AN LTT ACR CREW MADE A GAR IN RESPONSE TO A TCASII ALERT.

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GOT TA TCASII ALERT WHILE ON LOC AND GS TO ABE; PA. THE PARTICULAR PLANE WE WERE IN GIVES TA INFO ONLY; NO RA COMMANDS BECAUSE THE NEWER XPONDER HASN'T BEEN INSTALLED YET. THE ACFT SHOWING ON THE TCASII WAS 800 FT BELOW US IN LEVEL FLT A FEW MI AHEAD. WE WOULD HAVE COLLIDED HAD WE REMAINED ON THE LOC AND GS SINCE THE OTHER ACFT WAS FLYING TOWARD US. THE ALLENTOWN TWR DID NOT PICK UP THE OFFENDING ACFT ON THEIR RADAR; AND SINCE WE WERE IN THE CLOUDS AND COULDN'T PICK UP THE ACFT VISUALLY; WE ADVISED APCH CTL THAT WE WERE GOING MISSED APCH. THEY VECTORED US BACK AROUND TO LAND WITHOUT INCIDENT. ADDITIONALLY; THE ALLENTOWN APCH/TWR PERSONNEL WERE UNAWARE THAT WE WERE TCASII EQUIPPED. APPARENTLY THE DISPATCHER FAILED TO PUT IT IN THE FLT INFO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING PLT HAS SINCE FOUND A SMALL ARPT RIGHT AT THE LOCATION OF THE RPTED TCASII INCIDENT. VERY POSSIBLY THE INTRUDING ACFT WAS TAXIING WITH ITS XPONDER ON; OR FLYING IN THE TFC PATTERN UNDER VFR; BUT UNDER A LOW CEILING.