I’ve spent an embarrassing number of hours sorting out a player unit/ball count issue on a Williams Grand Prix and finally solved the issue. Since I never found the solution on Pinside, I thought I’d add my approach to sorting out the issue and the bit of knowledge I gained on the interrelationship between the score motor, ball count unit, and the player unit.

My issue was complicated because I had to make a number of repairs to wiring between the coin unit, ball unit, and player unit, including broken wires, broken jones plugs, and shorted jones plugs. Not to mention that the machine had no back cover, so the faded wires in the head all seem to be the same yellowish-brown color. So besides just solving the issue, I kept second guessing if I had soldered the broken wires to the correct positions.

The problem I originally experienced was a periodic failure to advance the ball count and or player unit at the appropriate time. The biggest issue was that I couldn’t figure out a pattern so everything seemed suspect. After sorting out the wiring issues above, I ensured the player unit was properly aligned and I readjusted the switches, but the problem was now reversed and the ball count was increasing too quickly.

I was still fairly convinced (and wrong) that the issue was the player unit since I had adjusted it dramatically and had already identified another short circuit. But in order to rule out the coin unit as a culprit, I set the machine to 4 players because that completely bypasses the coin unit (as seen in the wiring diagram below). It was still advancing extra balls (which ruled out the coin unit), but I finally noticed a pattern. Players 1, 2, and 3 stayed on ball 1, but on player 4 the ball switched to 2. Then when the player unit moved back to what should have been player 1 ball 2, it would advance to ball 3 and then instantly to ball 4 (but still showed player 1).

After observing this pattern a few times and observing the ball and player lights on the backglass, I realized that when the ball count was advancing from ball 1 to ball 2, that it was actually occurring after the player unit had advanced to player 4. If you look at the diagram of the score motor cams below, you realize that this doesn’t make any sense. The ball count should advance first (only if necessary) and then the player unit second.

What I realized is that I had adjusted score motor switch 1-C too narrowly so it was either staying closed or the cam had enough irregularity that it was prematurely engaging before the switch hit the divot on the cam. So as soon as the player unit switched to player 4 the circuit to the ball count unit could be completed and prematurely advanced the ball count by one ball. This couldn’t have happened on balls 1-3 because the path through the player unit can’t be completed when the coin unit is set to 4 players. (See the top three red lines on diagram below that represent players 1, 2, 3.)

Interestingly, the opposite issue was occurring when switch 1-C was too wide and the switch wasn’t consistently closing. Therefore, the ball count would not advance. However, in this scenario, since the ball count unit would never pulse, the E.O.S. switch on the ball unit would never close so the player reset relay would not engage. Therefore, even if the machine is set to 1 player, the machine will advance to player 2, 3, and 4 and get stuck on player 4 ball 1 forever since the ball unit needs to pulse to move to ball 2 and force a player unit reset to return to player 1. Again, this is because switch 1-C never closes.
Not sure if this particular machine is unique or if this is common on Williams machines, but it was unusually difficult to get switch 1-C dialed in so the ball count unit worked correctly even after I identified the problem.

Anyway, sorry for the long story, but if you are having similar ball count issues or player count issues on your Williams machine you may want to start with score motor switches rather than ruling out everything else like I did. Also, remember that setting the machine to 4 players is a quick way to rule out the coin unit and make the problem a little simpler.

williams score motor relationship to ball count and player unit (resized).png