B757-200
Flight Crew Last 90 Days 90; Flight Crew Total 15000; Flight Crew Type 12000
Aircraft Equipment Problem Critical
Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem; Flight Crew Regained Aircraft Control
Aircraft
Climbing through FL270 the aircraft started violently rolling left and right; uncommanded; and very near uncontrollably. There were no associated messages on the EICAS panel nor were there any lights illuminated on the overhead panel. What needs to be emphasized here is that it was very difficult to maintain control of the aircraft and there were no associated messages notifying us of a problem. We leveled off at FL270; slowed to 250 knots; notified ATC of our deviations and basically just worked to keep the aircraft straight and level. Quickly; the FO and I determined that the first thing we should do was to isolate the yaw dampers. After turning the right one off the problem persisted so we switched it back on and tried the left one. Once it was switched off the problem stopped. At that point; we notified ATC that we would be remaining at FL270 and would be returning to normal speed.  I called maintenance control and Dispatch to bring them up to speed on our issue. Maintenance just told us to follow our QRH procedure of which there was none due to not having any associated EICAS message; and Dispatch asked us if we felt comfortable continuing on to [a nearby alternate]; [to] which I replied yes because we were already halfway there and had 95 knots of wind on the tail. The rest of the flight was uneventful. Once on the ground; the mechanic meticulously followed his maintenance procedures inspecting the aircraft for stress and damage and informed us that maintenance engineering had pulled the data which apparently did not show a problem with the yaw damper nor were any exceedences recorded.  He ended up deferring the left yaw damper and we pressed on to [destination].
[Report narrative contained no additional information].
B757-200 flight crew reported diverting to a nearby alternate when they experienced violent nearly uncontrollable rolls in climb at FL270 from a malfunctioning yaw damper.