What Causes My Air-Operated Speedometer to Not Work on My Boat?
The conventional speedometer on your boat works because the pitot tube that runs from just beneath the outside of the transom -- the "back wall," where the outboard motor is mounted -- to the speedometer, is full of air. When water tries to push its way in, it compresses the air in the tube. As the air becomes more compressed, it forces the speedometer needle to climb.
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Watch What Happens When You Blow in the Tube
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The air-operated speedometer is a two-piece system. Either the water is not compressing the air in the pitot tube, the pitot tube is disconnected from the speedometer or the speedometer gauge is broken. Start the process to find out what the problem is by making certain the tube is connected and blowing in the tube while watching the speedometer needle. If the needle doesn't move, the tube is blocked or the gauge is bad.
The Speedometer Tube is Blocked
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Because water is trying to force its way into the pitot tube, a blocked tube will cause the air-operated speedometer to stop working. The blockage can result from seaweed or other aquatic plant life jamming the tube. The obvious solution to the problem is to remove the blockage, using a piece of wire to remove whatever is causing the problem. Some pitot tubes have a built-in screen to keep plant life or small aquatic animals from entering the tube and causing a problem. You can also find after-market pitot tube screen kits at boating supply houses.
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Pitot Tube Location
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If the speedometer moves when you blow in it, but not when you're boating, look at where the pitot tube is mounted. If it's too close to the motor, "prop wash" will cut of the flow of water. If the tube is in the water, pointed forward and under the boat, move it away from the propeller, farther under the boat or to the left or right side of the rear of the boat. If the speedometer needle moves a bit, move the end of the tube around to find the best mounting location.
Speedometer Gauge
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If the speedometer needle refuses to move after you have checked the pitot tube for a secure connection, blockage and correct placement, the gauge is the problem. Try blowing low-pressure compressed air through the tube to dislodge any debris far inside the tube, or replace the pitot tube. If the speedometer needle still refuses to budge when you're boating, though, you must replace the speedometer gauge.
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