The numbers in boxes are wire colors. These are coded, and there is a legend printed somewhere on the schematic.

First digit = solid wire color
second digit = tracer (little line stitched in)
Dash
Third digit = number of times this color combo has been used previously.

The first and second are for identification, and the third helps you understand that a wire --over here-- isn't the same as the one you're looking at --over there--.

If the start button does nothing, what happens when you hit the coin switch briefly?

As a Williams, there should be a limit switch on the credit unit (in the head) that tells the game credits are at max. Jumpering or adjusting this switch to closed will give you free play, if that's what you're after.

One thing to look at first are the switches mounted to the score reel - if the game actually started and scored correctly, it's also possible that the score reel (plastic) is not aligned properly. Check the zero position switches - when zero is displayed, the switches will actuate in a particular way. Look at the neighboring reels that DO show zero and it should be apparent.

Your game over problem is related either to the ball count unit or the game over relay (if this game has one).

Yes, if there are no credits on the credit reel, then the start button is effectively useless.

If you have your Jones Plugs set so that one coin = multiple credits, then it should put more than the initial game up on the credit reel.

Otherwise, it auto-starts - single player - why wait? The button is for playing replays, which is why it is labeled as 'replay button' on the schem.

If you want it to be free play, you'll need to observe the credit reel - you'll see one to two stacks of switches mounted to the backside of that reel.

If you manually press the credit add solenoid plunger (press one and look from the front of the game to make sure it says '1' - if it doesn't, press the other). When the credits tick all the way up, they will actuate one of the switch stacks. This is a configurable limit - so it might be 18, 24, 20, or other.

When the switch stack is actuated, you'll see a switch in the stack closed - this is the switch to jumper if you want free play.

The credit add and subtract solenoids will continue to fire as normal with this method, and you can reverse it easily if you choose to do so.

That means that either you have a switch that is out of adjustment on your score motor, or, more likely, the reverse issue to setting the machine to free play - the zero limit switch is being hit and telling the machine that there are NO credits on the machine.

I'll take a peek at the schematic snippet and see if it's there.

Yep it is - look at the top part of the schematic (farthest left portion).

That is the 110V section - you have a coil marked the Replay Relay - work your way down from that coil and you'll see the switches in line...

1) Lock Relay Normally Closed switch OR Game Relay Normally Open.
2) Replay unit zero break switch (that's the likely candidate mentioned above)
3) Replay switch (the button itself).

Check those and see if you can follow what I called out. Make sure you check with the game off as these are 110V switches. If your game does not have an off switch (it's probably just the kickoff switch) - unplug it before checking.

Whoops - yep, EMsInKC is correct regarding free play. My mistake.

The switches I mentioned above are in line with the replay relay (that lets you play a replay), and the stepdown coil on the credit unit is pulsed by the 4b switch he mentions above.

Except when he's wrong! Sorry for the confusion.