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
85 359 Fuel gauge question
Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by 85pete359, Jan 20, 2022.
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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.
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201 Thanks this.
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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 readingLast edited by a moderator: Jan 21, 2022
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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. -
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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 -
Last Call Thanks this.
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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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