T&T: Alternator Misbehavior

[Jim Healy]

My 1988 Trawler has a Cummins Diesel 4BT3.9M propulsion engine.  The OEM
alternator was a Delco unit stamped with Cummins p/n 1105170.  The
alternator failed, was bench tested, and confirmed to have no output.  I
replaced it with one that cross referenced to NAPA p/n 13-4076.  The NAPA
unit has a sticker that states its capacity at 81 Amps.  The pulley from the
OEM Cummins serpentine belt was transferred to the new unit.  The pulley
ratio is 2.95:1, so the alternator is at 5000 rpm when the engine is around
1700rpm.  My tachometers (two nav stations) are driven from the alternator's
regulator.



The new alternator seems to work just fine, but its behavior is strange when
the batteries are fully charged.  The boat has two 8D, AGM batteries.  When
they are partially discharged (I am generally able to keep them above the 50
per cent discharge point) the alternator works exactly right: over the
course of a 3 - 5 hours, the batteries charge, battery terminal voltage
rises a little (13.1 --> 14.4), and all is well.  But when the batteries
become fully charged, or when the batteries have been on a shore power
charger overnight, the output voltage from the alternator reaches around
14.5 volts, and the VDO automotive-type analog voltmeter begins to fluctuate
wildly.  Concurrently, the tachometers also fluctuate wildly, which is
disconcerting.  (Actual engine speed does not change.)  Other gauges (oil,
temp) do bobble a very little bit; but nowhere near the way the voltmeter
does.  This behavior was never observed with the Cummins OEM alternator, but
that alternator did not achieve output voltages above 13.6 volts, either,
which was always suspect.



I would appreciate thoughts and suggestions:  1) What is the cause of this
behavior?  2) Is there a problem here that needs to be "fixed;" i.e., am I
looking at something that will cause other problems down stream?  3) How
might this behavior be "fixed?"



Thanks in advance.



Jim Healy, aboard Sanctuary