AN ACR MLG CREW HAD AN ILS STICK IN THE 'TEST' POS CAUSING THE ACFT TO FLY THROUGH THE ILS COURSE.

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APCHING ATL FROM THE NW; SET UP FOR AN ILS TO 8L. ONCE VECTORING BY APCH CTL STARTED; I SELECTED THE ILS AND LEFT FO ON VOR TO MONITOR POS. VECTORED TO A 30 DEG OR SO INTERCEPT AND GIVEN A CLRNC TO MAINTAIN 5000 UNTIL BAHRR AND CLRED FOR APCH. SHORTLY THEREAFTER; APCH CTL STATED THAT WE HAD OVERSHOT THE LOC AND DIRECTED US TO TURN L IMMEDIATELY; RE-INTERCEPT THE LOC AND DSND TO 4500 FT. MY LOC WAS STILL SHOWING OFF TO THE R WITH NO FLAGS IN VIEW. I IMMEDIATELY COMPLIED WITH THE INSTRUCTIONS AND SWITCHED THE FO RADIO TO ILS. HIS NAV INSTS SHOWED THE LOC OFF TO THE L. CONFUSED; I TRIED TO RE-IDENT MY ILS BUT COULD GET NO IDENT. WE CONTINUED THE APCH USING THE FO'S ILS. I ASSUMED MY ILS HAD FAILED BUT COULD NOT FIGURE WHY I HAD RECEIVED NO FLAGS. I ENTERED THE DISCREPANCY IN THE FLT LOG AND LATER CHKED WITH MAINT TO SEE WHAT THEY FOUND. IT SEEMS THE ILS TEST SWITCH WAS STUCK IN THE TEST POS (DOWN/R) AND COULD NOT BE UNSTUCK. APPARENTLY; ME OR MY FO HIT THE TEST BUTTON WHEN WE SELECTED THE ILS ON MY SIDE. WE HAD PREVIOUSLY TUNED AND IDENTED THE ILS BUT HAD GONE BACK TO TRACK THE VOR. WHAT I HAVE SINCE DISCOVERED IS THAT; IN THE TEST POS; NO FLAGS ARE IN VIEW ON THE HSI OR ADI. IN OTHER WORDS; IT ALL LOOKS PERFECTLY NORMAL. THE ONLY WAY WE COULD HAVE KNOWN THE SWITCH WAS STUCK WAS TO HAVE BEEN CONTINUOUSLY MONITORING THE IDENT.