Cal,
Doug is right, the direction of rotation should not matter apart from
cooling. The output voltage is determined by the regulator which unless
you have a fancy external unit fitted (for smart charging etc) should be
fixed at around 13.8 volts. Overheating of the alternator could cause
this to vary but would not be immediate from cold and should be
noticeable as it warms.
I assume the rebuild included a test which should have checked the
output voltage under load. After all this is the most important thing to
test on a alternator. Ask the guy who rebuilt it what its output voltage
was under load. If he was not a marine guy it could be he made an
assumption that 12V was correct.
What is the voltage on the terminal without the engine running? Is the
12V a battery? What is the load on the alternator. If the load is
sufficiently high (more that the rated current of the alternator) it
could be that the regulator cannot maintain the 13.8 volts which would
be normal behavior. Could it be that you have a dodgy battery that is
boiling away? This could happen if one of the cells is has become an
equivalent of a short circuit. The battery's remaining cells will never
reach 13.8V instead it will boil or vent.
Don't just disconnect the load or battery! This apparently can be bad
(although I am not sure why they cannot be built to cope with this and
it could just be myth). Can you check the current flowing from the
alternator? This can be tricky since ammeters don't usually range high
enough. If it is very hot to touch that would clearly indicate that it
is under high load.
One trick is to test the voltage across the wire between the terminal
and the battery or whatever the first point of call is for that wire. It
should be a relatively low voltage even if at full load (<100mV). You
can get an idea of current by measuring the length of the wire and
looking up the resistance per meter (loads of web sites with this info).
You divide the voltage across the wire by the resistance of wire and get
current.
i.e. (metric values) if the wire is 25mm^2 copper the resistance per
meter is 0.0007 Ohms. If you have 10mV across such a wire or length 0.5
meters the resistance is 0.0007 * 0.5 = 0.00035. Therefore the current
is 0.01/0.00035 = around 30ish Amps.
It is only rough but you will get an idea of the current if there is
any. If this is near the rated output of the alternator you don't have
an alternator problem.
It may not even be a problem! My own boat has a funky multi-stage
charger that results in exactly what you are seeing during bulk
charging. Looking at the alternator you see a lower than should be
voltage produced by the charger loading it as much as possible. The
charger boosts the voltage of the resulting current to charge the
batteries.
The only other thing that occurs is that it could be a wiring loom or
connector problem. What other connections are on the alternator? You
didn't say what alternators you have. Have you tried swapping them over.
