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Matt,
A '77 most probably has an external regulator unless the replacement had an internal one, as I was told all replcements will have. If there is a small ceramic plate held in place with two screws at the bottom of the alternator, you have an internal regulator. If there are wires running up to the fender into a metal box, you have an external regulator. Don't be surprised if a rebuilt only lasts a year. My experience with them over the years is that they are not consistant in quality. If you need a replacement you will probably get an internal regulator so just forget about the one on the fender.
Check battery for 12.5 volts with engine off, then perform the following tests:
You can hook up a voltmeter (+) to the B+ terminal (the thick red wire) on the alternator and the (-) to a ground. At idle you should read 13.5 volts (approx.) and should increase with revving of engine about another 1 volt. Adding load like blower motor and headlights should reduce output but revving should increase it again up to about 13.5v. All readings should be steady, if they are alternator and voltage regulator are ok. If the voltage doesn't come back up after load is added the regulator is probably bad. If the voltage fluctuates at idle my guess is that the alternator is bad but see if it fluctuates at a higher voltage also. If it does I don't know what the cause could be. You might want to run the above idle test with the B+ wire disconnected to eliminate the car wiring as a cause.
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