[RichardJ]

First he needs to find out if he needs a bigger alternator. A bigger alternator will not give a lower Amp reading.
Only if the alternator can't supply the proper voltage at the battery would he need a larger alternator.

I don't know what " Jeep Fan mod" he is talking about. A 30 Amp fan would only draw 30Amps at startup for 1/2 second, then drop to 15-18 Amps.
Need to know battery voltage before and after fan kicks in and how it is wired. Would also be nice to know the specifics of the SI installation wiring.