When I first brought my Roller Coaster home, it did the same thing..score motor would start as soon as it was powered on. Turned out to be a messed up switch on the coin door. Carefully look at the switches on the coin door. You might have to remove the coin mech to see them. Not sure if this is the issue, but it is a good place to start.

If that does not work, Isolate the problem area. 1. Unlug coin door from bottom board. 2. Unplug playfield from bottom board. If still cycling, you have stuck/misadjusted contacts on a relay on the bottom board. It is possible that the motor run-on switch on the motor assembly is stuck (contacts arc-welded together) or mis adjusted.

Always verify that switch stack screws are tight before adjusting motor or relay switch blades. Tighten the screw closest to the contact points first, then the one closer to the wires.

Most common!
If the score reel contact points are mis-adjusted, the game will never complete its start-up sequence! This is definitely the most common problem in EM games. It's pretty easy to identify this problem too: press the "start" button on the coin door, and the score motor in the bottom of the cabinet "runs." It never stops, and the game never starts.

The reason the score motor is running is the game doesn't think the score reels are reset to the zero position. This happens for a bunch of reasons, but usually it's because the zero position switch(es) are out of adjustment or dirty (though sometimes it can be as simple as a wire broken off the score reel solenoid or score reel edge card or zero switch, or the solenoid is dirty and sticking).

From here--> http://www.pinrepair.com/em/index2.htm#reels

All good advice above, I'd start with the coin door since it's the easiest, then check the score reels, then start looking on the bottom control board. On that control board, if there is a reset bank, there is a very sneaky switch on the bottom that you can't see, p[retty common for it to cause problems.

Good Luck

In addition to the coin door thing other causes for score motor running:

1. Score reels are not resetting or not reporting reset to 0

There are leaf switches on each score reel that open/close when the wheel hits "0". These contacts can become corroded and not signal the relays to stop rotation. Also, the score motor can be junked up and not spin properly, never getting to "0".

2. A switch somewhere in the game is closed when it should be open

If the score reels are at "0" and the motor is still turning, check switches on the playfield and see if something is stuck down.

+1 on one or more score reels being the problem.

Yes, I agree with what others have stated so far.

Looking at all the different possibilities of repairing this specific problem which is quite common for any EM pinball machine will help you considerably now and in the future and help you get to know your machine.

Ken