How do you disconnect the speedometer cable from the back wheel of a 94 Vulcan 800?

On a mechanical (cable driven) speedometer, the problem's the drive unit, the cable, or the speedo head itself.

You stated the cable is good, so let's eliminate that.

You can attach the inner and outer cable to the speedo head, chuck the bottom end of the inner cable in a drill motor, and spin it to see if the speedo head responds.

If not, new speedo needed.

It both the cable and speedo head are good, then it's the speedometer drive (I believe it's in the rear brake backing plate) is probably bad.

Remove it and inspect/replace.

Remove the single screw or nut that holds the speedometer cas to the tank. carefully move the case forward and release from the front or the tand. disconnect the two or three plugs and the speedometer cable and remove the speedometer case. under the case you will find a bolt holding the front of the tank. remove it along with the rear bolt. Disconnect the fuel line and the petcock vacuum line on the left side and remove the tank straight up. As you remove the tank some models have a vent line on the right side under the tank that must be removed. Don't lose the rubber knobs that hold the tank straight. they are on the frame at the bottom of the tank. Good luck.......Jim

You might want to trace the wires and see if you have any bare wires that could be vibrating around and shorting out as you ride, they would be moving unless you where riding

the timing chains on the vulcan 800 are self adjusters, you do not need to manually adjust them.

The following wiring diagram applies for Kawasaki VN800 Vulcan motorcycle series. Herein you will find complete and clear description of ignition switch, radiator fan switch,temperature sensor, radiator fan, handlebar switches, rectifier, speedometer, headlight, spark plugs, tail brakelights, signal lights, alternator and battery.http://www.mediafire.com/?eztezdedw0n

Is the actual cable square? There should be a square hole that has to line up with your square cable. Slide the square cable into the square hole, then just screw the nut back on.

the problem is with the pin in the front wheel of your bike or the wire connected to meter is not connected properly.
the pin is responsibale to move the speedometer .
It rotated at the same speed as that of front wheel and involves gear arrangements.

speedo cable runs from front wheel gear up fork leg under fuel tank to speedometer

Look at the rear wheel from right side of the bike. Lower right of the hub, you'll see a #6 metric allen bolt that is a set screw for the opening that accepts the speedo cable.