DC10 WITH MECHANICAL PROB REQUESTS LOWER THAN CLRED ALT LEVELOFF FROM CTR. AT THE NEW CLRED ALT THERE IS A TFC CONFLICT WITH APCHING ACFT UNDER THE CTL OF A DIFFERENT FACILITY. TCASII PROVIDES THE SEPARATION NEEDED.

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ATIS RPTED RIME AND CLR ICE AT 12000 FT. AT 2500 FT MSL WE GOT A #3 PNEUMATIC SYS FAIL INDICATION. CAPT AND SO STARTED WORKING THAT PROB WHILE I FLEW AND HANDLED COM. INITIAL LEVELOFF ALT WAS TO BE 10000 FT. AT ABOUT 8000 FT IN CLB ZOB CLRED US TO 13000 FT AND GAVE A FREQ CHANGE TO A DIFFERENT ZOB CTLR. WHILE CONTINUING TO CLB AND MAKING FREQ CHANGE I GOT A QUICK BRIEF ON OUR PROB. KNOWING THIS WOULD AFFECT OUR ABILITY TO ANTI-ICE THE R WING WE QUICKLY AGREED TO STAY BELOW THE ICING RPTED AT 12000 FT. I COMMUNICATED OUR INTENTION TO LEVEL OFF AT 10000 FT DUE TO A MECHANICAL TO THE NEW CLE CTLR. BY THE TIME HE CAME BACK WITH THE OK TO MAINTAIN 10000 FT WE WERE VIRTUALLY THERE. I LEVELED AT 10280 FT AND BEGAN A SLOW DSCNT BACK TO 10000 FT. AT THIS TIME THE TCASII SHOWED A TARGET AT 11 O'CLOCK POS ABOUT 4 MI; OUR ALT; CONSTANT BEARING; DECREASING RANGE. WE GOT A 'TFC; TFC. REDUCE DSCNT.' I BEGAN A CLB AND TOLD CLE WE HAD A CONFLICT; THAT WE WERE CLBING TO 11000 FT. THE OTHER ACFT SHOWED A DSCNT AND PASSED ABOUT 1000 FT DIRECTLY UNDER US. CTR TWICE SAID THEY HAD NO OTHER ACFT ON THEIR SCOPE. HE LATER SAID HE ONLY COVERED THE AIRSPACE FROM 13000 FT UP. ALTHOUGH THE TCASII PICKED UP THIS SIT RATHER LATE AND DID NOT GIVE AS FORCEFUL A RECOMMENDED ACTION (SUCH AS 'CLB; CLB.') AS I WOULD HAVE EXPECTED; IT DEFINITELY GAVE US THE INFO WE NEEDED TO PREVENT AN UGLY SIT. SUPPLEMENTAL INFO FROM ACN 467138: ALT OF INTRUDER WAS ALSO 10000 FT (CO-ALT) AS THE DISPLAY CHANGED TO A RED RA (SOLID RED SQUARE ON TCASII) CAPT INQUIRED ON RADIO TO ZOB ABOUT PROX OF ACFT AT 12 O'CLOCK POS; CO-ALT. ATC SAID NO ACFT INDICATED. AS THE INTRUDER GOT INSIDE 2 MI; BASICALLY HEAD ON; CAPT INSTRUCTED FO TO CLB. AT THIS TIME; MY FO STARTED TO CLB THE ACFT AS WE GOT AURAL WARNING 'REDUCE DSCNT' AND RED PREVENTATIVE VSI ARC. FO CLBED ACFT AND WE BELIEVE WE CLBED OVER INTRUDER BY 1000-1400 FT. WE INFORMED ATC THAT WE BELIEVED WE HAD HAD A NEAR MISS SIT. WE NEVER GOT A 'CLB; CLB' RESOLUTION OR A 'CLR OF CONFLICT' ADVISORY FROM TCASII. AFTER DISCUSSION WITH ATC REPRESENTATIVES IN DTW; THEY CONFIRMED 'OPERROR' BY CTLR. THE DECISION TO CLB WITHOUT A DIRECT CLB COMMAND MAY HAVE AVOIDED A DISASTER. ATC INFORMED US THAT THE SIT HAD DEGRADED TO HEAD ON; 300 FT ALT DIFFERENCE AT 1.6 NM.