Working my way through a Grand Prix I just bought. The biggest issue is the ball will not advance to Ball 3. I am not the best with schematics so I am trying to solve the problem by process of elimination and slowly parsing out how things are related.

I rebuilt both left and right bonus steppers and they are working 100% and counting down bonus, kicking the ball out of the left and right saucers. Switches are cleaned and adjusted on both bonus units.

The alternating relay, which hits the ball count unit to start ball 3, is definitely firing/locked on after ball 2 drains.

One of the switches on the right bonus relay is arcing a little bit after ball 2 drains, but I can't tell if the right bonus relay is the problem since the right bonus is counting down correctly.

What else to check? Ball index relay looks ok, should I inspect the outhole relay?

Thanks for any advice.

paulace thanks so much, I will review your thread. Anything to help me understand this procedure. The ball count unit on the relay board is stepping up smoothly with the plunger and resetting appropriately. Will try to observe how the ball count unit works going through ball 1, 2 etc.

The game will advance to ball 3 if I manually trigger the outhole relay. So something is not activating that relay…

Case closed. I tested a bit more, then went back to the schematics. The ball wouldn’t kick out on ball 2 or 4. So right bonus related issue was a prime suspect. I went back and checked the right bonus unit zero switch was misadjusted and wasn’t making contact at the reset position. On to the next issue