A B737-300'S AUTOPILOT APCH MODE WAS SELECTED ON TOO EARLY AT AN ARPT WHERE THE GS WAS UNUSABLE BEYOND 5 DEGS. THE ACFT PITCHED DOWN LOSING 500 FT BEFORE RECOVERY AT 1800 FT.

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WE WERE ON APCH AFTER BEING VECTORED AROUND NUMEROUS TSTMS FOR RWY 23. I HAD NOTED ON THE ATIS THAT THE GS WAS INOP BEYOND 5 DEGS R OF COURSE AND BRIEFED THE CAPT ON THIS WHEN WE FIRST PICKED UP ATIS. THE WX HAD GONE THROUGH NUMEROUS CHANGES FROM HVY TSTMS IFR TO NOW 3 MI LIGHT RAIN AND 1500 FT OVCST. THE CAPT BRIEFED AN ILS. WE WERE BELOW THE BASES AND HAD GND CONTACT AT 2300 FT MSL. WE WERE WELL R OF COURSE AND HAD BEEN TOLD TO PROCEED DIRECT TO OM AND MAINTAIN 2300 FT UNTIL ESTABLISHED ON THE APCH. THE CAPT WAS IN LNAV MODE AND I SWITCHED MINE TO THE LOC. WHEN WE WERE ABOUT 4 MI FROM THE FAF STILL R OF COURSE; THE CAPT ARMED THE APCH THE AUTOPLT PICKED UP THE GS AND BEGAN AN IMMEDIATE DSCNT DESPITE NOT BEING ON THE LOC. I TOLD HIM THAT WE NEEDED TO GET BACK UP TO 2300 FT BECAUSE WE WEREN'T ON THE LOC YET. HE BEGAN TO RECOVER AND CLB AFTER DSNDING TO 1800 FT MSL (1000 FT AGL). I HAD SWITCHED TO TWR AT THIS TIME AND WE WERE JUST COMING IN OVER THE MARKER TO R OF COURSE. LOOKING OUTSIDE I PICKED UP THE FIELD AND HE STATED HE HAD IT AS WELL DESPITE US STILL BEING AT ABOUT 2100 FT MSL. WHEN CHKING IN; TWR TOLD US WE SHOULD BE AT 2300 FT TILL ON THE APCH. I TOLD HIM THAT WE HAD A VISUAL ON THE FIELD AND WERE CLBING. HE THEN SAID OK YOU ARE CLRED FOR THE VISUAL CLRED TO LAND. WE SHOULD HAVE ASKED FOR A LESS DEG VECTOR TO INTERCEPT RATHER THAN GOING DIRECT TO THE MARKER AND NOT INTERCEPTING TILL OVER THE MARKER. THE CAPT SHOULD HAVE NOT ARMED APCH MODE TILL WE WERE ON THE LOC SINCE THE GS WAS INACCURATE MORE THAN 5 DEGS R OF COURSE. I SHOULD HAVE KEPT INSISTING THAT WE GET IMMEDIATELY BACK TO 2300 FT UNTIL ON THE LOC.