Hi Crowebar, Welcome to pinside!!

On the Coin Unit stepper (probably in the bottom of the cabinet), there's a toothed ratchet wheel with a pin sticking out of the side. When you add players, the toothed ratchet wheel turns so that pin moves and opens a switch. The switch should open after you've added the 4th player, but it's out of adjustment and is opening when you've added the 3rd player.

The switch has a White&Blue wire on one terminal and a White&Brown wire on its other terminal.

What Howard posted.

I've seen this issue a number of times. The manufacturer put that in to keep customers from accidentally pressing the start button one time too many and losing a credit if the game was already up to four players.

One of those 2 steppers behind the score motor is almost certainly the Coin unit.
If you'd like to try a phone call, send me your cellphone number in a private message and I'll make a first reply with a text message.

Crowebar,

You've at the wrong tree.

The switch that Howard was referring to is located on the side of the coin unit stepper that has a copper-colored gear and horizontally oriented springs. The first pic below shows a coin unit from a different Williams 4-player. It is usually the stepper farthest to the rear of the main cabinet. The switch that is designed to open after the 4th player is added is shown in the 2nd pic. Howard suggests that the switch is misadjusted and is opening after the 3rd player is added. That switch is in series with the credit unit stepdown coil, so it prevents a credit from being deducted when the switch is opened.

IMG_1194 (resized).JPGIMG_1201 (resized).JPG

When playing a 4 player game, the ball count step up coil circuit does not go thru the coin unit disc. Instead only the player unit disc advanced to player 4 will complete the circuit. Clean disc and wiper. inspect positioning of wiper finger alignment to the rivets

Crowebar, It is possible that the ball count had been failing to advance when the machine was in 4-player mode, so a previous owner perhaps hacked the switch adjustment on the coin unit to prevent the machine from going to 4 player???

PBDave likely is on to the issue. For educational purposes, check out the schematic at area D /5-6 and you'll see the circuit that goes through the Player Unit and bypasses the Coin Unit when it is in the 4-player position. Should be looking for a Blu-R wire and a R-W wire in and out of the Player Unit. Then, inspect the Owners Manual on pages 16 & 17 and you'll find that circuit on the uppermost terminal strip at lugs A & B.

Like Dave said, try to clean the rivets inspect that the wipers are centered on the rivets. If you take it apart, mark the revolving disc position with respect to the fixed board first. Also inspect for any wires that may have broken off the solder points on the terminal strip. If that doesn't fix it, the machine might have a bad wire on a Jones plug.

Lee

Glad to hear you got it going 100% and that the family enjoys the game too. Aztec is a fun game with a variety of tragets to shoot for.

Sounds like you did ok with cleaning the contact surfaces on the stepper unit. For some additional stepper cleaning tips, check out section 2D in Clay's EM repair guide. If you scroll down, you'll see some photos of the Williams coin unit switch that you just worked on. http://www.pinrepair.com/em/index2.htm#steps

cheers,
Lee