Well my i5 doesn't have a "northbridge" voltage to monitor, however, with turbo on it's basically like enabling C6 state (or maybe C3... anyway...) which works by completely shutting down cores that are unused. So in idle you can expect no voltage. This is just a power saving feature.
I kind of have it working but the other way around, turbo is off but I'm using the x21 multiplier, and I enabled C6 state... which then enabled turbo. Seems like they're pretty much the same thing.