A B737-400 ailerons became very sluggish and heavy in flight similar to manual reversion. An emergency was declared and the flight diverted to a nearby airport where an actuator cable was found failed and the aileron feel sense unit was also replaced.

Narrative

Approximately 200 miles from destination we were at FL260 and had just switched Centers. Aircraft started turning left while on autopilot. When unable to turn back to the right on automation the pilot flying disconnected the autopilot and flew manual. Pilot flying commented on how heavy and sluggish the controls felt. Pilot not flying informed ATC that we might have issues and please give us a heading while we sort this out. We complied with QRH Jammed/Restricted Flight Control procedure. About this time pilot not flying took control of the flight controls to get a feel of the problem. To the pilot not flying it felt very similar to manual reversion in the simulator. Pilot not flying then got a phone patch to Maintenance Control and asked for assistance to diagnose the problem. At this time the pilot flying said flight control pressures were getting worse. The pilot not flying terminated the phone patch and made an immediate command decision to declare an emergency and proceed to the nearest suitable airport. We were offered a nearby airport which we turned down and Dispatch suggested nearby larger airport. Pilot not flying briefed the (A) Flight Attendant for normal landing; nature of divert; then gave a PA to the passengers explaining precautionary landing for a flight control issue. [We] requested emergency vehicles to be on the ready; ran applicable checklists. Approach and landing [accomplished] safely and taxied in. Needed GPU; APU was on MEL. Passengers met by Agent and rebooked. I do not know why this happened. Because of the way the controls felt. Maybe the feel/centering Unit or electrical. Left flight spoiler was deployed and remained in the up position.