There is a cup and a magnet in there. A spring to return the pointer to 0. The cable spins the cup. Magnetic coupling provides the torque to act against the spring. The faster the cup spins the more torque is available to move the pointer. So the way to check it is, pull the cable from the back of the speedo head. Pull the inner cable out and see if it is damaged. The ends are squared off to engage the drive at the trans and the cup. Are they good? is the inner cable nice and smooth? If yes to both then lube it with some light grease and replace in to the outer housing. Slide it all the way back and twist it with your fingers until you feel it engage the drive gear in the trans. Push it in to fully engage. There should be enough sticking out to engage the speedo drive. That drive is the back of the cup. There is a way to lubricate the head but a little WD40 will work, squirt a bit into the place the cable goes to and reattach the cable. Drive it a see what happens. You might need to pull the head out but at least you will know that the cable is OK. If you put the inner cable in and when it engages you can still turn it the problem is the drive gear at the trans. Now you can start at the right end to fix it I hope. Enjoy.

Steve H