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
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).