Quoted from Manny65:

Congrats on your first pin (it won't be your last as they have a habit of multiplying) and also on a great job in identifying the issue.
When balls are continuously launched either the diverters in the subway aren't working and/or the opto switches at the VUKs or trough aren't working. Now J118 provides 12V unregulated power to the opto PCBs that the opto switches hang off, with J118-2 Grey-Yellow being 12V and J118-3 Black being Ground. Given that the black ground wire has come off the opto switches won't be functioning and the machine cannot determine the true state. Turn the machine off and try reattaching the ground wire to J118-3, note that it's best to use a proper IDC punch tool (eg https://www.pinball.co.uk/tools/molex-idc-tool-0-1/ and https://www.pinball.co.uk/tools/molex-idc-tool-0-156/ these are the 2 sizes used in the machine)
FYI https://pinwiki.com/wiki/index.php/Williams_WPC is also a great reference and helps walk you through various issues and how to troubleshoot.
Let us know know how you get on

Quoted from Eric_Manuel:

I know you are excited to play your machine. I certainly would suggest technically going through your machine FIRST. (why?) It's nearly 30 years old. And this Blog is fantastic to read, yes, all 162 pages which will answer every question you have and more. The TIE BACK MOD is a must, if it's not already done on your machine. Look for burned up connectors in the backbox. You've got a worthy and great machine to now care for, and immediately playing it will be frustrating and possibly damaging unless checking it over first.

Quoted from Manny65:

There is no reason why you can't start putting some games through your machine (once you've reconnected the 12V ground wire). On the AA batteries, immediately swap them out for some new ones (preferably lithium as they are not prone to leaking like alkaline batteries), down the track you can decide whether you want to move the batteries to a remote holder (if not already done) or replace the battery with either a coin style battery or NVRAM. As for the tieback mod, while this is good practice and recommended, if you double check the solder connection of the existing tieback wire (the thinner purple-green wire) on the drop target, there no reason why you can't power on your machine to test it and even put some games on it. The issue is that solder connections on coils fatigue over time especially due to the vibrations from the coil/plunger firing and those wires can come off, typically with most coil wires this doesn't cause an issue (unless the 50V wire touches some else) however if the tieback wire comes off on a STTNG then it'll damage the 8 driver board. So if your game doesn't have the tieback mod, just ensure for the time being that the existing factory installed tieback wire is solidly connected.

Quoted from Eric_Manuel:

There are several solutions for the Tie Back, I did them all after burning up the mini 8 driver board and locking on a subway coil (burned up). Certainly looking closely through the machine (inch by inch with a bright light) will allow you to look for burned up connectors, etc. Under the playfield, look for hanging loose wires or wires no longer attached to a nearby location.

Look for things which are broken, possibly missing (loose parts in the bottom of the cabinet). You will find several people in past blogs detail their restoration process. Yes, get those batteries OFF the board. If it doesn't have leds, get the old bulbs out. They draw power and create heat. Remove and examine each fuse for correct size. Some will fall apart as they are removed. And check for grounding straps, there should be one from the backbox down to the inside of the cabinet. Make sure you've got a grounded plug on the machine. Also the machine should have LX-7 firmware, viewable upon power up.