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Old 26 April 2015, 07:30   #1
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Yamaha speed sensor

Recently I invested 100 EUR in a speed sensor kit for my Yamaha F100. Installation was pretty straightforward, the engine is pre-wired, the kit contains the sensor, hose, plastic connector and tie-wraps. The instructions leaflet omits the F100, but I followed instructions for the F90.

Unfortunately the speed display still shows a big 0.

Do I have to do anything like toggle a switch to get this working?
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Old 26 April 2015, 11:21   #2
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On the leading edge of the lower unit, there is a tiny hole (run a thumbnail along the edge, where it grabs is probably where the hole is.) That hole feeds a plastic or rubber tube that goes up the leg to the tubing from your kit, and makes its way to the console and gauge. As you move forward, water gets forced into the hole, which pressurizes the air trapped in the upper part of the tube. The gauge reads that pressure and converts it to speed. Any wiring you did was just to get power to the gauge (at least if it's like mine.)

So, you can do a few things:

1) Take a small drill bit or piece of wire and clean out the pitot hole on the LU. It has a habit of picking up mud, algae, or bits of shell.

2) Wherever you plumbed the kit in to your motor, disconnect it, and pour some water into the lower tube. Make sure it comes out the pitot hole.

3) blow some air up the upper tube; should cause a reading on the gauge. No idea what pressures are involved, but I would hazard a guess it's not much.

Luck;

jky
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Old 26 April 2015, 11:44   #3
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Should be about 1 psi at 5 knots.


ρ ≈1000 kg/m
V ≈ 2.6 m/s
q = ρV ≈ 1000*6.76 Pa ≈ 0.98 psi

If you need more, 4 psi for 10 knots or 16 psi for 20*.


*possibly. Not sure if the equation will work at that speed.
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Old 28 April 2015, 03:19   #4
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In theory the formula is correct, there can be small deviations if the pickup is not a true pitot tube but a tiny hole in a round object causing turbulence.

My first thought was similar to the reasoning of jyasaki, so I checked the entrance but found no clogging of any kind there. It may be higher up the thin hose or near the small plastic fitting that connects the extension hose.
The boat is in the water now, so instead of compressed air I will try to use a can of butane to blow away whatever is blocking the tube.
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Old 28 April 2015, 05:26   #5
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Easy check. Disposable syringe about 50ml. onto the plastic tubing on the back of the speedo. If it gives a reading then you've got a blockage or a leak somewhere between the pito hole and the instrument head, if not there's a problem with the instrument.
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Old 28 April 2015, 08:05   #6
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Quote:
In theory the formula is correct, there can be small deviations...
But it is good enough to confirm that you could get a reading by blowing into a tube, and that you don't have to blow so hard your head explodes .
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Old 28 April 2015, 10:03   #7
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You'll only blow about 2 PSI it should move, but only just, if it's a 60mph speedo it'll just give a tiny indication.
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Old 29 May 2015, 11:58   #8
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For various reasons it took a while; lots of other stuff to attend to and very disappointing weather.
I have blown, sucked and pried, all to no avail. Until today, when we were on a longer trip and after about one hour the "-" on the speed display turned into a 3, then 4 and 5.
It seems something like clay blocks the passage in the lower gear housing and it takes some time before the water can soften it.
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Old 01 June 2015, 10:19   #9
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Poke a piece of wire into the pitot hole or ream out with an appropriately sized drill bit. Then reverse flush with a hose. Should take care of it.

jky
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