When it works with the wiper disk off, is the power on or is the power off? In other words is this a mechanical problem or an electrical problem?

Assuming it's a mechanical problem, what happens with the wiper disk on, and the plunger for the reset solenoid pushed in? Does it try to reset at all? Does it reset if you help it along a bit? With the reset plunger pushed in you should be able to rotate the wiper disk back and forth easily with no snags or catches.

It may be that the wiper disk is binding somehow. Here are some possibilities:

1) There's not enough tension on the return spring wrapped around the shaft. This is probably only an option if it was removed for cleaning. The return spring usually needs 3-4 turns to get the wiper disk to return to the home position from all of the other positions (not just from the ball 5 position).

2) One or more of the rivets that the spring loaded wipers ride over is worn and catching a wiper as it tries to ride over it. In extreme cases some rivets may have troughs worn right through them.

3) One or more of the spring loaded wipers on the wiper disk isn't moving smoothly in and out of its sleeve. Clean springs are nice but unless the wiper itself can slide smoothly in and out of its sleeve it may bind up and not retract enough to ride over the rivets. With the wiper disk off check each one to see if it catches at all moving in and out of its sleeve.

/Mark