1988 should have internal voltage regulator(in alternator)
There should be a 3 wire connector on the back of the alternator
1 white short jumper wire
2 Yellow wire
3 Green wire
Key off
Green wire should be 0 volts
Yellow wire should have Battery Voltage, if battery has 12.4volts then yellow wire should have 12.4volts
If not then fuse or fusible link is blown
On the back of the alternator is the B+ connector, 1 or 2 larger wires connected
You don't need to disconnect these, but test for voltage, should have Battery voltage same as Yellow wire.
If not then Fuse or Fusible link is blown
Now turn key on
Green wire should now show battery voltage, only with key on.
If all test as said then start up engine, Battery should now show 14+ volts, 14.5-14.9 volts if alternator is working.
If is doesn't then replace NEW alternator, it is bad
Just FYI on the charge light
A 12 volt light bulb will light up if you give it 12volts and 0volts(ground), if you just give it 12volts then it won't, if you give it 12volts on both connections it won't, has to have a high and low volt side to light up
The green wire on alternator runs to the Charge light bulb in the dash
The other contact on the Charge light bulb is connected to ignition switch
When you turn on the key 12volts will run to the bulb and out the green wire to the alternator, if it is plugged into the alternator and alternator is not producing voltage(engine off) then it is a Low volt, so charge light lights up
When you start the engine and alternator is working then charge light has high volts on both connections so is OFF
The green wire is more than just the charge light though, it is actually the ON/OFF switch for the alternator.
An alternator would drain the battery when engine is off if it wasn't shut off.
The green wire provides "startup power" for the alternator, and shuts alternator off when voltage is cut on that wire.
If a charge light flickers it means voltage is dropping low on one side of the bulb, below battery voltage, so alternator is not producing full power