To answer a few of the comments that have appeared (all of them good),
there are separate alternators for the twin engines; they always respond
together, i.e., they both work or they both don't work; we have tried
running a stand-alone ground wire from the negative terminal of the
battery to the engine (and have separately measured the resistance of
the existing connection and found it to be near-zero ohms; our tests
continue, and we continue to think there is some grounding issue, but so
far, we're not any closer to a solution than before.

Thanks for the comments! They generate discussion that may lead to the
answer.

J. Morrin

J. Morrin wrote:

My partner and I have a very tough problem that's confusing 2
experienced mechanics and both of us. We have twin Cat diesels each
with a motorola alternator, and one of the alternators is brand new.
Here's the problem: We fire up one or both diesels and notice that we
are getting no battery charging - nada. We shut down the engines, and
check everything (we've already replaced any of the batteries that were
even close to being questionable and, as I said, we even bought a new
alternator). We re-start, and sometimes the gauges show both
alternators functioning and putting out close to 14 V and functioning
normally. We'll shut the engine down, re-start several times, and all
seems well. Forty minutes later we start up and neither alternator is
charging. As you might guess, the thing that has us confused is the
intermittent nature of the problem. (We've never had an occasion where
the system changes behavior while it is running -- it if works when it
first starts it continues to work, and if it doesn't work when first
started, it "stays" not working.) If it were the alternator diodes not
functioning, it is my understanding they would either work or not work,
and ditto on any number of other hypothetical problems. I guess I have
to admit that I can't imagine how the circuits could be mis-wired or are
shorting so as to create this kind of behavior. I don't feel too bad
yet (other than total aggravation) because the diesel experts claim
they've not seen anything like this either.

Any ideas out there? This is one real stumper, and I'd like to know any
possible explanation for this behavior.

Thanks.

J. Morrin