Had a similar problem a while back with a World Cup (same era) where the left pop bumper stayed energized and fried itself. Ended up replacing both transistors to that solenoid (the TIP and the 4401) on the driver board and got it to work. I recommend checking those transistors if you haven't done so already.

Link to my post: http://pinside.com/pinball/forum/topic/help-with-williams-system-3-world-cup-not-playing

#33 is the switch readout number looking at the manual. The solenoid in question should be #17 on 2J12 and the associated transistors are q2 and q1 if I read the manual correctly online. I would start with the transistors at q2 which should be the tip120 and q1 the tn4401.

Edit: Looks like the Disco Fever manual calls it 2P12 and pin 7 is the left jet bumper (#17), while the schematic refers to it as 2J12 still pin 7.