ACR RESPONDS TO A TCAS ALERT NEAR DTW.

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WE WERE DSNDING TO 11000 FT WITH DETROIT APCH. ACFT (FOKKER 70) WAS IN ALT CAPTURE PHASE AT APPROX 11300 FT. RECEIVED TCASII TA. TCASII TARGET INDICATED INTRUDER WAS 1900 FT BELOW OUR ALT; CLBING. WITHIN 1-2 SECONDS RECEIVED 'CLB' RA. INTRUDER WAS NOT VISUALLY IDENTED. SINCE AUTOPLT VERT SPD MODE WAS DISABLED DUE TO ALT CAPTURE PHASE; I DISCONNECTED AUTOPLT AND CLBED MANUALLY ACCORDING TO TCASII PRESENTATION. AT APPROX 11400 FT RECEIVED TCASII 'MONITOR VERT SPD' MESSAGE. APPROX 1-2 SECONDS LATER RECEIVED TCASII 'CRL OF CONFLICT' MESSAGE. I RE-ENGAGED AUTOPLT AND SELECTED VERT SPD DSCNT TO RETURN TO ASSIGNED ALT OF 11000 FT. FOR REASONS UNKNOWN; AUTOPLT DID NOT CAPTURE ALT AT 11000 FT. AT APPROX 10600 FT I DISCONNECTED AUTOPLT AND CLBED MANUALLY BACK TO 11000 FT. THERE WAS SOME COCKPIT CONFUSION DURING THIS SEQUENCE OF EVENTS DUE TO FOLLOWING REASONS. WE RECEIVED AN AIRSPD ASSIGNMENT; ALT CHANGE; AND RA ALL AT ABOUT THE SAME TIME. COULD NOT UNDERSTAND REASON FOR RA WHEN TCASII INDICATED INTRUDER WAS 1900 FT BELOW OUR ALT. NECESSITY FOR DISCONNECTING AUTOPLT DUE TO VERT SPD MODE INHIBIT. AUTOPLT DID NOT CAPTURE ALT WHEN DSNDING BACK TO ASSIGNED ALT. CTLR QUESTIONING REASON FOR RA WHEN THERE APPEARED TO BE ADEQUATE VERT SEPARATION BTWN ACFT. CTLR DID STATE HE 'HAD NO PROB' WITH OUR MANEUVER. NOTE: TCASII SOFTWARE VERSION 6.04A IS INSTALLED IN OUR ACFT.