Heat oven to 325F. Grease and flour 10 in tube pan. In large bowl, cream sugar, 1 1/2 c. Margarine and 1 1/2 tsp vanilla until light and fluffy. Add eggs, one at a time, beating well after each. Cut up yams; add to vreamed mixture, beating until well mixed. Lightly spoon flour into measuring cup; level off. Add flour, baking powder, soda, salt, cinnamon and nutmeg; mix well. By hand, fold in pecans, raisins and drained pineapple. Spoon bater into prepared pan;spread evenly. Bake at 325F for 65-75 minutes or until toothpick comes our clean. Cool upright in pan 10 min. Invert onto serving plate. In medium saucepan, combine brown sugar, dash salt, 1/4 c margarine, reserved 1/4 c. Pineapple liquid, milk and reserved 1 T yam liquid. Bring to a boil; boil threee minutes, stirring constantly. Remove from heat; beat in powdered sugar adn 1 tsp vanilla until smooth. Cool about 10 min. To allow frosting to thicken slightly, beating occasionally to maintain smooth consistency. Beat well, quickly spoon ocer cake, allowing some to run down sides. Garnish with pecan halves. (Note: to substitute yams, use 1 1/4 c. Mashed, cooked sweet potatoes)