is the alternator turning the right direction, it will of course work either way
but the fan is optimized to run in one direction only,

its possible you have toasted a diode or two, but these units seem very durable to
some hard work, i know i have beaten mine have to death in testing, and it is still doing its thing.

another thought, are you using the oem regulator?
if so as the alternator gets hot it will derate the volts and in turn lower the amps
as well.

maybe your load if it is batteries are getting charged up enough to where they only need half of what you originally could get out of the alternator?

are you sure of your meters accuracy?  is it possible the alternator is putting out
much more than your meters are telling you? i know i have had all sorts of issues
with meters and getting accurate readings, most especially on the dc amps using a shunt,

all else fails , take it off and take it in for testing, usually alternator starter repair places will spin it up and do a full test for no fee, or only a slight fee.

the rectifier bridge in the new series 555jho has the diodes in a sort of harness
assembly that welds the leads in rather than seperate wires that you could simply part from the stator to check each individually, so its hard to make sense which diode you are checking and which you aren't with these new rectifiers.

i highly doubt you have a rectifier problem unless you have been running it backwards so that the fan cannot pull in enough air and been running it at continous high amperage of over maybe 120amps, which is doubtful with you ratio.

its possible that when you tipped up the deck to do the work, maybe some crap got dislodged in the float bowl and is now caught in the main jet? starving a bit on fuel will certainly cut power out of the engine and result in power being down on the alternator.

do you recall any short circuits while running? if it happened you would recall them because they are very hot indeed.

people use alternators like these to run 3/32" welding rod, so its hard to imagine
how you might have hurt it,, however

because you are using a reostat, the field will stay high for heavy loading ~120amps
if you suddenly removed the load, or lost connection the voltage will spike to very highvoltage, the diodes these new alternators are avalanche type and will clamp to ground at ~30vdc,
this doesn't seem to hurt them intermittently, but it might if it last for very long or at very high voltages,, its possible then to have damaged one or more of the rectifiers.

if this turns out to be the case:

let me know, as there are replacement diodes available for that unit that can handle
more current than the oem's and are not avalanche type, and have a PIV rating of 200volts. they are quite reasonably priced, but require removal or your diodes from the heat sinks, and press in the new ones, make up new leads and solder in, etc.
takes a bit, but is a good fix.

had you kept the regulator in place it is fast enough to save the rectifier in case of a rapid load dump or loss of load.

getting ahead of myself here, first things first
gotta find out whether its the engine thats down on power or a failed rectifer

so ask yourself these questions

did i have a short?

did i break connection to the load while the alternator was putting out high current?

if it turns out you are turning backwards there are straight cut bidirectional
fans readily available, and i understand the optimized reverse direction fans
on special order.

if you choose to use the straight fan, limit the current to no more than 140amps
cold and about 120hot and you will be golden,, the straight cut fans were for prior
versions up to 140amp units.

if you can speed up and now get full power, i am thinking you have an issue with the motor. usually if the alternator has lost a phase speeding it up makes very little difference, unless you speedup a couple thousand rpm at the alternator.

it is very unlikely that you have smoked the stator, not impossible but very unlikely. more likely a diode turned loose if the alternator turns up to be the problem.

this problem is why i am not crazy about using a reostat to control the field
as the only control handle for these high output alternators, it requires a lot of babysitting. a controller such as the balmar would have an optional alternator temp sensor and it would have caught the unit and derated it to 50% if it overheated.

this is assuming of course its the alternator and not an engine problem.

and that is a big assumption at this point,

ok now another question

how fast were you running the engine in the first tests, 3600rpm or so?

you mention that the output goes back up to 100amps if you speed up the engine?
if you are running at 3600rpm how much faster can you speed up? doesn't seem likely you would get maybe another 500rpm out of the engine without it sounding like it is going to blow.

if this is the case, it seems likely to me the alternator is fine, and i am looking
back at the engines ability to pull the load.

sorry to hear your having issues with this setup, everything was looking so good
and promising.

likely something simple we are missing here.

The DC readings are bouncy,

My instincts say something is wrong if bouncy. You know this, does not help define if electrical or mechanical. May help if we understand exactly what you were measuring.

terminal 1 to 2
terminal 2 to 3
terminal 1 to 3

going from ground to the AC terminals might not tell you what you need to know.

do this with some load on the alternator, and report back your results

the unit is delta wound fwiw

also i looked at mine and the fan is clockwise rotation, so check your rotation
i am thinking maybe you are turning the unit backwards, if so the fan will pull some air but is optimized to pull much more air turning the right direction.

when you get this thing sorted out, be sure and double check the rotation of the
alternator and see that it is spinning the way the fan needs to go.
if it isn't you should look into remounting so that it can turn the right way
or get a straight cut fan for it. i have a couple is you need one.

also you might want to look into one of the temp switches like they use in electric dryers, and epoxy it to the stator and connect the field in series with it, so that
if the alternator gets too hot it will shut down the field,
kind of crude but effect at protecting the alternator, they make them in many temp ranges,, look up wwgrainger at i think grainger.com i know they have them.

good luck

bob g

ps, i really don't like these alternator for much over 100 amps for continuous duty
unless you are in very cool climates, then maybe 120amps
i figure far better to get a consistant trouble free 100amps that it would be to
have a hotrod at 140 or more that needs additional cooling, sensor/protection, babysitting and frequent repairs.