My speedo does not work !!! Help Please

[Stan]

My speedo does not work !!! Help Please
Tried spraying water into all openings in the lower unit to remove mud or anything…..no good.
Tach works fine.

Where / how does the specdo work in this unit ?
Thx

1989 Donzi 22 Classic
Mercruiser 454 / Bravo 1
Outdrive Serial # OC689928
Drive Unit Serial # OC504526

[gcarter]

A picture may be helpful.
Front and behind the dash.

[RedDog]

Where is your pick-up port? On my Bravo drive it is on the leading edge of the drive. Wood and other junk can clod the port. Take a drill bit the size of the port and thread it in with some pliers (or if daring, a cordless drill) to clear the junk.

[Stan]

A picture may be helpful.
Front and behind the dash.

I don'DSC00183.JPGt have a behind the dash pic.

[gcarter]

Stan, they do eventually wear out.
Your speedo is a Bourdon tube type gauge (Google it), and the internal linkages do break.
However, the tube from the transom may be blocked. You might remove the tube from the
rear of the speedo and see if you get water through. If you have a drive w/a built in pitot,
and it's blocked, you can install a separate pitot mounted on the transom near the drive.
If water comes through the tube to the rear of the speedo, it's probably cooked.

[Stan]

I will check it out this weekend.


Vintage member ..............are you saying that with a Bourdon tube type gauge.....water goes all the way from the outdrive to the back of my dash
and the rear of the guage !!!????

[Pat McPherson]

Yes, there is a water tube that goes all the way from the drive to speedo. I found a crack in mine needed to be fix to get the speedo to read again.

[Morgan's Cloud]

I will check it out this weekend.


Vintage member ..............are you saying that with a Bourdon tube type gauge.....water goes all the way from the outdrive to the back of my dash
and the rear of the guage !!!????

Pat McPherson

yup
Yes, there is a water tube that goes all the way from the drive to speedo. I found a crack in mine needed to be fix to get the speedo to read again.


Huh ? I thought only a small amount of water went up the tube from the pitot and pressurized the remaining air in the tube thus activating the mechanics of the gauge .

[Pat McPherson]

Pat McPherson

yup
Yes, there is a water tube that goes all the way from the drive to speedo. I found a crack in mine needed to be fix to get the speedo to read again.


Huh ? I thought only a small amount of water went up the tube from the pitot and pressurized the remaining air in the tube thus activating the mechanics of the gauge .

The gauge reads water pressure not air pressure. If you disconnects the tube from the gauge a run the boat, lots of water will pour on your legs...

[Morgan's Cloud]

The gauge reads water pressure not air pressure. If you disconnects the tube from the gauge a run the boat, lots of water will pour on your legs...

This is certainly news to me . I have seen speedo installation instruction manuals that recommend removing the tube from the speedo when the boat is initially launched so that a 'small amount' of water can creep up the tube for a foot or so but not to let water make its way all the way to the speedo head .

How would the water make its way all the way to the speedo on a new boat that was never water tested at the factory , do they somehow bleed themselves ? Also , why is it often recommended to test a possibly defective/malfunctioning speedo with a blast of compressed air up the tube ?

I can assure you that there's no water in my speedo itself and it seemed to be very accurate the last time I ran the boat.

[joseph m. hahnl]

If the gage is bad it can be repaired for the same cost of a new one. But seeing as you would need to replace the others gages to match the new speedo The cost is well worth it

[txtaz]

The gauge reads water pressure not air pressure. If you disconnects the tube from the gauge a run the boat, lots of water will pour on your legs...

The gauge reads air pressure. It's a bourdon tube and the water pressure at the pitot increases the air pressure in the tubing up to the gauge.

In all my boats, I have never "bled" water into the speedo tube. I just drop them in the water and go. I guess technically it would give a better reading having a hydraulic buffer in the line but who is going to pull the tube from the speedo every time they put in?

Taz

[Carl C]

I don't know if it reads water or air pressure but if the hose pops off water does indeed squirt all over the place!

[gcarter]

Been busy in KY for a few days.......
The gauge will register what ever is in the tube.
However it's easier for most folks to test the tube
w/water than air if they don't have a compressor.
BTW, I woul dn't blow 125# air up the tube w/it attached to the gauge.
I have one regulator that will run down to "0" pressure. I tested
my speedo by slowly increasing air pressure. You'll see a result
with as little as 10 PSI.
But, if it's not working, by all means disconnect the tube
from the gauge and see if you can get water or air through the tube.

[txtaz]

The gauge will register what ever is in the tube.

True but water and air compress at different rates and have different densities. The gauge is designed for air. Bourbon tubes are shaped like a hook. The difference in surface area on the outside of the hook is greater than the inside. When pressure is applied inside the tube, more pressure is exerted on the outside due to larger surface area and the hook tries to straighten out causing needle movement. The greater the density of the medium will result in more movement and the gauge would not be accurate.
The point is mute though. It is a closed system and bleeding the air from the bourdon would be ridiculous. You could have water all the way to the spedo head and it would still read the same.

Taz