Does your Player Up Unit advance when the ball drains in a multi player game?
Fireball Ball Count (resized).jpg
On a one player game pulses come up through the Score Motor 3B switch to advance the Ball Count Unit at the end of each ball. The Coin Unit is in the position shown in the schematic and the Player Up Unit isn't involved. On a multi player game the Coin Unit wiper has advanced to another position depending on how many players have coined up. In that case the pulse has to get through the Player Up Unit and through the Coin Unit (except in a 4 player game).

If your Player Up Unit doesn't advance when the ball drains pulses never get through the Player Unit to the Ball Count Unit.

Williams used essentially the same circuit. More explanation of how it works is in this topic: https://pinside.com/pinball/forum/topic/williams-grand-prix-revival

/Mark

I think both are shown in their reset positions although the contact board on the Player Up Unit might be rotated slightly so that the wipers are more centered above the contacts. Also note that one and probably both springs on the Coin Unit have been mangled and should probably be replaced.

If multiplayer games aren't advancing the Ball Count trace a path through the two units shown in reply #2. Unplug the game, reset both the Coin and Player Up Units, then manually advance each unit one step. That should put them at Player 2 in a 2 player game, ready to advance the Ball Count when the ball drains. With your meter you should be able to trace a low resistance path (of an ohm or less) from the grey-red wire into the Player Up Unit, to the white-brown wire between the two units, to the orange-white wire between the Coin Unit and the Ball Count Unit step up solenoid.