DESCENDED BELOW MINIMUM WHEN ALT HOLD FEATURE OF AUTO PLT DISENGAGED.

Narrative

I WAS FLYING MY PRIVATELY OWNED SMT FROM CARLSBAD (CRQ) TO GILLESPIE (SEE) TO DELIVER ACFT TO MAINT FAC. SEE WAS RPTING 600-2 IN FOG. THE LOC-D APCH WAS AN 1135'-ONE AND ONE-QUARTER MIN. I FLEW AN AUTOPLT COUPLED APCH. I REQUESTED APCH TO CALL THE OUTER MARKER AND TWR TO CALL THE MM FOR ME; SINCE I HAD HAD SOME DIFFICULTY WITH MARKER BEACON RECEIVER. TWR CONFIRMED A 3-MI FINAL AND I DSNDED TO THE MDA 1520' (1135'); WHERE I ACTIVATED ALT HOLD. SHORTLY THEREAFTER; BEFORE REACHING MAF; AUTOPLT ALT HOLD FAILED AND ACFT DSNDED TO 900' AGL. I DISABLED AUTOPLT AND LANDED NORMALLY. I OBSERVED CEILING AT 900-1000'. AUTOPLT TESTED AND FOUND TO BE WORKING NORMALLY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: TWR DID NOT QUESTION THE RPTR AS TO HIS DSCNT BELOW MIN. WHEN HE BROKE OUT; THE ACFT WAS NEAR THE ARPT AND A CIRCLE TO LAND WAS MADE. HAS RECEIVED NO INQUIRY REF THE INCIDENT FROM THE FAA. HAS ELECTRIC ON THE ACFT THAT COULD DISABLE THE ALT HOLD FEATURE; BUT BECAUSE OF THE RAPID TRIM RATE; DOES NOT USE. IN ORDER TO TRIM MANUALLY; IT IS NECESSARY TO DISENGAGE THE AUTOPLT. LENGTHY DISCUSSION ON TCA PROB AND AWARENESS OF MANY PLTS AS FAR AS ASRS.