I chased a similar charge system problem on my Mainship and finally rewired it.
Here's a link to my post re: Charging System Mods & website
w/ a summary of what I did.
I purchased the MS and suspect the boat never charged - owner only ran a few hrs on a small lake and didn't anchor out - shore charger recovered the batt'ys when returned to dock.
Diode isolators are a problem and I took the isolator out and connected Alt output directly to start batt'y (via starter wire in my case) and still no output.
Removed alt & had an auto electrical shop test it - they found the alt OK w/ good 14.2V output starting to put out at approx 800-900 RPM's
He told me the Hitachi's won't work w/ diode isolators (others have also confirmed this) and that you need 3 good connections to make the Hitachi work - Batt'y +, Good Ground and 12V "ignition" to either the R or L terminal - I forget which - it's one of the small wire terminals at 90 degrees.
When I checked my wiring I was not getting the 12V ignition which energize the rotor until the alt output & V Reg takes over.
I traced my wiring and found it was connected incorrectly through an eng rm fire system controller - after correcting this connection my alt worked fine.
Diode isolators have an inherent voltage drop at reduce the effectiveness of the charging... best to remove them - you may want to consider something like the ProMariner ProIsoCharge Controllers
Sterling ProSplit R
both are available for several multi eng & batt'y bank configurations - other TFers have reported good luck w/ them - my situation was a little tricky and I chose the Sterling CVSR per my linked write-up.
The Sailing Today article on my website is also available for download in the TF Library - Misc section - pretty good review of the options available to replace diode isolators
I have wiring diagrams for the Hitachi Alt / Reg that explains how they work if you are interested.
- Making sure you have direct 12V batt'y connection to the Bat Terminal - Good Ground and do have 12V to either the Ror L terminal
- If still no output - remove alt & have an auto elec shop test it - possible one or more of the internal diodes are not working properly
- Consider one of the alternative combiners / charge controllers to replace the diode isolator
Let me know if the Hitachi schematic would be of help I can post or email it