there are a pair of wipers on the player unit that close a circuit at ball 5 (or ball 3) to aid in declaring game over. When its in that state press two fingers on these wipers and see if completes. if it does, clean them up

Here are the Relays and Switches that run the motor. Start by finding which one is closed at the end of the game.

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Hi Enaud +
HowardR shows the snippet "relays making the motor to run" - I guess in Your pin the "O-Ball-Return-Relay" is constantly pulling and the fault is "the Coin-Unit is faulty not stepped down" - so the Q-Game-Over-Relay cannot pull-in. Or then, maybe the Coin-Unit is stepped down and just the activation of Q-Relay does not work (?).
Please find in Your pin the BX-Last-Ball-Relay - in an 1,2,3,4 player game: The last ball on last player is played, lost - enters the Outhole - bonus is counted down , the O-Relay pulls-in - NOW the BX-Relay does actuate (in Your pin ?), the Coin-Unit is stepped down (single steps) (in Your pin ?) - then the Q-Relay actuates - well, should actuate.

I like http://www.pinwiki.com/wiki/index.php?title=EM_Repair and I like http://www.pinrepair.com/em/index.htm --- in there is a great video about the AX- etc. Interlock-Type Relays - AND something that puzzled me every time I watched the video - a hint to make me look-up in the schematics of "Jet Spin". Watch in the video http://www.pinrepair.com/em/index3.htm#axrelayvideo - (time) 1:04 --- a bit of "July 1977 Jungle Queen" --- 1:15 to 1:27 he steps up and down on the COIN-UNIT - SINGLE STEPS - 1:26-1:27 he steps DOWN, SINGLE steps. This always puzzled me as I am a "rather a Williams guy used to TOTAL-Resetting the Coin-Unit".

Want to try (?) - wear rubber-gloves and/or do not touch electrical stuff, solder-lugs --- when Your pin starts doing the fault: MANUALLY step down the Coin-Unit (I believe there is an Zero-Position on the Unit so in a One-Player-Game You should be able to step down a step).
Maybe the pin gets to "Game-Over" --- well, the O-Relay must be pulling and closing its switch with wires-soldered-on "Black-Yellow" / "Brown-White-Red". Also the BX-Relay must have actuated and therefore have closed the switch with wires soldered-on "Brown-White-Red (((connecting wire from O-Relay)))" / "Maroon-mingled-with-Yellow". Greetings Rolf