Alternator charging at 18 volts

Something is shorting the field control line to ground or it is shorted in the alt itself. Measure the voltage across the field terminals (two small ones) with the engine running and all accessories off. You typically see around 5V, +/- a few. No more than 10V. If you see the 14 or 18 (or whatever the charging voltage is) then it is shorted.

What codes are you getting?

Happened to me last summer. I even tried turning on all accesories and high beams to take it to get fixed, and blew both headlights on the way,LOL. The internal regulator in my SMEC was toast...got an external one mounted on the rebuilt alternator for an extra $1.50