69 273 headlights wont turn on

OK.
The 12.17 V at the battery suggests the alternator wasn't supplying power. And it tells us the battery is low.
It also tells us the battery has a half decent connection to the starter and the key switch.
If voltage was measured at the alternator output stud as well as at the battery we would know for sure the alternator is or isn't producing power.

In general, disconnecting the battery while the engine is running is not a good thing to do.
These cars have an ammeter in the battery feed/charge line. The ammeter will show if the battery is charging, discharging, or neither.
If the engine is running and the ammeter shows discharge, then the battery is supplying the power.


For diagnostics, start with the battery, make sure its fully charged.
Put the voltmeter probe or clip on the voltage regulator's feed (ignition) terminal.
It should be 0 V with key off, and system voltage with key in run.

IF the there is system voltage at the regulator, then check for voltage on at the regulator output.
With input voltage well below the setpoint, the voltage on the output (field) terminal should be the same.
If there's no voltage at all, the regulator is probably at fault. Could be an easy fix (sticking points) or something that needs replacing.
If there is voltage at the regulator's field terminal then check the alternator's field terminal.