for it to send power to the switch?
     
    201 Thanks this.
  5. Catmando

    Catmando Road Train Member

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    Your #14 wire is the signal wire from your fuel tank sending unit there is 1 for left tank & 1 for the right tank you need to check those with a Ohm meter .. also check the ground wire on each sending unit because they tend to break or loose their ground they are ion top for the sending unit also
    Also more than likely the sending units are non-functional so just replace them with the tube style sending units the sending units are easy to test with a Ohms meter just ground and check the terminal pole the signal wire #14 is bolted to for Ohms reading
     
    Last edited: Jan 21, 2022
    Roger McG and 201 Thank this.
  6. SmallPackage

    SmallPackage Road Train Member

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    There shouldn’t be any 12v going to switch. The 12v goes to a marked post on back of gauge. The switch is there just to regulate what tank sender needs to be read. If you only read off one tank switch isn’t needed.

    Switch diagram:
    Top terminal= RH tank sender
    Middle terminal= “Sender” marked post on back of gauge
    Bottom terminal= LH tank sender
    Make sure gauge has good ground. On them old aluminum trucks with all the built up oil and crude you may need to run an extra wire from ground post to a good under dash chassis ground spot. They sometimes just have a weak ground to the gauges.
     
  7. 85pete359

    85pete359 Light Load Member

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    Ok yes thanks very much this is exactly what I wanted to know I appreciate that brother
     
  8. 85pete359

    85pete359 Light Load Member

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    Thanks brother
     
  9. Catmando

    Catmando Road Train Member

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    Hope I was of some help ... I kinda question your wire #14 thou .. because I thought the signal wire numbers were something like 06 & 08 .. buts thats going off memory I have all the PDF files for the wiring and air line Schematics. for a 359 I just didn't pull them up before I replied
    Your fuel tank switch just switching the power from left tank to right tank is all it does. There is no diverter valve because it draws fuel frome the bottom of both tanks.. unless some one has changed it. its a pretty simple set up
    If you need more info just let me know
     
  10. 85pete359

    85pete359 Light Load Member

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    The set up is exactly how you described, the signal wires is #1 and # 2 1 is for the left tank and 2 is for the right, but yes sir you were very helpful I appreciate you taking your time out today, it’s always easier to fix something when you know how it works lol
     
    Catmando Thanks this.
  11. Catmando

    Catmando Road Train Member

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    I've fix my share of wiring issues in 359s
    JMO but the fuel gauge it's self generally works it usually the ground s or the sending units or seems like the wires tend to break close to the the sending units .. anymore I just pony up the money for the newer tube style sending units and install them regardless what I find wrong because they seem to crap out soon after I fix the other issues... they were not very good 40 years ago even ...lol
     
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