PLT OF AN ATX C402 CARGO FLT DSNDED EARLY ON AN ILS APCH CAUSING ATC TO INTERVENE AND ADVISE HIM OF DSCNT BELOW THE OM GS ALT. THE RPTR ALREADY HAD FELT SOMETHING WRONG WITH HIS GS INDICATION AND WAS SELECTING #2 ILS INDICATOR WHEN HE NOTICED AN ERRONEOUS INDICATION AND HAD STARTED TO LEVEL OFF.

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WHILE PREPARING TO SHOOT THE ILS APCH; I WAS VECTORED ONTO THE LOC. I INTERCEPTED THE LOC AND WAS TRACKING INBOUND AT MY ASSIGNED ALT OF 2200 FT MSL. THE GS NEEDLE WAS IN THE NORMAL; HIGH POS; AND THE LOC NEEDLE WAS CTRED. THE GS NEEDLE THEN BEGAN TO MOVE TOWARD CTR; INDICATING GS INTERCEPT. I LOWERED THE LNDG GEAR AND BEGAN MY DSCNT. AS I DSNDED; THE GS INDICATOR INDICATED THAT I WAS ESTABLISHED ON THE GS. I SOON REALIZED THAT I HAD NOT YET RECEIVED AN OM BEACON; AND THE GS INDICATOR SEEMED TO NOW BE PERFECTLY MOTIONLESS. I QUICKLY TUNED THE #2 RADIO TO THE LOC FREQ TO DOUBLE CHK THE GS INDICATION. AS I SUSPECTED; THE #2 RADIO INDICATED THAT I WAS BELOW THE GS. I QUICKLY LEVELED OFF AT APPROX 1750 FT MSL. AT THAT TIME THE APCH CTLR ADVISED ME OF MY ALTDEV; AND ADVISED ME THAT I HAD NOT YET CROSSED THE FINAL APCH FIX. AT THIS POINT; I HAD ALREADY BROKEN OUT OF THE CLOUDS AND HAD THE ARPT IN SIGHT. I CANCELED IFR AND PROCEEDED TO LAND VISUALLY. I HAVE SINCE MADE IT A RULE TO ALWAYS TUNE BOTH OF MY RADIOS TO THE NAVAID WHEN SHOOTING AN APCH. ALSO; IF I SUSPECT AN ERROR IN INDICATIONS; I WILL ASK THE CTLR FOR MY DISTANCE FROM THE MARKER. I FEEL THIS ALTDEV WOULD HAVE BEEN AVOIDED IF I HAD APPLIED THESE PRACTICES THEN.