I have the two player Pioneer so probably similar. The path to ADD BONUS unit coil runs thru D "500 point & Add Bonus" relay and switches on score motor 3C "Adds bonus & hole scoring" and 1A "Scoring" so that's where I'd start looking.

3c and 1a are on the score motor. A is the bottom disk and C is the top disk. The score motor has 4 stacks of switches, 1, 2, 3, and 4. There should be a little yellow sticker with black numbers identifying them.

Don't discount the step up coil itself. It's easy to take a resistance reading across the leads and rule it out. It should read the same as the other similar coils in the game.

I had an issue with the ball in play unit on an Abra Ca Dabra the other day where it would not advance. I was looking at other stuff first because you rarely see coils go but there was a break in the stepper coil winding and the coil needed to be replaced.

made that way

need a pic of the switches at "max"...

That's built in to stop it--this is a step up/step down unit right? Not a continuous rotation?...your max switch should open to stop...it may be maladjusted so it is getting opened early and stopping it early.

Maybe you are missing a tooth in the gear

I can't tell from pics--does the post sticking out of the gear open the switches in that last position? That first switch looks a little close still.

You said the missing tooth is keeping the bonus from maxing out. How close is it getting to maximum? What are you expecting maximum to be? 15 is what the game is designed for?