So, you need to chase some wires, but if you're patient and have the schematic, you can narrow it down.
EMs are pure "state" machines. State is set when the ball is in play. Once the ball is drained, there should be a "click" in the cabinet, somewhere in the bottom, that trips the relay for game play ball state, cycle a cam roll, and kick out the next ball or end the game. It will depend on where the ball count state is sitting.
So, some questions - was this working, and then stopped, or has it never worked? If it worked and then stopped, what changed? What area were you working on?
I find it is critical in EMs to know their sequences. What do they want to do to get into a play state, and what do they do when that state ends?
With the playfield up, so you can see the cam, or rolling brain, put it in test mode and kick out several cycles - get to know the sound of the banter. Then, look on the schematic for the hold relay, or whatever sets the state for a ball in play (I have not looked at your machine's schematic, but they all work about the same here). Once a credit is put in, and the start button is pressed, the game sets the state for all the relays to be tripped by actions.
See if you can trip, by hand without getting shocked, the relay that is wired to the drained ball sensor. Be sure to kick in a point or two FIRST - I know my two Williams EMs grant the player a free reshot if a point isn't scored and the ball drains.
If tripping the relay signals a correct ball lost and Ball 2 is cycled, then you have a contact and or mechanical/wiring problem from the ball drain stack to the relay. You schematic should tell you which solder points on the relay stack get this input, and you can now trace back.
Take the ball drain stack out of the equation, and without power, ohm out the stack's wires to the relay - does that work? If yes, then you have a bad set of leaf switches under the drain? If not, then there's a wire break, or perhaps a mechanical failure from the playfield's jack to the bottom box's jack. I find, on EMs, these jacks to be very, very tricky, easily broken, and problematic. Disconnect the jack and then short (with the game on, credit in, and point scored) the points on the bottom box jack. If it works, then the issue is from the jack to the playfield.
In EMs, I use a process of elmination like this, from the state relay backwards. If the state relay can be tripped by hand to send a new state signal, and it works, then the issue must be either a fried state relay, or at some other electrical point back to the playfield, and one has to check each stop along the way.
This is what makes shooting EMs FUN! Heck, I leave the scope and multimeter on the bench, break out the aligator clip jumper wires, the schematics, and go to town isloating things untul it all sings! EMs do sing, you just have to get to know the pace and sounds of their song, and then you'll be able to spot a sick EM from 100 yards!
Good luck,
Scooter