I think it's the "A" relay in the bottom of the cabinet that controls the GI in the backbox.
The three relays are A,T & Q T is the Tilt Relay and Q is the Game Over Relay.
I think the "A" relay is usually closed and when energized it turns off the backbox. Controlled by Q26 on the Driver Board.
I've had a few Sys 3's and one of mine had a bad coil. I replaced the whole thing. Check Pinball Resource for parts.
G'luck