In addition to the good advice above, I like to check these items first.
There are usually five tilt mechanisms on a Williams EM.
Weighted tilt - under the playfield
tilt_weighted (resized).jpg
Ball roll tilt - Left front cabinet
Plumb bob tilt - Left front cabinet
tilt_ball_roll_and_plumb_bob (resized).jpg
Kick-off tilt - Middle of bottom mech board
tilt_kick_off (resized).jpg
Slam tilt - Inside coin door
tilt_coin_door_slam (resized).jpg
Check the first two to make sure that the switches are not closed. These are normally open 'make' switches.
Check the ball roll tilt. The ball should be toward the front of the bracket and the switch on the back of the bracket should not be touching the bracket. The plumb bob should not be touching the bracket.
Most of the Project Pin EMs I buy are missing both the ball, 15/16" on some games and 1 1/16" on others, for the ball roll tilt and the plumb bob for the plumb bob tilt. I always replace these so the pin is like the way it was when it left the factory. You don't have to, but I recommend that you do. No hurry on that though.
The last two are the most important. The game will not even turn on if these are activated.
The kick-off switch is a normally closed 'break' switch. Make sure that the contacts are making a good strong connection. Clean the contacts with Isopropyl alcohol and 500 grit sandpaper or finer.
The slam tilt is a normally closed 'break' switch. Make sure that that the contacts are making a good strong connection. Clean the contacts with Isopropyl alcohol and 500 grit sandpaper or finer.
Check the switches on the coin door. There are two types of switches, two blade and three blade switches. The two blade switch is a normally open 'make' switch. The three blade switch is normally closed and normally open - a 'make/break' switch.
expo_coin_door_switches (resized).jpg
These contacts are filthy! I need to clean them!
oxo_coin_door_switches (resized).jpg
These switches are often mangled by people trying to add games to the credit wheel. Ha! I just discovered a problem with my OXO! The wire should be in the 'up' position! When it is in the down position, like in the picture, the score motor might run forever and you likely won't know why!
If the power cord has been spliced, it should be replaced. Check the connection of the power cord to the wires on the mech board to be sure that the wire nuts are not missing and tight.