SMA EXPERIENCES INST FAILURE WHILE IN IMC. MAKES UNCTLED UNAUTH DSCNT TO BELOW CLOUD BASES.

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IT WAS TO BE FLT FROM NEW BERN; NC; ALONG V-139 TO GRAND STRAND AS THE FIRST LEG OF AN IFR XCOUNTRY TRAINING FLT. CONDITIONS WERE RPTED AS BEING 3000-4000 FT OVCST; BUT NO AIRMETS; SIGMETS; OR FREEZING CONDITIONS WERE EXPECTED ALONG OUR RTE OF FLT. THE FIRST PROB ENCOUNTERED WAS WHEN PRE-FLTING THE ACFT. NO ELECTRICAL PWR WAS SUFFICIENT TO TEST THE NAV LIGHTS OR RADIOS AND NO PWR TO START THE ENG. THE DECISION TO HAND PROP THE ACFT WAS TAKEN AND TO PROCEED ON WITH THE FLT. APPROX 30 MINS INTO THE FLT THE SMA WAS LEVEL AT 6000 FT IN IMC AND WORKING WITH THE LCL APCH CTR. I THEN NOTICED THE STUDENT WAS HAVING PROBS IN STAYING ON COURSE; SO I TOOK CTL OF THE ACFT AND REGAINED OUR CORRECT COURSE; ONLY TO FIND THE COMPASS AND DIRECTIONAL GYRO PRECESSING APPROX 10 DEGS EVERY 5 MINS. I THEN NOTICED THE SUCTION GAUGE READING LOWER THAN NORMAL (APPROX 4 INCHES). BELIEVING THAT WE HAD A PARTIAL VACUUM FAILURE THE DIRECTIONAL GYRO AND ATTITUDE INDICATOR WAS COVERED; SO AS NOT TO BE A DISTR. WHILST THE STUDENT CONTINUED TO FLY THE ACFT PARTIAL PANEL I BEGAN TO RECHK OUR POS ON THE CHART. WITHIN MOMENTS OF LOOKING AWAY I NOTICED THE SMA WAS NOW LOSING ALT AT APPROX 1500 FPM WITH THE AIRSPD INCREASING RAPIDLY AND THE TURN COORDINATOR INDICATING A R TURN. I BEGAN RECOVERY FROM THE NOSE-LOW UNUSUAL ATTITUDE; BUT FOUND IT DIFFICULT TO MAINTAIN WINGS LEVEL USING THE TURN COORDINATOR. AT APPROX 4000 FT; THE SMA ENTERED VMC. THE ACFT WAS STILL IN APPROX 30 DEGS OF BANK TO THE R (ALTHOUGH THE TURN COORDINATOR CONTINUED TO INDICATE WINGS LEVEL) AND 10-15 DEGS OF PITCH DOWN. ON ROLLING WINGS LEVEL; VISUALLY; I RPTED TO THE LCL APCH CTR WE HAD HAD A SERIOUS PROB WITH OUR INSTS; WERE LEVEL AT 4000 FT AND REQUESTED VECTORS BACK TO NEW BERN AT 3000 FT TO REMAIN VMC. THE CTLR APPROVED THE DSCNT FROM 4000-3000 FT AND BEGAN TO VECTOR THE SMA BACK TOWARDS NEW BERN. THE TURN COORDINATOR CONTINUALLY INDICATING A TURN. ON APCH TO LNDG IT WAS THEN DISCOVERED THAT NEITHER THE FLAPS NOR LNDG LIGHT WORKED. LOOKING BACK I NOW BELIEVE THAT THE PARTIAL VACUUM FAILURE CONTRIBUTED TOWARDS THE INCIDENT; HOWEVER; I WAS TOO QUICK TO DISREGARD THE DIRECTIONAL GYRO AND ATTITUDE INDICATOR BECAUSE ALTHOUGH PRECESSING MORE THAN NORMAL; SOME INDICATION WOULD HAVE BEEN BETTER THAN NONE AT ALL. THE TURN COORDINATOR I BELIEVE HAD INSUFFICIENT ELECTRICAL PWR TO READ ACCURATELY. IF NO PWR WAS AVAILABLE INITIALLY TO PWR THE LIGHTS AND RADIOS; THEN POSSIBLY INSUFFICIENT PWR WAS AVAILABLE TO 'EXCITE' THE ALTERNATOR TO WHICH WOULD HAVE SUPPLIED ALL THE ELECTRICAL INSTS INCLUDING THE FLAPS AND LNDG LIGHT; ALL WHICH REQUIRE A LARGE ELECTRICAL LOAD. REALIZING THE FACTORS WHICH LED TO THE INCIDENT BEGAN ON THE GND; I NOW KNOW THAT; IF NO PWR IS IN THE BATTERY TO SUPPLY SMALL ELECTRIC LOADS; THEN IT'S POSSIBLE INSUFFICIENT PWR WILL BE AVAILABLE FOR THE ALTERNATOR TO WORK; THUS MAKING IT ILLEGAL AND DANGEROUS TO FLY IN IMC.