A CL-600 FLT CREW ON AN ILS APPROACH TO ORD WERE 300 FT LOW AT WAVIE; DUE IN PART TO THE PROCEDURE BEING FLOWN IN RAW DATA BECAUSE OF AN INOP FMS.

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ON THIS FLT WE HAD A MEL'ED FMS; AND HAD BEEN FLYING THE ENTIRE FLT IN 'RAW DATA.' AS THE PNF; I HAD SPENT A GOOD PORTION OF THE FLT EXPLAINING TO MY FO ABOUT THE LIMITATIONS OF WHAT WE CAN AND CANNOT DO WITH THE FMS INOP. (UNABLE TO DO RNAV ARRS; DIRECT AN INTXN; ETC.) WHEN WE WERE CLRED FOR THE ILS RWY 28 APCH INTO CHICAGO; WE WERE TOLD TO MAINTAIN 5000 FT MSL UNTIL XING WAVIE INTXN. MY FO; THINKING THAT THE APCH MODE ON OUR FLT DIRECTOR WOULD NOT WORK BECAUSE OF THE INOP FMS; DISCONNECTED THE AUTOPLT AS WE WERE CLRED FOR THE ILS APCH. THIS WAS A MISUNDERSTANDING; AS THE FLT DIRECTOR WOULD WORK; AND WAS REQUIRED TO BE USED BY OUR OPS SPECS FOR THIS APCH. I ARMED THE APCH MODE FOR HER; AND CONTINUED MONITORING HER FLYING AS THE AUTOPLT WAS OFF. I DID NOT NOTICE THAT WHEN THE GS BECAME ALIVE; AND INTERCEPTED; THAT WE WERE NOT QUITE TO WAVIE INTXN YET...AS THE FLT DIRECTOR STARTED TO COMMAND A DSCNT; WE FOLLOWED IT DOWN; CAUSING US TO BE ALMOST 300 FT LOW AS WE CROSSED WAVIE INTXN. I THINK SOME OF THE FACTORS TO THIS EVENT HAPPENING WERE THE INOP FMS AND MY FO'S LACK OF KNOWLEDGE ON ABNORMAL OPS WITHOUT THE FMS. ALSO; WHEN THE APCH CLRNC WAS GIVEN; THE ALT RESTR WAS NOT SPOKEN VERY CLRLY. WHEN LATER QUERIED BY ATC I DID REMEMBER THE INSTRUCTION; BUT I FEEL IT COULD HAVE BEEN SPOKEN MORE CLRLY (NOT MUMBLED BY THE CTLR).