Diodes do not belong on an EM machine coils and must be removed.
Diodes exist on SS games to prevent the electrical back-lash of the collapsing magnetic field when the coil is deactivated. This protects the transistors which drive the coil.
When power is applied to a coil, a magnetic field develops which pull the solenoid plunger or moves the relay armature. When the power is removed, the magnetic field collapses within the coil which creates a high voltage spike. (You know what happens when you move a magnet on a coil.. your lawnmower spark plug knows).
The reverse current caused by the collapsing field will destroy the driver transistor on a SS game, burning it out and cause the coil to be permenently switched on. This reverse induced current is arrested and stopped at the source... at the coil diode.
(Incidently, if you ever get a shock from an EM coil, it won't be from the 24 volts powering it. You'll get a jolt from the reverse current as the field collapses within it.)
Diodes conduct in one polarity and short on the opposite polarity. Since EM games use AC which is a constantly changing polarity, the diode will short in half the AC cycle and either blow a fuse, or the diode itself will break down and short out (Essentially making itself a solid wire) or become open.
Unlike a driver transistor on a SS game, the electrical backlash of a diode-less coil won't effect the switches on a relay. I consider things like this a real advantage to working on EM games.
Sometimes people order or get replacement coils which come with the isolation diode pre-installed. Sometimes previous owners leave the diode in place but cut one leg. Again, it should be removed from the coil in an EM machine.