Your alternator isn't doing well because of what?
I mean, unless you've calculated that 90amps just isn't enough, it could be the brushes (replaceable, cheap, and easy) or the regulator (less-cheap, still easy).
What is the issue you are having exactly? Pics help immensely.
All good questions, and I can only describe all of my troubleshooting. Other than normal loads (stereo, a/c, etc.) I also have LED trail lights, a HAM radio and probably the biggest potential draw, a second battery system using a LiFePO4 battery and a
Redarc controller. It's that second battery system that started the digging and after testing everything within it (everything works like it's supposed to), I turned my attention to the alternator. The symptom was as soon as I turned on the charging circuit (truck battery to
Redarc/LFP battery) I would get a voltage drop at the
Redarc input, meaning it was trying to draw more amps and they simply weren't available. A breaker would trip and no more charging. FYI the
Redarc will draw up to 40 amps, the HAM something between 10-20amps on transmit, and then all the other stuff I'm running.
At idle, the voltage drop was quick, 30 seconds or so. My wife raised the engine RPM to 2k and I would see the voltage rise at the
Redarc and things would remain relatively stable for a time, but overall still drop slowly.
So then I purchased a charger with a built-in alternator test function (needed the charger anyway) and hooked it up. With no load and engine idling, it tested ok. then I added all lights, both radios and a/c (left the second battery disconnected) and tested again, and this time the alternator failed. It simply can't keep up with the demand, even without the second battery system. A rebuild will probably help, but I figure with the power draw and the potential to add more accessories, an upgrade seems like a good idea.