speedometer no longer working (Merc stern drive 454 c.i.) ( 1989 Formula 223 SS)

[Skidood]

Hi.

I recently found the rubber tubing on the back of the speedo had come off. I re-installed it, and the speedo is still not working. If I attach a piece of tubing to the speedo and blow on it, the needle only moves up to around 5 MPH with me trying to apply as much pressure as I possibly can using my mouth. I know from previous experimentation on a work-related thing that when you blow as hard as you can into the end of rubber tubing, you are applying around 1-2 psi. The line going back to the outdrive is not plugged, I can blow air through it fairly easily. I'm very surprised that it would require this much pressure to move the needle, considering the little hole on the front edge of the lower unit is only about 1/8 in diameter. What's going on? The speedo is a VDO 0-75 MPH.

 

[Bondo]

Hi.

I recently found the rubber tubing on the back of the speedo had come off. I re-installed it, and the speedo is still not working. If I attach a piece of tubing to the speedo and blow on it, the needle only moves up to around 5 MPH with me trying to apply as much pressure as I possibly can using my mouth. I know from previous experimentation on a work-related thing that when you blow as hard as you can into the end of rubber tubing, you are applying around 1-2 psi. The line going back to the outdrive is not plugged, I can blow air through it fairly easily. I'm very surprised that it would require this much pressure to move the needle, considering the little hole on the front edge of the lower unit is only about 1/8 in diameter. What's going on? The speedo is a VDO 0-75 MPH.

Ayuh,.... What's the Other end of the tube lookin' like,..??

Is the pitot clear, 'n hooked up proper,..??

 

[Watermann]

So have you taken the boat out since reattaching the pitot tubing to the speedo? Be sure you cut the flared end off the tubing and maybe even use a cable tie to secure it. Check for kinks in the tubing and for obstructions in the pitot on the stern.

 

[NYBo]

Do you have a pitot mounted on the transom or is the pickup built into the lower unit of the stern drive?

 

[GA_Boater]

I found this on the Web. So If you can blow 2 PSI and got 5 MPH, you did well.

1. If adjustable air pressure is available, standard calibration speedometers should react to the following pressures: 5 PSI = 20 MPH; 11 PSI = 30 MPH; 19 PSI = 40 MPH; and 30 PSI = 50 MPH.
2. A standard speedometer using the Mercury/ Mariner speed pickup in the engine foot will read approximately 10% faster than with a Teleflex pitot mounted to the transom. Speedometers with the suffix "M" after the Part Number are calibrated for use with in-foot pickups. To test as in (1) above: 5.6 PSI = 20 MPH; 12.6 PSI = 30 MPH; 22.4 PSI = 40 MPH; and 35 PSI = 50 MPH.

Also if the tube was off while operating the boat, water should have been squirting out of the tube. I think your trouble is a blockage in the pickup on the lower unit, which is not an uncommon problem. Try clearing the hole and the inner pipe.

 

[Skidood]

Ok thanks everyone...the line must be plugged or something...I did take the boat out for a run after re-connecting the tubing and the speedo did not move. Also, when the line was still disconnected I did not notice water coming out from under the dash. The pickup is in the front edge of the drive. Since I can blow air through the line going back to the drive, I wonder if the line is broken back there somewhere. Anyway I am still shocked at the pressures seen inside this line. Really gives you some insight as to the resistance of slicing through the water.

 

[tpenfield]

The pitot channel within the outdrive may have become plugged up. I had to get a long 1/16" drill bit to pull the gunk out of my outdrive after many years of use. On the Bravo outdrives the channel runs straight in from the leading edge for about 6-8" and then runs straight down from where the pick-up tube connects.

As stated, the pressure to get the speedo to move is substantial. I have used compressed air in the past to test my the entire system.