Check J6 on the interconnect board, it's probably crispy

System 11 machines have a big issue with the GI over heating J6 to the point of actually burning not only the connector but the board itself.

Someone may have replaced the connector and left the board damage.

Take the board out and check it out, if it's black replace the board.

Sure you can try to hack it, solder on wires and all that crap, you're better off replacing it (they weren't built to last)

If a visual inspection passes, reflow the header pins on the whole board, or seriously replace the board! it's a little over 100 bucks, a fraction of the machine cost and it will up the resale value.

I won't pay top dollar for a machine unless I go through the back box and verify that it hasn't been hacked to bits. 100 to 200 off for each hack.