Go to www.google.com and do a search on "testing alternator". There is a good check sheet for testing the alternator, battery, etc. If the mileage is not excessive on your 89 300E, I would suspect the regulator which is held in the back of the alternator by two screws. Costs about $40.00. Just did mine. If you need a new alternator go to http://autoexpress.safeshopper.com/125/cat125.htm?602
John O