How trouble shoot Data MArine knotmeter?

[zippy]

My Data Marine knotmeter has not worked since I launched the boat this spring. It had worked fine in the fall. The paddle wheel spins freely and the display works, but only shows 0.0. I checked the wiring for any obvious breaks, but all looks well. Are there other checks that could help me pinpoint the problem?

Thanks

[Rick]

Don't know what model you have yet if you remove the case and use a DVM having a Hz display mode (frequency) you can measure between the black negative wire (leading from inside where the transducer wires are lead to the pcb) and the center of the coax wire. Move the paddle wheel and you should see a few Hz response on the DVM.

If you don't have a frequency mode the next best thing is to use the ac input voltage mode. Again, spin the paddle wheel and look for an ac voltage. The meter in this mode will not read accurate low frequency very well so use a sensitive scale like 100mV to look for a response.

No response indicates either a bad coax or transducer. If you have a good response then the problem is in the instrument. I have some schematics of some of the knotmeters and can help troubleshoot further on the instrument depending upon your ability to troubleshoot and repair. Otherwise you will have to send it in for repair.

If the problem is not the transducer or cable be sure to inspect the connector at the instrument as well as the inside wires of the instrument for any indication of corrosion or water intrusion and remove it with Corrosion Guard or a similar product. Do not use WD-40 or just water. These will cause other problems later even thought many mariners routinely use WD-40 in ignorance (yeah, I know that it is better than nothing just like a monkey wrench is better than not having a proper box wrench).

[Radio University]

You can also contact the guys that service the units here:

http://www.dmimarine.com/

I am sure they can help you. Steve Meleo is very good troubleshooting and repairing these units.

[Talbot]

one reason for an impellor to stop providing information is if the small metal bit falls outof the blade, as it is this which provides the pulses to power the speed calculations.

[hellosailor]

"if the small metal bit falls outof the blade, " That's a magnet, not just metal. It also pays to take a multimeter, disconnect the cable at both ends, and use the continuity settings to check for a break in the cable. If moisture gets in, that can kill it with no visible sign.

[Talbot]

That's a magnet,

No really? - and there was me trying to keep the answer as non-tech as possible. It doesnt matter to zippy if its metal or a magnet (or both!), It does if it is not there!

[hellosailor]

Zippy?

I only mention the magnet because if it HAS come out, it can easily be replaced with another magnet or a new wheel ordered. But if someone tried to fix a missing magnet by sticking in a bit of "metal"...that wouldn't help any.