How does a Speedo work?

My bro in laws 1998 Maxum Merc/Alpha 3.0 has a dead speedo, how do they work? Is the hose going into the middle of the back of the gauge just a hollow tube and the needle is moved by water pressure? If I pulled the hose off the gauge, would water come out or air? Any way to test the gauge? Blow air thru the line? The hole in the front of the lower unit is not clogged, but could there be something in there that would cause it not to work?

Re: How does a Speedo work?

its the pressure of the water - that then 'converts' to air pressure to operate the speedo.

no water should be in the guage.

have you followed the tube back to the 'pitot' - its like a little arm mounted on the boat - usually at the base of the transom - and sits below the water level.

this has the tube attached at the top - and then there is a little hole at the end of the arm - that the water rushes into and causes an airlock in the tube - and subsequently a pressure increase that then turns the speedo needle.

you can blow into the tube to see if it moves the speedo arm - but with mouth pressure you will only get it to twitch a bit.

hope this helps

cheers

adam

Re: How does a Speedo work?

liquidlew said:

The hole in the front of the lower unit is not clogged, but could there be something in there that would cause it not to work?

Click to expand...

Yes, if the fitting on the front of the anti-ventilation plate is not attached or not attached properly, it won't work.

libellav, he's got a Mercruiser, they have the speedo integrated in the outdrive. No device like you are describing. Volvo has the same thing now, too.
But you can still try to blow air through the tube when you take it off the back of the speedo gauge to see if there is a restriction.

Re: How does a Speedo work?

I used an air compressor to blow through the hose at the transom. I could see the gauge going up and down.