Hopefully i can help..
What engine does it have?
Whats been checked or replaced if anything so far upto this point?
Hi..there thanks for your reply..
You need to check exactly whats coming out the output terminal on the back/wiring side of the alternator - you need a good multimeter to check for 13-14.5v roughly direct from the alternator itself..if its fluctuating direct at the alternator as per the meter - dropping in and out..then i maybe in agreement as per the alternator,voltage regulator being at fault.of course to you should check the condition of the belt and wether the alternator pulley is binding/siezed up or anything else on that belt pulley wise..only way to check that is to take off the belt and spin the pulley or each one by hand to ensure it is able to spin freely..Also check of course too the wiring condition on the rear of the alternator is tight and connected properly and shows no signs of corrosion/damage...Also follow/trace the harness back for any problems too,if all the above is ok then it most likely a wiring fault/on the charging side that needs to be traced for shorts on that wiring..A routine check of ensuring the battery terminals are secure and clean again show no signs of corrosion..and with the meter you should have at least the same as per from the alternator at the battery too..but as above theres a few checks to make before going down the route of fitting another alternator..I do hope this helps