FLC OF ACR MLG ACFT RESPONDED TO A TCASII RA AND DEVIATED FROM ASSIGNED ALT.

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CLBING THROUGH 3200 TO 5000 ON 060 VECTOR AFTER TKOF 11L MSP DIRECT BDL DEP CTL CALLED TFC 12 O'CLOCK 4 MI; MODE C 4200. I LEVELED AT 3700 AS PRECAUTION AS TCASII CALLED 'TFC' THEN 'DSND'. I BEGAN FIRM DSCNT; NOTING RED INTRUDER 12 O'CLOCK INSIDE THE 4 MI DOT CIRCLE. I FIND IT INTERESTING THAT UPON LOOKING AT TCASII ; I REMEMBER THE RED ARC CURVING PAST THE BOTTOM (6 O'CLOCK); THE RED INTRUDER AS CLOSE AS 1/2 MI AS THE TCASII CALLED 'INCREASE DSCNT.' MY IMPRESSION WAS THAT INTRUDER MUST BE DSNDING INTO US SINCE THE ORIGINAL 500 FT SEPARATION CALLED BY ATC POSSIBLY SHOULDN'T HAVE BROUGHT ON AN RA. I ESTIMATE OUR MAX VERT SPD 2000 FPM FOR A MOMENT. WE LEVELED AT 3000 AS RED INTRUDER TURNED GREEN ON TCASII AT OUR 6 O'CLOCK AND TCASII CALLED 'MONITOR VERT SPD' AND 'CLR OF CONFLICT.' LATER PHONE CALL TO DEP CTL DISCLOSED THEY VECTORED A NEARBY VFR ACFT TO THE 'INTRUDER' RETURN; A LIGHT SINGLE; AND CONFIRMED IT LEVEL AT 2000 BELOW THE CLOUDS. THEY WERE UNABLE TO INFORM THE 'INTRUDER' THAT HIS MODE C WAS ERRATICALLY SENDING ERRONEOUS ALT INFO; VARYING FROM 5700 TO 3000; THUS THERE WAS NO REAL MISS JUST OUR LUCK TO BE THERE WHEN 'INTRUDER' SENT 4200 FT.