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Speedometer wire colors coming out of lower unit 250hp?

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  • Speedometer wire colors coming out of lower unit 250hp?

    hello i cant get to my lower unit to visually check out speedometer wires as boat is hanging in a sling over water.
    trying to determine if i have a break in line from lower unit to gauge on center console.
    gauge is yamaha y5 with trip/time/batt,trip always shows 0(zero),time and batt work fine, also backlight works fine.
    enclosed is pic of gauge front and back.
    any help appreciated.

  • #2
    Wires? What do you mean by that? Speed is determined by water/air pressure through the pitot hose. The most common culprit is a clogged pitot hole on the leading edge of the drive. Google for that - plenty of info on it. Clear it with a little drill bit. Or, the clog has moved further up or the hose has split, allowing water to spill out into the boat somewhere. Follow the pitot hose into the boat - there's often a little splice/connector somewhere where you can pull it apart and blow into the tube to see if speed registers. I don't remember for sure, but there may be a little connector just above the anti-ventilation plate - some engines have that - I can't remember if my Yamaha has it. You can disconnect there and blow both ways to check for clogs. Figure out a way to get to it while on the lift... with a skiff?
    Last edited by DennisG01; 1 day ago.
    2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
    1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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    • #3
      thanks Dennis for your reply.
      i understand the pitot tube.water pressure.
      what i dont know if how that water pressure is translated to the digital gauge on the dash.
      did some modifications and wire movement of wire harness in stern of boat and since then the speedometer trip/mph not registering,stays on zero.
      is there a wire harness that connects the water pressure/pitot tube to the back of the digital gauge?
      thanks
      john

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      • #4
        That particular gauge works solely off of air pressure. Air pressure to the gauge itself. Via the hose barbed fitting. Air pressure of course created by water pressure.

        The electrical part is only related to the display. So that the pressure and other information can be seen.

        Power up the gauge. Connect a short section of hose to the hose barb. Blow like hell into the hose. You might see a mile an hour or so on the display.
        Last edited by boscoe99; 9 hours ago.

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        • #5
          I think the small tube runs all the way to the gauge,
          but maybe a new motor might have a pressure to electrical transmitter.
          is there a reason you did not post your complete motor model #

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          • #6
            If it's like my F150 (as Boscoe states), the hose starts in the mid-section and continues upward into the lower cowl (probably thru the swivel steering, vertical tube- so it'll come out low).

            It then exits out of the engine maybe 18", (It's not super long for riggine purposes), goes into a barb,and is then connected to a hose thru the hull, etc..

            If the hose is pinched (you were just working there), that would dis-able it.

            The inlet on the LU leading edge (you may already know), is SUPER SMALL and will clog up over time if used in salt water.


            You can confirm the barb to the speedo works by attaching some air (compressor) to the hull end of the hose and the speedo will work (key on).

            On the LU, just ahead of the shifter shaft is the plastic nipple with o-ring seal that connects in that area (see below pic).







            If clogged, you probably want to work from both ends and if using a drill bit (on the leading edge), ONLY by hand- it is small...


            To keep mine working (from corrosion), I cut a sharp "V" in a very short piece of old washer machine hose. Hook it up to the garden hose/nozzle and
            shoot fresh water into the pitot hole. Might get a half cup of water spit back out if clear. To clear the line FULLY of water, I "T'eed in a schrader valve
            in the center console speedo hose. With a very small electric air compressor attached to the schrader valve, turned on, it'll blow ALL the water out of the lines and LU.
            Doesn't take 5 minutes and the speedo, 13 years old, still reads down to 4 MPH. The compressor, when hooked up and running, will show about 17 MPH (with the lu/system open)

            .

            Last edited by TownsendsFJR1300; 9 hours ago.
            Scott
            1997 Angler 204, Center Console powered by a 2006 Yamaha F150TXR

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            • #7
              Hoses get old and brittle - it's possible moving it around cracked it. Locate it and check it out. If you don't see anything obvious, take the boat for a ride and observe the hose - water will be squirting out if it's failed. The other possibility is that it's just a coincidence and the pitot hole is clogged.
              2000 Yamaha OX66 250HP SX250TXRY 61AX103847T
              1982 Grady Weekender/Offshore (removed stern drive & modded to be an OB)

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