You can have your alternator checked for free either in the car or on the bench at most parts stores (Auto Zone, Advance Auto Parts, etc.). Replacing the voltage regulator is a cheap fix that can last for many miles or very few miles, but worth a try if the bench test does not show a bad diode or other component. I replaced my voltage regulator once and it lasted about 12 months. If you buy a voltage regulator and it does not work - you're stuck with it as most parts stores will not refund your money.

Before working on the alternator disconnect the battery negative terminal to prevent an accidental short. I have to jack my 300D up a bit to get enough room to access the alternator, but I have worked on it without jacking the front end up. You want the belts snug, but not too tight because too tight can ruin the bearings. Your Haynes manual should have good info about R&R'ing the alternator.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.