In a small saucepan, combine soy sauce, brown sugar, and plum jam.

Heat over low heat until sugar is dissolved, stirring occasionally.

Reserve 1/4 cup of the sauce for basting.

In a large plastic resealable bag, combine the remaining sauce with black cherry soda.

Place the pork tenderloin in the bag, seal, and marinate in the refrigerator for at least 4 hours, or overnight.

Preheat grill to medium heat.

Lightly oil the preheated grill.

Discard the marinade.

Cook the tenderloin for 15 to 20 minutes, or until an instant-read thermometer inserted into the center reads 145 degrees F (63 degrees C).

Baste with the reserved sauce during the last few minutes of grilling.

Remove the meat from the grill and allow it to rest for 5 minutes.

Slice into 1/4 inch thick medallions and serve.