Contacts on player relay clean and well adjusted?
Ball counter clean?

There is a schematic on ipdb.org of 8-ball.

Welcome, Million2one!

I don't know Williams games very well, and after looking quickly at the schematic wizardblom referred to, I'm not any better off. Just guessing the obvious here, but the player stepper (or relay(s)?) aren't acting properly, and continue to register 2 players regardless of 1 or 2 player game. Take a look at those relays? Maybe?

I have the same issue. If i am on 3 balls and hit the game start once for one player, it will go 1, 1, 2, 2, 3, 3 then game over. It never actually switches or lights the secobd player when it does this. On a 5 ball game it's 1, 1, 2, 2, 3, 3, 4, 4, 5, 5. A two player game works with no issues.

Hi Million2one
great - ipdb has the schematics: https://pinside.com/pinball/forum/topic/counts-balls-on-1-player-like-its-on-2-player#post-4843998 - I show the place where the action is shown. You play a ONE-Player-Game - You play a ball, then loose the ball - the pin should step to the next ball for player-1. See on top of the JPG, "blue lines" - the ball is lost - the Outhole-Relay pulls-in, stays pulling, "Switch on Outhole-Relay" is closed - You have played the ball and have made some points with this ball, Ball-Index-Relay has pulled-in and is steady pulling, "Switch on Ball-Index-Relay" is closed - the Outhole-Relay pulling-in made the motor to do a turn of 180 degrees, motor is running and at a time closes Score-Motor-(SCM)-Switch-2A - You did not gain an Extraball, "the Shoot-Again-Relay" is not pulling so its switch is closed - You have not yet reached "Game-Over": The "Ball-Counter-Step-Up-Coil" fires.
ALL this (mentioned before) works --- but in a ONE-Player-Game the "Ball-Counter-Step-Up-Coil" should fire TWO times. See "my brown lines" - the turning motor should close SCM-1A to do an step. Your fault is "in my brown wiring". Schematics are drawn abstract --- see the bottom of the JPG - maybe Your pin is wired like this.

Look in Your pin - does the "2nd Coin Relay" faulty pull ? (and the switch is entitled to have moved) - check for "broken off wire" along my brown wiring. Have the pin unplugged - manually actuate the "2nd Coin Relay" - look at the threebladed Make-and-Brake-Switch - blades O.K. ? use fine sandpaper and clean the contacts. Lastly: Check motor-Switch-1A. Greetings Rolf

08-Ball-Work-03 (resized).jpg