There are only 3 things to go wrong.
The alternator is bad (but it tests OK?)
The regulator is bad (but it tests OK?)
The wiring that connects the alternator to the regulator & to the rest of the system is bad. (no way you've checked it all <img src="/forums/images/graemlins/wink.gif" alt="" /> )


Assuming the bench tests were done properly:
If the battery is dropping below 12 volts with the engine running:
1- check the voltage at the alternator B+ terminal. If it's normal, your wiring between the alt & battery is defective.

Try using your jumper cables to connect some metal near the Alt to battery neg & see if that helps. Then try the B+ terminal to positive. BE CAREFUL! don't short anything out while you are testing.

2-check the voltage at the alternator B+ terminal. If it's *not* normal, your wiring between the alt & regulator is defective.(assuming external regulator on an 82?)

Use an ohm meter (engine off) to check each wire between the regulator & alternator for continuity.

If those tests fail to find the problem, you now know not to trust whoever did the bench test. There are only so many possible problems in a charging system, & most of them are in the alternator or regulator....

Dave