The first thing I would do Ian is check the connectors on the alternator. I went through the process of taking my alternator off and finding an alternator shop in Tahiti, got charged $30 for a simple test to be told it was fine.. and it turned out to be dirty connectors on the wires that just needed a bit of a clean.

I had a battery issue crossing the Atlantic. I switched off everything, disconnected the individual house batteries and tested each one. One was dud but the other four were pretty good. I reconnected excluding the dud battery and all was fine again. I do not know enough about alternators to advise on that issue.

Ian

Ocean Hobo SN96

Ben,

I am not very familiar with the wiring of a Maramu, but I am guessing these are both 12V alternators feeding a 12V battery bank?

Is this the original engine?
How long have they not been working?
Did they both die at the same time?  While the engine was running?  
Can you describe how you did the measurements you are making that tell you the "output voltage" and the "voltage supply"?
On the alternator with the external regulator, can you measure the Field voltage coming from the regulator?  (Usually the terminal will be labeled F, or FLD, or DF, or EXE, or 67)

If you are measuring across the B+ and B- terminals of the alternators and you get very low readings that are DIFFERENT from each other, that strongly suggests they are disconnected from the batteries.  A loose connection?  Or maybe a common fuse? Are there any other electrical problems you notice?

If that is the issue, you might have bigger problems.  Running an alternator that is not connected to a battery can destroy the diodes.  

There might be a lot going on here, especially when you say your batteries are "busted." How bad are they?  It is also difficult ot be sure especially on an older boat where many wiring changes could have been done by previous owners of varying degrees of skill.

Bill Kinney
SM160, Harmonie
Port Louis, Grenada
http://cruisingconsulting.com

Hi everyone, 
It looks like we have yet another problem with our maramu.. Tonight as we turned on the engine we realised that both alternators (the original one on the engine and a bigger one which was added later I guess) were not charging the batteries. When I checked with a voltmeter, the output was 7V for one and 1.4V for the other one?!? Anyone has any idea where to start with this?
The bigger alternator has a brand new charge controller. 
Our batteries are busted and were meant to be replaced tomorrow.. 90Nm away from where we are.
We have been running the shaft alternator a lot over the past few days (I still didn't figure out how that one is wired to the system). 
We have been running the engine and therefore the alternator a lot over the past few days as our batteries a no longer holding charge. 
The voltage supply to the alternator is 12.4V

Anyone has an idea? 
Cheers 
Ben