Coat a saucepan with cooking spray. Add margarine; place over medium-high heat until margarine melts. Add onion and celery; saute until tender. Add carrot and broth. Bring to a boil; cover, reduce heat, and simmer 20 minutes. Remove from heat; cool 10 minutes.
Transfer mixture to container of an electric blender; cover and process until smooth. Return to saucepan. Add milk; cook 3 minutes or until thoroughly heated. Stir in dillweed, salt, and pepper.