What was found
ON ILS APPROACH TO DTW, APPROACH MODE ARMED AT 4000 FT, SHOWING HIGH ON SLOPE. AUTO PILOT ON. UNCOMMANDED DESCENT TO 4000 FPM NOTED. AP OFF. AP REMAINED OFF FOR REMAINDER OF FLIGHT. UPON UNCOMMANDED DESCENT, AVIONICS LIGHT NOTED. YELLOW GS NOTED EADI. SLOPE INDICATIONS ERRATIC. AP DISCONNECTED. REMAINED OFF. OPEN CIRCUIT IN GS ANTENNA. REMOVED AND REPLACED GLIDESLOPE ANTENNA IAW SF340 MM CH. 34-30-10. VOR LOCALIZER AND GLIDESLOPE CHECKS GOOD IAW SF340 MM CH 34-30-00, 34-51-00.