FLC OF ACR MLG ACFT RECEIVED TCASII WARNINGS DURING FINAL APCH IN IMC CAUSING MUCH APPREHENSION IN WHETHER TO RESPOND OR CONTINUE THE APCH THINKING THAT IT WAS FALSE SINCE ATC HAD NOT ADVISED ANY TFC.

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A FORMAL INVESTIGATION HAS BEEN OPENED. I HAVE BEEN CLRED OF ANY FAULT. WE WERE NEVER ADVISED BY ATC OF THE TFC SO THE TCASII ALERT WAS A TOTAL SURPRISE. WAS ON AN ASSIGNED HDG OF 090 DEGS; 5000 FT; WHEN TOLD TO DSND TO 3000 FT AND TOLD TO EXPEDITE TO 3000 FT. PASSING 4000 FT; WAS GIVEN A HDG OF 070 DEGS TO INTERCEPT THE ILS RWY 4R LOC (FO WAS PF). WE WERE GIVEN 2000 FT FOR THE INTERCEPT AND WERE SOLID IMC AT THIS TIME. WE WERE ON A 70 DEG HDG WHEN THE CTLR ASKED US TO TIGHTEN OUR TURN TO THE 070 DEG HDG. I RESPONDED THAT WE WERE 'ON THE 70 DEG HDG AND INTERCEPTING THE LOC AT THIS TIME.' HE SAID 'GOOD JOB (ACR FLT NUMBER); CONTACT EWR TWR ON 118.3.' I SIGNED OFF FREQ AND SWITCHED TO TWR. DSNDING THROUGH APPROX 2800- 2700 FT AND 1-1 1/2 DOT HIGH ON THE GS; WE RECEIVED A TCASII ALERT AND ALMOST AN IMMEDIATE RA ALERT. I DO NOT RECALL OUR RATE OF DSCNT BUT THE RA RESULTED IN OUR IVSI INDICATOR BEING RED FROM PLUS 6000 FPM TO MINUS 3000 FPM. THE RED SHOWED AN RA TARGET AT PLUS 200 FT INITIALLY AND BASICALLY IN ONE PIECE OF SKY. BOTH THE FO AND MYSELF WENT INTO THE 'WHAT THE HELL IS THIS MODE!' WE WERE STILL IMC AND APPROX 2 MI OUTSIDE THE FAF/LIZAH. THE PLUS 200 FT QUICKLY CHANGED TO MINUS 200 FT TARGET ON THE RGO (RADAR). WE CONTINUED THE DSCNT AND CLRED THE TFC AND FLEW INTO VMC WX CONDITIONS AT 2100 FT TO 2000 FT MSL. I CHKED IN WITH TWR AND WE WERE CLRED TO LAND WITH HIS FIRST XMISSION. I ASKED HIM IF HE HAD ANY TFC AT 2500 FT JUST BEHIND US. HE SLOWLY RESPONDED THAT HE WAS 'JUST TAGGING SOMETHING THERE NOW.' LNDG ATC WAS NORMAL. I IMMEDIATELY CALLED THE NEWARK SECTOR OF NY APCH CTL AND THEY WERE ALREADY AWARE OF THE SITUATION. THE FAA HAS OPENED A FORMAL INVESTIGATION.