Xs400 ground circuit going “hot”?

[Streetfighterkz]

I’m working on a 80/81 Yamaha XS400. I’m trying install a Koso TnT combo gauge. I tested the main power hook-ups and I’m getting a “hot” ground with the key on. If I disconnect the 3-wire voltage regulator, the circuit switches back to a ground circuit. Right now the harness ground is not bolted to anything as I got the bike partially disassembled.

Am I missing something? I really don’t want to fry a $200+ gauge if I can help it.

Thanks.

Later, Doug