Knotmeter not working?

[Gdindy]

I have 1990 26S, Bulkhead knotmeter, I have power to the unit, the impeller moves freely but registers no speed when underway?? I can take the cover off from inside the cabin and access it. I know their is a beige long wire that runs from the Aft storage fuel compartment through the boat and to the Knotmeter. It appears to be a 4" hole through hull design, I checked at West Marine and they do not have any knotmeters like this anymore? Any suggestions on getting this one to work??

Thanks in advance

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