AN ACR B737 FLC EXPERIENCED UNCOMMANDED RUDDER INPUTS. THESE CTL DISPLACEMENTS WERE APPARENTLY CAUSED BY WATER GETTING INTO THE YAW DAMPER UNIT IN THE FORWARD ELECTRONIC COMPARTMENT.

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DURING CLB SW OF ALLENTOWN; PA; RUDDER APPARENTLY BEGAN UNCOMMANDED EXCURSIONS. NO FEEDBACK WAS FELT IN RUDDER PEDALS. PLT CTL INPUTS WERE NOT EFFECTIVE IN DAMPENING YAW AND ROLL OF ACFT. MY IMPRESSION WAS THAT MY CTL INPUTS -- ESPECIALLY RUDDER -- WERE NOT EFFECTIVE. ACFT MOTIONS RESEMBLED DUTCH ROLL. HOWEVER; ACFT WAS AT A LOW ALT FOR DUTCH ROLL; ALL CTLS WERE PWRED; AND YAW DAMPER WAS ENGAGED. AUTOPLT WAS NOT ENGAGED. WHILE WE WERE ATTEMPTING TO CTL THE UNCOMMANDED ACFT YAW AND ROLL AND DETERMINE JUST WHAT WAS HAPPENING; ACFT SPD AND ALT WERE ALLOWED TO VARY. ATC WAS NOTIFIED OF CTL PROB AND RESULTANT EXCURSIONS. YAW DAMPER INPUTS DO NOT FEED BACK INTO RUDDER PEDALS; MAKING IT DIFFICULT TO DETERMINE EXTENT OF INPUTS; BUT THEY SEEMED TO GREATLY EXCEED THE 2-3 DEG AUTH THE YAW DAMPER IS ALLOWED. THE YAW DAMPER WAS TURNED OFF AND B HYD CTL TO THE RUDDER WAS TURNED TO STANDBY ELIMINATING PWR THE YAW DAMPER MIGHT USE. NORMAL CTL RETURNED. QRH CHKED FOR OTHER CREW ACTION. ATC NOTIFIED. FLT CONTINUED TO ORLANDO. MAINT RPTED THAT WATER HAD SHORTED OUT THE YAW DAMPER AND CAUSED PROB. THIS MIGHT EXPLAIN OTHER RPTED B737 CTL PROBS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID THAT THE FO WAS FLYING WHEN THEY FELT THE ACFT YAW. THE MOVEMENT WAS UNUSUAL AND THE CAPT TOOK CTL WITH THE MOVEMENT BECOMING MORE PRONOUNCED. HE SAID THAT AT FIRST HE FELT THAT HE HAD MADE A MISTAKE BY TAKING CTL BECAUSE THE YAW WAS PROGRESSIVE. WHILE HE WAS DISCUSSING THE SIT THE FO; AGAINST THE CAPT'S WISHES; SHUT OFF THE YAW DAMPER. THIS PROC IS STANDARD PRACTICE FOR UNCOMMANDED RUDDER INPUTS IN THE B737; BUT THE RPTR SAID THAT HE WAS RESISTANT BECAUSE OF HIS LONG EXPERIENCE IN THE B727 WHERE THE YAW AND DUTCH ROLL MOMENTS ARE DIVERGENT. THE CREW THEN COMPLETED THEIR CHKLIST FOR THIS CONDITION AND CONTINUED TO DEST. MAINT PERSONNEL FOUND THAT THE COVER ON THE YAW DAMPER UNIT IN THE FORWARD ELECTRONICS COMPARTMENT WAS DISPLACED ALLOWING WATER FROM THE PREVIOUS NIGHT'S RAIN TO LEAK INTO THE ELECTRICAL SYS. THIS APPARENTLY CAUSED THE YAW DAMPER TO DISPLACE THE RUDDER CAUSING THE RATE GYRO TO SIGNAL A CORRECTION WHICH WOULD THEN CAUSE ANOTHER DISPLACEMENT ACCORDING TO INFO THE RPTR OBTAINED FROM MAINT PERSONNEL.