Speedometer?

[wargle70]

I have an 1988 90hp mercury. There is a small plastic tube running from a fitting on the forward side of a collar near the top of the lower unit. The tube runs to the speedometer on the dash. This has got to be what causes the speedometer to register. The engine end of the tube doesn't appear to be open to the water. Can anyone explain how it works the speedometer. I also have the speed/temp paddle wheel sensor that runs to my Eagle sonar. Just can't figure out how the speedo works.

 

[wrench 3]

Re: Speedometer?

The tube connects to a hole in the front of the lower unit housing. The faster the boat goes, the more water pressure is pushed into the hole. The speedometer is just a pressure gauge with speed readings on it.

 

[Arkyhunter46]

Re: Speedometer?

These tubes get clogged easily with debris from the water. Get a small drill bit and work it in the gap BY HAND to remove the debris. Sometimes you have to add a LITTLE compressed air from the speedo tube end to assist in pushing it out.

 

[wargle70]

Re: Speedometer?

Thanks for the replies. Seems pretty simple. I am only familiar with the leaver arm style, but I guess they all work the same way.

 

[MH Hawker]

Re: Speedometer?

Or you can use one of the small hand pumps.

 

[wrench 3]

Re: Speedometer?

Thanks for the replies. Seems pretty simple. I am only familiar with the leaver arm style, but I guess they all work the same way.

If this is the "leaver arm style" that you were referring too. It still works on water pressure to a tube. There is a hole in the leading edge of the bottom of the arm that connects to the tube. The spring arm is just so that it won't get broken if it hits something.

 

[BatDaddy1887]

Re: Speedometer?

Once you get that fixed, you could do a test with a GPS app on your phone to see if the speed matches.