WHILE DRIVING TO WORK SMELLED BURNING ODOR FROM AC VENTS. WHEN STOPPED AT WORK, SMOKE WAS POURING OUT FROM UNDER THE HOOD. SHUT OFF IGNITION AND RAISED HOOD. SPEED SENSOR DEACTIVATION SWITCH WAS SMOLDERING. REACHED IN WITH RAG TO DISCONNECT ELECTRICAL PLUG TO SWITCH AND PLUG FELL OFF IN HAND (MELTED). *NM