when the shop tested the alt did they give you a sheet with the voltage? which is what it produces when full fielded. You might try this: pull both female terminals off of the alt male field terminals & jump one terminal to the nearby alt case (ground) then hook the voltmeter to the alt (batt stud/nearby alt case) & idle it at a good fast idle & jump 12V to the other alt male field terminal for several seconds, just long enough to see what the voltmeter reads & have all lights/accessories OFF for this. Is this the same 15+V reading you are dealing with? You might also grab a coil of wire & some alligator clips from the HELP section at your parts house & make some jumpers & pull both field leads from the alt & pull the triangle reg connector & jump 12V to the top reg nub and 12V to either male alt field terminal then connect the other reg (side) nub only to the other alt male field terminal & jumper from the reg case to ground & start it & at a fast idle see what the voltmeter AT the alt reads. this takes the wiring out of the picture & only leaves the reg or the alt. & electronic regs almost always either go open (no charging) or full field (max voltage the alt can produce).