99 379 Fuel gauges with selector switch

Discussion in 'Peterbilt Forum' started by simonsocm, May 10, 2021.

  1. simonsocm

    simonsocm Light Load Member

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    Im trying to find a wiring diagram for my trucks fuel gauge and sending units. It's got a selector switch for left and right tank level. The left one doesn't do anything, like it's disconnected. The right one only seems to go from half down to a little less than a quarter tank. I don't mind running new wires if I need to but I'd like to know how it was wired from the factory first.

    My vin is below if you need it.
    1xp5dr9x8xd488775
     
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  3. SmallPackage

    SmallPackage Road Train Member

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    If the senders in the tank are good when the switch is in neutral (mid) position the gauge should read empty. Flip to selected tank and should read real level. 99% of the time it is bad senders or grounds on the senders. The middle wire on switch should go to the sender terminal on gauge. The other two go to the selected sender on tank.
     
    Last edited: May 10, 2021
    blairandgretchen and Bean Jr. Thank this.
  4. simonsocm

    simonsocm Light Load Member

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    Where would the grounds be located for the senders? May as well look at those first since the right side seems like the resistance is a bit off and the other I'm guessing has an open or bad sender because it's always showing empty. If I can get at least the one working that would be great.
     
  5. SmallPackage

    SmallPackage Road Train Member

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    The grounds are usually a wire that is screwed to a seperate lug on the sender plate or it is held on with one of the screws that mount the plate to the tank. That wire could either be attached to the frame somewhere close or run all the way back into the cab wiring with the sender wire and be connected to a master ground. Either way check the connection on sender for corrosion. It won't hurt to run a new ground to the chassis if need be. The tanks well not ground without them because of the insulated anti-scuff protectors used on the straps.
     
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  6. simonsocm

    simonsocm Light Load Member

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    Awesome. Really appreciate the info. I'll probably see if putting another ground on the passenger side will fix that sender for now then and probably do the same on the driver side to see if it even changes anything.
     
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  7. SmallPackage

    SmallPackage Road Train Member

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    Generally on most common senders the center screw connection is always the sender wire to switch or gauge and the other one on lug or mounting screw is ground. Some are not color coded and can be the same color so mark them when removing as to not switch them.
     
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  8. lester

    lester Road Train Member

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    I believe if you remove the signal wire from the sender the gauge should sweep all the way to the left (empty). If you ground the signal wire the gauge should sweep all the way right (full).

    That is how to check your wiring and gauge. If it does those things wiring is good and problem is in the sender itself.

    I'd guess more than likely senders are bad or floats have fallen off.

    I could be wrong on that test of gauge and wiring but I know that works for a lot of other gauges
     
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  9. simonsocm

    simonsocm Light Load Member

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    Well it looks like one of the senders is bad. Other looks like it may be ok but I'm not sure what the resistance range should be so I'm unsure if it's bad or not.

    The one that's good reads about 160 ohms and it's currently around half a tank. Gauge reads around a quarter. The presumed bad one was open.

    When grounded out the gauge sweeps the full distance between full and empty.

    I'm thinking the "good sender" may have high resistance but the other is completely shot. Crappy thing though is my tank sender selector switch on the bad sender reads something ridiculous like 1000 ohms as well. Good thing that side is bad already.
     
    Last edited: May 10, 2021
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  10. blairandgretchen

    blairandgretchen Road Train Member

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    My ‘95 was a bad sender on passenger side, float had fallen off.

    Tank needed to be near empty to rotate and replace sender unit.

    The previous posts are all spot on.
     
  11. simonsocm

    simonsocm Light Load Member

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    Did you end up just loosening the straps to rotate it? Did you remove the pickup and return as well?
     
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