After changing the fuel lines and reinstalling the fuel filter in my 86 NA, the speedometer has stopped functioning. The needle sits at the very bottom and there is no deflection. We did notice that there is one disconnected black wire hanging from a single one socket connector. The black wire runs parallel to the fuel filter. When we ground this wire to the body, the oil pressure gauge needle goes straight to the very top of the gauge. Is it possible that we dislodged this from a nearby harness, and which one would it be?

 

After unwrapping some of the wires in the harness mentioned above, we found that the unconnected black wire connects via the single socket connector to a yellow wire with a red stripe and red bands. The harness runs parallel to the fuel filter.

After doing some reading, I have found that apparently the speedometer is connected by cable (and therefore this wire dangling should not be the cause of the issue). That being said, I still want to know exactly what that wire is supposed to do?