Low voltage at gauge, no charge?

The reason I always (if possible) suggest checking power with the key on, is in case of a bad switch or in the case of a headlight switch, the built in breaker

It is very possible you could check power at the B1 terminal and it's OK (you think) but then with the switch on, no lights!! Could be the switch is bad, or could be a bad connection in the wiring BEFORE the switch is droppin power off when you try to load it.