It should be impossible for the flippers to energize unless the flipper switches are closed or there's a short

For the flipper to lock on without pressing the flipper buttons the short has to be between the cabinet switch and the coil. This wire will be shorted to ground somewhere.

StratDoc I am experiencing the exact same problem on Williams Jungle Lord. I disassembled the cabinet to repair, did not remove the boards in the head and when I reassembled and turned on the upper and lower left flipper coils fire when the game is powered on. Also the left drop target coil energizes. I also disconnected all the solenoid connectors on the driver board but those coils are still firing. My game was playing fine before the disassembly.

There are 2 large molex connectors for disconnecting the p/f on a High Speed, and they can be connected in reverse. Can this happen on a Black Knight also? When reversed it will cause a lot of damage to the boards so don't just reverse them to see if things are better.

Yes they can. The connectors are USUALLY one set white and one set dyed black but never go by connector color. Always check the WIRE colors to avoid damage.

I think if you got then wrong that you'd have some smoke though... I assume the game is still booting if you take the coil fuse out?

Try disconnecting 8j3, and then find the flipper wires in those plugs, and check continuity of each side of the plug to ground to narrow the short to either the pf or the cabinet/boards

Whichever fuse stops them from locking on, just so you can make sure it's booting properly without smoking your coils

I ran into this same type of problem when trying to fix a Highspeed....It's been awhile and I can't remember what caused it. I have the game completely torn down right now...

I want to say it was something to do with a screw that was too long running through a switch on the playfield.Or a playfield screw that made two contacts....or something along those lines.

The previous owner ran a drywall screw through the leafs cutting through, making contact with all leafs, then it was too long on top of that and the end was touching a plate or pad.

I keep trying to think of what caused it to lock...but it's been a while...Not positive the screw caused that issue or another one... I know I'm not much help. But it's just something to keep an eye open for. That and coin door wires have caused funny things to happen to a machine for me. Stripped coin switch(?) wires that were twisted together were grounding on the frame. And one or two of the solenoids were acting funny. No clue why or how it caused it. But once those were fixed the coils worked again. That was on a 6803 game.

Did you check the two large connectors? If they're swapped it might not be worth repairing, but this being a Williams it could be not booting for tons of other reasons

Ouch. There is a vid post showing what all is wrong with mismatched connectors. It's not good.

These will get you started.

https://pinside.com/pinball/forum/topic/space-shuttle-wont-boot-error-code-7-williams-system-9

https://pinside.com/pinball/forum/topic/accidentally-switched-connectors-on-williams-system-9-how-bad-is-it

Don't be to sure on this.

I attribute this to poor engineering. I had a similar problem when I brought home a Stern Big Game and cross plugged the backbox; When I pressed the right flipper the extra ball light came on. I got lucky and nothing fried.

It is the first time I have experienced a wire harness where two similar Molex's are nested next to each other. Proper engineering would call for the Molex male and female connectors to be reversed for one Molex connector when two "same-count" Molex connectors live next to each other.