A B737-400 IN CRUISE AT FL330 WITH YAW DAMPER ON UPON ENGAGING AUTOPLT THE ACFT BEGAN TO YAW L AND R. WHEN AUTOPLT DISENGAGED THE YAW DAMPER DISCONNECTED. TESTED YAW DAMPER AND RE-ENGAGED AND OPERATION NORMAL AND RE-ENGAGED AUTOPLT AND ACFT BEGAN TO YAW IMMEDIATELY.

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AT TOP OF CLB (HAND FLOWN) ENGAGED AUTOPLT AND IMMEDIATELY THE ACFT BEGAN TO YAW L AND R. DISENGAGED AUTOPLT AND YAW DAMPER DISENGAGED SIMULTANEOUSLY. THIS SHOULD NOT HAPPEN WITH NORMAL YAW DAMPER. RAN THE UNCOMMANDED YAW CHECKLIST THEN THE YAW DAMPER ABNORMAL CHECKLIST. RE-ENGAGED YAW DAMPER AND I EXPERIENCED NO YAW. THEN RE-ENGAGED THE AUTOPLT AND ACFT BEGAN TO YAW IMMEDIATELY AS RPTED ABOVE. DISENGAGED AUTOPLT; YAW DAMPER DISENGAGED AND FLEW TO SFO. FERRIED WITH APPROPRIATE MEL'S. MET BY HONEYWELL; OUR ENGINEERING AND MAINT PERSONNEL. THEY ADMITTED THIS ACFT HAS A NEW DIGITAL YAW DAMPER AND HAD THE SAME PROB DURING FLT TEST BUT COULD NOT DUPLICATE PROB SO RELEASED TO REVENUE SVC. SUBSEQUENT FLT TEST PRODUCED SAME RESULTS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED WHEN THE AUTOPLT WAS ENGAGED THE L TO R YAW WAS MINOR TO MODERATE AND COULD BE FELT VERY SLIGHTLY IN THE RUDDER PEDALS. THE RPTR SAID THE OTHER ODD THING WAS WHEN DISENGAGING THE AUTOPLT THE YAW DAMPER TRIMMED OFF. THE RPTR STATED THIS AIRPLANE HAD TWO PREVIOUS MAINT RPTS ON THE AUTOPLT AND SOME UNITS WERE REPLACED. THE RPTR SAID WITH THE YAW DAMPER TURNED OFF THE AUTOPLT FUNCTIONED NORMALLY THE REST OF THE TRIP TO SFO AND ON THE MAINT FERRY TO BOEING. THE RPTR SAID ON ARR AT BFI THE CREW WAS DEBRIEFED BY HONEYWELL ENGINEERS WHO ADVISED A DIGITAL YAW DAMPER COUPLER AND PRESSURE REDUCER HAD BEEN INSTALLED IN THIS ACFT. THE RPTR STATED THE HONEYWELL ENGINEERS ALSO ADVISED WHEN THIS MODIFICATION WAS TESTED IT HAD SOME PROBS BUT WHEN FLOWN AFTERWARD THEY COULD NOT DUPLICATE THE RPTS. CALLBACK CONVERSATION WITH RPTR ACN 466588 REVEALED THE FOLLOWING INFO: RPTR STATED THE YAW DAMPER WHEN THE AUTOPLT WAS ENGAGED INITIALLY WAS ONE HALF RUDDER INPUT WITH WINGS 10 DEGS OF BANK. THE RPTR SAID THE RUDDER PEDALS DID MOVE SLIGHTLY. THE RPTR STATED THE YAW DAMPER OP WAS NORMAL WITH NO AUTOPLT ENGAGEMENT. CONVERSELY; THE AUTOPLT OPERATED NORMALLY WITH NO YAW DAMPER ENGAGEMENT. THE RPTR SAID THIS SAME CREW MAINT FERRIED THE ACFT TO BOEING AND WERE ADVISED BY THE HONEYWELL ENGINEERS OF TWO MODIFICATIONS; A PWR UNIT PRESSURE REDUCER AND A DIGITAL YAW DAMPER COUPLER.