Is the switch working? Does it fire in test?

Try testing the u3 chip on the SDB. https://pinside.com/pinball/forum/topic/ca3081-pinout-drive-transistor-number-table

Q12 are pins 9 and 10 on U3

So red lead on pin 10, then measuring 9 and 15?

I'd say if you can confirm the whole circuit between the coil and the transistors, then keep moving towards the mpu. Maybe even try swapping U10 and U11 on the mpu to start. When I fist got my Harlem it had bad pia chips that were causing strange solenoid behaviors.