Assuming a Delco alternator, since my CJ's had Delcos than Motorolas, Prestolites, or Motorcrafts combined...
1980 CJ's came with Delco SI series alternators.
#1, 7.5 volts when the key is 'On',
NO VOLTAGE when the key is 'Off'.
#2 should show battery voltage at all times.
That would be about correct with the resistor in place.
It would be about battery voltage if you were using a Diode instead of the brown factory resistor wire.
Here are a couple of things you might want to check first...
1. Fusible link.
The 'Batt' wire on the back of the alternator is connected to the Starter Solenoid, Battery cable side.
That is a 10 Ga. wire, and there is a safety device connected to the starter solenoid called a 'Fusible Link'.
It's about 4" long and will have a ring terminal at one end to connect to the starter solenoid, and a plastic collar connected to the primary charging wire connected to the 'Batt' terminal on the alternator.
If that Fusible Link has burned or corroded into two, then the voltage produced by your alternator isn't reaching your battery.
2. Sense or Sample Wire.
This is the 14 Ga. 'Red' wire in the two wire plug.
It does two jobs, one is 'Sense' or 'Sample' the 'Line' voltage for the alternator regulator,
The other is supply current for the alternator rotor.
This wire on AMC engines usually loops back around to the back of the alternator and connects to the 'Batt' terminal
on the alternator. This is 'OK' for testing, but Delco (the people who manufactured your alternator) and I don't recommend you run your alternator this way everyday.
On some installs, that #2, 'Sense' or 'Sample' wire connects back to the starter solenoid via a 'Fusible Link'.
That Fusible Link should be 16 Ga. and connect to the Starter Solenoid Battery Cable terminal.
You should check both wires for LOAD (not just voltage) at the alternator.
Here are a couple of diagrams that should help chase the wiring...
Here is a wiring diagram WITHOUT an 'Idiot' light,
And WITH a Sense wire 'Jumper', the way most AMC engines are wired...
Take note of the Fusible Link Location!
Here is a diagram with an 'Idiot' light, and with a Sense wire ran back to the starter solenoid...
Take note of both Fusible Links!
NOTICE IN BOTH DIAGRAMS, There is a DEDICATED GROUND WIRE!
Since every electron your vehicle produces or consumes with have to be produced by that alternator,
GIVE IT A DEDICATED 10 Ga. Ground Wire!
For quick testing, you can make one of these for under $10.
This is the two wire plug from the alternator, but set up with a Diode and a 'Quick Jumper' to the back of the alternator.
(find the diode part number in the diagrams above)
Plug this in, connect the Ring Terminal to the back of the alternator on the 'Batt' terminal,
And touch the open end of the diode to the 'Batt' terminal.
You alternator should begin to charge instantly.
If not, the only other thing that can be wrong is the wire or connector ends from the 'Batt' terminal to the Starter Solenoid, including the 'Fusible Link'.