I've been working with Tommy James on his alternator problem and the
question came up (it always comes up) - do we need the diode and relay shown
on the Mazda alternator wiring diagrams.  It appears that if you are using
the Mazda alternator (mine is Bosch rebuild) then you do indeed need the
diode (and the relay if you want an lamp indicator) in the circuit going to
the "L" (Lamp?) terminal of the alternator.  You certainly need the "L"
circuit to provide voltage to the alternator for the following reason.

With the engine off, a current from the battery  flows in the "L" circuit
closing the relay and thereby turning on the lamp (to indicate no charging
when the alternator is not running), and then throught the diode to the
alternators field coil (internally).  This current provides the "initial"
starting current for the field coil of the alternator.  Once the alternator
starts producing its own voltage then the regulator switches to using the
alternator's internally generated voltage for regulation.  The "L" wire then
has essentially equal voltage on both sides of the diode which turns it
"off" stopping the current flow through it and the relay causing the "lamp"
to go out.

For this reason, it is probably not wise to put a lamp directly in the "L"
circuit of the alternator (if it is one of the type described) because
should the lamp burn out or otherwise cause an open circuit, the alternator
field coil will apparently not get the initial starting current and will
fail to generate voltage.
Thanks to Tommy James for causing me to research this a bit more as I had
always wondered whether the "L" cirucit was needed if you didn't use the
"lamp" feature.  It turns out it is on this alternator!
.
.
Ed Anderson
Matthews, NC
RV-6A N494BW
eanderson@carolina.rr.com