When a score motor keeps running you would first ask the following questions:
Are the score reels completely resetting?
Are any of the coin switches stuck in the closed position?
Are the stepper units completely resetting?
Problems with these items would include mechanical issues (sticky disc, worn or broken parts) or a problem with the circuit that sends power to the reset solenoid... If these items were working, then you would move to the schematic that showed the circuit that included the score motor.
(The following assumes that someone has a basic understanding of how to "read" an EM schematic and how electricity flows through EM pinball circuitry.)
Then you would ask the next question: "What switches are sending power to the score motor?"
Gottlieb EM machines have a group of switches that I call "primary switches." These primary switches are the only way power can be sent to the score motor. Let's look at a popular Gottlieb EM machine from 1976 called Royal Flush. This machine has ten primary switches that can send power to the score motor:
C, D, E, G, O, P, S, W, AX, 1C
So, whenever you have a constantly running score motor, you would want to examine if these switches were clean and adjusted properly. Also, you would check the switch stacks for possible shorts. If everything looked good, then you would want to observe what switches were actually closed when the score motor was running.
Let's say the E switch was closed when the score motor was running. Then we would look at the "second group" of circuits and switches that sends power to the E relay coil. There are seven switches that are in the circuits that send power to the E relay coil:
Bounce switch - needs to be closed to supply power to the circuits / not applicable
Anti-cheat switch - needs to be closed to supply power to the circuits / not applicable
Score motor switch 2B - might be mis-adjusted and stuck in the closed position
E relay switch - lock on switch / activated when power reaches the E relay coil /might be mis-adjusted and stuck in the closed position
Score motor switch 1C - might be mis-adjusted and stuck in the closed position
Ball return switch - might be mis-adjusted and stuck in the closed position
A relay switch - might be mis-adjusted and stuck in the closed position
If everything looked good with the second group of switches, but you saw A relay switch was closed, which is a normally open (NO) switch, then you would look at the "third group" of circuits and switches that send power to the A relay coil.
As you work your way through the different groups of circuits and switches - primary switches, second group, third group, fourth group, etc - it gets less likely that the problem is a result of switches in the third group or proceeding groups since it would be less likely that multiple switches would have problems at the same time. From my experience, when you have a constantly running score motor, the problem is with the primary switches or second group switches.
If available, the game operation sequence or theory of operation can be helpful in finding the problem. Other clues can be seen on the backglass or card tray apron - Is the game over light on or off? - Is the match light on or off? - Is a ball in play light on or off?
Using the schematic and examining the primary switches is the key to solving a problem with a constantly running score motor.