LESS THAN STANDARD SEPARATION WHEN WDB-ACR OVERTOOK SMA ON ILS FINAL. OPERATIONAL ERROR.

Narrative

I WAS VECTORING ACFT FOR THE ILS RWY 10. WX WAS MVFR; PREVAILING VIS 4 MI WITH 2 MI ON FINAL. THE FIRST ACFT WAS SMA X; AND THE FOLLOWING ACFT WAS WDB ACR Y. SMA X WAS CLRED FOR THE ILS 3 MI FROM THE FAF; AIRSPEED 140 KTS; AND WAS INSTRUCTED TO MAINTAIN HIS PRESENT SPD UNTIL THE MARKER. ACR Y WAS CLRED FOR THE ILS; 10 MI FROM THE FAF. SMA X CALLED SHORTLY AFTER AND RPTED OVER THE MARKER. I SWITCHED X TO TWR FREQ. I SAW THAT I HAD ENOUGH SEP BTWN X AND Y (APPROX 6 MI). I INSTRUCTED ACR Y TO MAINTAIN HIS PRESENT SPD (180 KTS) TO THE MARKER AND TO CONTACT TWR. ABOUT A MINUTE LATER; THE TWR CALLED ME AND SAID; 'I HAVE LESS THAT 3 MI BTWN X AND Y; WHAT DO YOU WANT ME TO DO?'. ACR Y WAS ISSUED 180 DEG HDG AND 3000' AND WAS VECTORED FOR ANOTHER ILS. AFTER REVIEWING THE DATA EXTRACTIONS; WE REALIZED THAT SMA X DID NOT COMPLY WITH THE SPD INSTRUCTIONS. SMA X HAD REDUCED TO 90 KTS PRIOR TO THE MARKER AND THEN HELD 75 KTS INSIDE THE MARKER ON FINAL. I FAILED TO DETECT THE SPD ON SMA X WHICH RESULTED IN MY SYS ERROR.