Fix your charcoal grill so you have 2 rows of briquets. Place loin lengthwise between rows of briquets, not directly over them. This way the loin can cook with indirect heat. Turn every 6-7 minutes basting with honey mustard dressing until internal temperature reaches 145 - 150 (use meat thermometer) this usually takes around 50 minutes. Take off grill and let rest at least 10 minutes, it will keep cooking so don't let it reach over 150 before you take it off or it will be dry.
Slice and enjoy:)