72 C20 voltage regulator question

[540ChevyC20]

Hi all,

Hoping to get some help with a charging issue I'm having with my '72 C20. It uses an external voltage regulator (EVR). For some background, I recently completed an engine swap (old 402 out, new 540 in) during which I put brand new wiring harnesses in under hood. Alternator is also new. When I start the engine, the alternator does not kick in. When I disconnect the ground wire from the EVR to the radiator support, the alternator kicks in and starts feeding the battery. That's the opposite of what I'd expect given that the EVR needs a good ground. I found this out by accident while checking for voltage at various wires (tan at the EVR plug, white at the alternator plug) as I've seen suggested elsewhere on the forum. I've also tried a new EVR. I've consulted the wiring diagram but am not sufficiently knowledgeable on how charging systems work, so would appreciate if anyone here could help me troubleshoot. Thanks in advance!

Grant