I fought a similar issue with a Williams StratoFlite a while back. It turned out it was a mechanical issue rather than a sticky switch or something. The re-set bar that reset the relays was just not pushing it back far enough and thereby leaving the bonus light on.

The bar on my StratoFlite was worn at some of the pivots I believe and it gave me fits trying to figure it out. In the end, at the recommendation of another pinsider who had a similar issue, I ended up putting a cable tie where the bar contacted the relay just to give it that extra couple of millimeters of throw. It never did it again after that. I see that your double bonus is on the end on the Coil side, where my trouble was the other end of the bar at the end as well. Good luck!!

Just a thought... I fought a bonus stepper on a Williams Stratoflight a while back for about a year. Bonus would count up and down but at start of a new ball it had to advance from 0 to 1. That's where mine was failing. After messing with spring tension, assembly and reassembly a billion times it turned out to be the coil.
Did it work...yes. But not strong enough. That was the catch. I had cleaned it before but it had a metal sleeve which was stuck in the coil. Once I put new coil with a plastic sleeve in it worked like a champ. Never failed again.

Just mentioning this after reading about the spring tension and finding middle ground. I did that battle too many times and never won it.

This is the thread about the coil problem.
https://pinside.com/pinball/forum/topic/bonus-step-up-coil-issue

Great to hear!!!

You can thank EMsinKC for helping me find this problem two years ago.

Just curious... When it try's to score 500 and you only get 2 or 300 or whatever.... Is the coil actually firing 5 times? Or does it fire just the 2 or 3?
It just sounds suspiciously like a bad coil/ plunger and it only managing to partially complete its job. If it fires 5 times and you only get 2 or 300....Maybe try watching it and see...

Sorry ..yes... Score reel coil. I just ask because alot of times it is just worn parts not performing properly. Just watch it and see if it tries to fire 5 times. If it does and still only get 2 or 3 hundred at least youll know where to look.

Very interesting. I'm perplexed that the reels try to move although weakly and I would assume bad coil but you are saying it resets at start of game correctly suggesting a good stroke on the plunger. Hmmmmm

If you are getting a strong reset on the reels, I guess a good place to start looking is by following the contact points for regular scoring. It is a fairly simple path. Check and clean and ensure good contact at these points.
100Pt relay Switch 1D This will send to the Player unit for the appropriate player then to the appropriate players score reel coil to step up the reel.

crop (resized).png

i think I would look hardest at the 100 pt relay 1 D first off. Seems to be the first point of commonality.

crop (resized).png