Something I wrote several years ago. Might explain the function of the 74 Ohm resistor:
I have an 1985 Volvo Penta AQ225D (Chev V-8)
with a Paris-Rhone 14V 50A A13N1M alternator with integral
Paris-Rhone 14V YL135 voltage regulator. The voltage at
the battery with engine running is (and always has been)
only 13.9V, which results in an inadequately charged
battery even after a few hours of running. Using a
hydrometer confirms that the battery is only at
60% to 70% charge after running.
My boat sits for months at a time in dry storage at Lake
Powell. I use a regulated solar panel maintainer (set at
13.5V) to keep the battery from self-discharging. A
hydrometer check after months of storage shows that
the solar panel keeps the battery from losing charge,
but it doesn't add any charge, either. Without the
solar panel, and the high daily ambient temperatures
at Lake Powell (80 to 110 deg F), the batteries
self-discharge to the point that they wont crank the
engine in three to six months.
Common practise for long-term maintenance of Lead-Acid
batteries calls for an "Equalization" charge, which
periodically (like once a month) raises the battery
voltage for a few hours, during which the battery
charges heavily, and evolves gas so that the rising
bubbles stir the acid. This also raises the S.G.
to the 100% charged level.
Since there is no 120VAC power available where the boat
is stored, and the solar panel will not deliver more
than 200mA, the only source of such an "equalization"
charge is the alternator in the boat. In order to
recruit it, I will have to "trick" the voltage regulator
into causing the alternator to put out a higher-than
-normal voltage (like 14.8 to 15.2V) for several hours,
and then revert to a more normal running voltage of 14.25V.
The Volvo regulator is "factory" set to what I consider
too low to begin with (13.9V). Ideally, I would like
to raise the basic running voltage to 14.25V, and add
a switch which would temporarily raise the voltage
to something like 15V...
The Paris-Rhone 14V 50A A13N1M alternator has only two
terminals (B+ and 61+) which interface it with the
engine/boat wiring. The main output terminal B+ goes
to the battery positive (thru a 60A circuit breaker),
and the 61+ terminal goes through a 74 Ohm resistor
to the Ignition side of the key switch.
AFIK, the 61+ terminal is sometimes used to feed one
end of a "idiot" lamp wired just like the 74 Ohm
resistor described above. Since this boat has a
voltmeter, no idiot light is used. It appears to
me that the resistor provides some sort of "bootstap"
bias to provide field current to the alternator until
it begins producing enough output to supply its own
field current?
I temporarily shorted this resistor with the engine
running, thinking that it something to do with
setting the regulator voltage, but all it did was
to make it so that the engine would continue running
even if I turned off the ignition switch. This
indicates to me that terminal 61+ is capable of
sourcing enough current (~2A) to run the ignition
module. With the resistor in the circuit, and engine
running, I measured about 20mA flowing out of
terminal 61+ toward the ignition...
So for you alternator gurus? Is there something I
can do externally with terminal 61+ to trick the
internal regulator to increase its output? If
terminal 61+ is the (internal) source of the voltage
being sensed by the integral voltage regulator, then
I thought about connecting a power resistor from
terminal 61+ to ground, expecting that the additional
current being drawn out of terminal 61+ would "load down"
the voltage being sensed, causing the regulator to
"see" a lower voltage, and thereby crank up its output?
All other alternatives seem to involve getting inside
the alternator to intercept the internal connections
between the alternator and the voltage regulator?