Vehicle Speed sensor disconnected
This question is very wide. But, I would suspect the speed sensor.
The fan relay could be bad or the fan motor could be bad.
If speedometer itself is working then gears inside the assembly are damaged.
If the speedometer needle is going crazy and not working properly, it is probably the speedometer mechanism that needs to be replaced. It could also mean the vehicle is having a transmission slippage problem if the needle is bouncing back and forth.
The VSS (vehicle speed sensor) may be at fault.
If the speedometer and tachometer are not working in a 1994 Nissan Kingcab 4X4 V6, it could be that a fuse is burned out. It could also be that their mechanisms in the undercarriage are corroded or dirty if the vehicle was used for 4-wheeling off road.
May have replaced the wrong sensor or you have a bad wire in that curcuit. gm Jim
My knee-jerk answer would be a TCM (Transmission Control Module) problem, but have a shop hook up a scan tool and they can tell you what is wrong.
Your speedometer may have failed, or the speedometer cable that connects to the transmission, or the plastic gear in the transmission may be broken.
The speed sensor is bad.
Speedometer gives a wrong reading for Yukon XL 2003?
it could have a bad speed sensor