On July 23, 2000, at 1731 central daylight time, an Aerospatiale ATR 42-300, N421TE, operated as Trans States Airlines Flight 7495, piloted by an airline transport pilot, experienced a flight control difficulty while in cruise flight near Champaign, Illinois. An uneventful landing was made and the airplane sustained no damage. The 14 CFR Part 121 passenger flight was operating on an instrument flight plan. Visual meteorological conditions prevailed at the time of the incident. No injuries were reported. The flight originated from the Michiana Regional airport, South Bend, Indiana, at 1640 eastern standard time, and was en route to the Lambert-St. Louis International Airport, Saint Louis, Missouri. While in cruise flight at 14,000 feet msl, the flight crew was unable to trim the aircraft for level flight. According to the pilot's original report of the incident, approximately 80 to 100 pounds of aileron force was required to maintain level flight. The flight crew declared an emergency and diverted to the University of Illinois-Willard Airport, Champaign, Illinois, where an uneventful landing was made. Following the incident, a mechanic for the airline determined that the, "...right section of [the] aileron control switch for trim to be very intermittent (mostly inoperative)." The mechanic also stated that the trim indicator showed 1/2 trim to the left. The mechanic stated that he was able to center the trim by using the intermittent switch. A ferry flight was performed so that the switch could be replaced. According to a report filed by the airline, the switch had accumulated 26,747 hours time in service at the time of the incident. The switch was examined by the manufacturer under the supervision of the Bureau Enquetes-Accidents. The malfunction of the switch was confirmed. The switch was found to have been contaminated with, "... salt, dust, hairs and other organic materials." Corrosion of the switch contacts was also found. The pilot stated, in a written document dated after his original statement, that he may have overestimated the required force to hold wings level. He stated, "...it was probably 50 to 70 pounds of force to hold wings level.