FLC OF AN MLG ACR ACFT STOPPED THEIR DSCNT ON A VISUAL APCH WHEN THEIR TCASII RA ADVISED TO CLB.

Narrative

WE WERE CLRED FOR THE EXPRESSWAY VISUAL TO RWY 31 AT LGA. AT THE MASPETH TANKS WE TURNED TO 085 DEGS AND FLEW THE PRESCRIBED PROC. WE WERE VISUAL BEHIND AN MLG Z. FO WAS FLYING THE ACFT AT 180 KTS BEGINNING THE DSCNT AND CONFIGURING TO LAND. WE HEARD TWR ASK IF AN ACR Y FLT HAD US (ACR X) IN SIGHT. TO AN AFFIRMATIVE RESPONSE HE WAS ALSO CLRED THE EXPRESSWAY VISUAL TO FOLLOW US. WE IMMEDIATELY RECEIVED A TCASII RA TO DSND WHICH WE WERE ALREADY DOING. I CHKED THE TCASII AND IT SHOWED TFC AT ABOUT 1/2 MI BEHIND US; 600 FT ABOVE; DSNDING. THE RA INCREASED TO 400 FT SEPARATION WHILE FO IMMEDIATELY INCREASED DSCNT TO 1500 FPM. RA CHANGED TO 'MONITOR DSCNT' THEN DSND; DSND AS FO SLOWED DSCNT. I CALLED TWR WHO IMMEDIATELY BROKE THE ACR Y MLG Y OFF OF THE APCH. MY FO LOOKED OUT HIS SIDE WINDOW AND SAW THE ACR Y RAISING ITS GEAR AND TURNING AWAY. TWR TOLD US WE HAD 1000 FT SEPARATION; ALTHOUGH OUR TCASII SHOWED ONLY 400 FT AT ONE TIME DURING THE WARNING. OUR APCH WAS COMPLETED TO A NORMAL LNDG. APPARENTLY; APCH CTL HAD CLRED ACR Y TO FLY THE EXPRESSWAY APCH WHEN HE CONFIRMED US IN SIGHT. THE REDUCED SEPARATION MUST HAVE OCCURRED BECAUSE ACR Y SAW THE ACR Z AHEAD OF US AT ABOUT 3 MI AND MISTAKENLY IDENTED HIM AS US. IF OUR TCASII HAD NOT ALERTED US TO THE ACR Y DSNDING ON US; OUR DSCNT WOULD NOT HAVE BEEN AS RAPID AND PROBABLY WOULD HAVE RESULTED IN A MIDAIR. ACR Y TCASII EVIDENTLY FAILED; ISSUED INCORRECT INFO; WAS NOT OPERATIONAL OR WAS NOT INSTALLED. CONTRIBUTING TO THE INCIDENT WAS MISIDENT OF OUR FLT BY ACR Y.