speedometer all wonkey

Korkie

Korkie

Active Member
Feb 2, 2016
1,331
Leonardtown, MD Potomac River / Chesapeake Bay
Boat Info
2006 340 Sundancer, Garmin 7610, xHD 24 Radar, VV703, SeaStar Hydraulic Steer, Shadow Drive AP
Engines
Mercruiser 496 Mags
Bravo III Drives
2006 340 496mags with Br III drives. Speedometer at times reads fine and works properly, then it will just peg at 70 and stay there, other times it will stay at 0. Changes between these three readings while out. I am pretty sure the info comes from the stbd outdrive but have no idea how to trouble shoot. Any insight would be appreciated. Thanks, K
 
I plugged the speedo pickup on my outboard and installed a Faria GPS speedometer. Loving it.
 
Even though it's a larger boat, you're probably correct in that it still uses the pitot hole in the stbd drive. Otherwise, you would have a paddle wheel attached to the transom.

Typically, the issue with a pitot pickup is that it gets clogged with debris from the water. Using a small drill bit, by HAND, ream the hole out. The hole goes in a good inch or so and you won't damage anything by hand - just keep twisting/pullng back out/repeat till you feel the bit bottom out (can be done while boat is in the water).

From there, follow the leading edge of the drive upwards and in the 'cavity' above the anti-ventilation plate there is a quick disconnect/plastic hose for the speedo. You can disconnect there and further trouble shoot and check that the pathway is clear.

If you're not familiar with how a basic pitot/speedo system works, it's actually quite simple. Starting at the pitot hole in the drive there is essentially, a tube all the way to the speedo gauge. As water enters the pitot hole, the air in the tube gets compressed, which in turn spins the needle. It's that simple. Knowing that, it might make it a little simpler for you to understand and diagnose.

However... you have an extra part to this system in your boat since you have Smartcraft gauges. Your speedo is controlled by the Smartcraft guage, which gets a signal from a pressure sensor. The pressure sensor, which could also be your issue, is located at the top/back of the engine (you KNEW it wouldn't be located right up front, correct? ;) ). The water/air from the pitot hole in the outdrive stops at this sensor. from here on it is all electronic, but you can blow backwards from the sensor (and out the outdrive) to verify a clear pathway, as well.

Things to check...

-Make sure the sensor/wires aren't located too close to spark plug wires. Verify wires are intact. Clean contacts.
-Since you used the word "pegs" I assume the MPH you are referencing is via the analog speedo gauge. Check the digital speed readout in the Smartcraft display. Your analog speedo gauge is dumb - it simply does what the SC gauge tells it to (as does your other analog gauges). If the digital speed is correct, the issue is between the SC gauge and the analog gauge (and the wires/connections). If the digital speed is wonky, as well, try resetting your SC gauge. I 'think' there is a calibration that can be done - but that's more intricate than I recal from memory - you would need the SC manual for that.
-Not always, but often, a warning icon/message would show up in the SC gauge about the pitot circuit or pressure, indicating a fault with the sensor. Short of getting the system looked at/scanned by a Merc tech, the last thing I would do is to just go and replace the pitot sensor - especially since all of the other things are free.
 
Lazy Daze said:
Even though it's a larger boat, you're probably correct in that it still uses the pitot hole in the stbd drive. Otherwise, you would have a paddle wheel attached to the transom.

Typically, the issue with a pitot pickup is that it gets clogged with debris from the water. Using a small drill bit, by HAND, ream the hole out. The hole goes in a good inch or so and you won't damage anything by hand - just keep twisting/pullng back out/repeat till you feel the bit bottom out (can be done while boat is in the water).

From there, follow the leading edge of the drive upwards and in the 'cavity' above the anti-ventilation plate there is a quick disconnect/plastic hose for the speedo. You can disconnect there and further trouble shoot and check that the pathway is clear.

If you're not familiar with how a basic pitot/speedo system works, it's actually quite simple. Starting at the pitot hole in the drive there is essentially, a tube all the way to the speedo gauge. As water enters the pitot hole, the air in the tube gets compressed, which in turn spins the needle. It's that simple. Knowing that, it might make it a little simpler for you to understand and diagnose.

However... you have an extra part to this system in your boat since you have Smartcraft gauges. Your speedo is controlled by the Smartcraft guage, which gets a signal from a pressure sensor. The pressure sensor, which could also be your issue, is located at the top/back of the engine (you KNEW it wouldn't be located right up front, correct? ;) ). The water/air from the pitot hole in the outdrive stops at this sensor. from here on it is all electronic, but you can blow backwards from the sensor (and out the outdrive) to verify a clear pathway, as well.

Things to check...

-Make sure the sensor/wires aren't located too close to spark plug wires. Verify wires are intact. Clean contacts.
-Since you used the word "pegs" I assume the MPH you are referencing is via the analog speedo gauge. Check the digital speed readout in the Smartcraft display. Your analog speedo gauge is dumb - it simply does what the SC gauge tells it to (as does your other analog gauges). If the digital speed is correct, the issue is between the SC gauge and the analog gauge (and the wires/connections). If the digital speed is wonky, as well, try resetting your SC gauge. I 'think' there is a calibration that can be done - but that's more intricate than I recal from memory - you would need the SC manual for that.
-Not always, but often, a warning icon/message would show up in the SC gauge about the pitot circuit or pressure, indicating a fault with the sensor. Short of getting the system looked at/scanned by a Merc tech, the last thing I would do is to just go and replace the pitot sensor - especially since all of the other things are free.
Click to expand...

wow...great write up.
 
Thanks Lazy Daze - Will check all this out this weekend. And yes - very nice instructions. Only thing better would be if you stopped by and did it for me. lol
 
I used to struggle with the speedo's on two prior boats that had the pitot tube. Constantly clogging.

In this day and age...I would not spend any time repairing it. You can get a very free speedo app for your phone. Nice big numbers, easy to read, 100% accurate all the time. Get a suction cup ram mount for the helm and be merry.

I've got an app on my phone called (shockingly) "Speedometer". Just numbers, nothing fancy, super simple.
 
I have speed displayed on my Garmin 7612 so seeing actual speed is covered. The issue with the SmartCraft not functioning reading properly significantly throws off things like distance to empty, usage, etc. When the speed is pegged at 70, the distance to empty with 120 gallons is like 1200 miles. One could only wish this was true. Will clean as described above and go from there. Thanks for the input :)
 
Korkie said:
I have speed displayed on my Garmin 7612 so seeing actual speed is covered. The issue with the SmartCraft not functioning reading properly significantly throws off things like distance to empty, usage, etc. When the speed is pegged at 70, the distance to empty with 120 gallons is like 1200 miles. One could only wish this was true. Will clean as described above and go from there. Thanks for the input :)
Click to expand...

That makes complete sense. Sorry to over simplify! Nice work on that mileage though....10mpg is quite impressive for a 340 :)
 
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