Jay is right probably. I used a Hall Effect Sending unit and it had 3 wires. Battery, Ground, and the Sending wire to send pulses to the speedometer. Once it got a break in the insulation and it did the same thing yours is doing so I got a little heavier wire and ran 3 new ones and then fixed them so they couldn't rub or be hit by any flying debris from the ground. And as I said above, it is still working perfectly.
Ron
