FAULTY AUTO FEATHERING (COARSEN) COMPUTER CAUSED ERRONEOUS ENG INDICATIONS WHICH RESULTED IN AN ENG SHUTDOWN AND RETURN TO LAND. RECURRING EVENT IN THIS SAAB 340 ACFT IS TAIL PIPE HOT TEMP INDICATIONS CAUSED BY MOISTURE IN THE WIRING; PROBABLY THE CANNON PLUG.

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AFTER TKOF RWY 20 CHA; CLBING THROUGH 400 FT; AUTO TRIM LIGHT ON CWP ILLUMINATED. THE YAW DAMPER WAS DISENGAGED AND THE AUTO TRIM LIGHT WENT OUT. A FEW SECONDS LATER THE TAIL PIPE HOT LIGHT ILLUMINATED. MEMORY ITEMS (PWR LEVER TO FLT IDLE) WERE EXECUTED DUE TO THE EVENT. THE APPROPRIATE CHKLISTS WERE THEN STARTED. WHILE THE FO WAS USING THE CHKLIST; I WAS TRYING TO THINK OF WHAT I COULD DO THAT MIGHT CORRECT THE TAIL PIPE HOT WARNING. THE SAAB HAS A HISTORY WITH THIS PROB; NORMALLY DUE FROM MOISTURE GETTING INTO THE CANNON PLUG THAT CTLS THIS INDICATION. I RESET THE TAIL PIPE HOT CIRCUIT BREAKER TWICE AND THEN THOUGHT I WOULD PUSH THE PWR LEVER FORWARD AND THEN RETARD IT AGAIN. WHEN THE PWR LEVER WAS PUSHED FORWARD; THE TORQUE ON THE AFFECTED ENG BEGAN TO DROP 30-40 PERCENT; INDICATED BY THE TORQUE GAUGE AS WELL AS A STRONG SURGE TO THE L. THINKING THE ENG HAD FAILED AS WELL AS THE TAIL PIPE HOT LIGHT STILL ILLUMINATED THE ENG WAS SHUT DOWN. MEMORY ITEMS WERE USED AND THEN THE CHKLIST WAS USED TO FINISH THE ENG SHUTDOWN PROCS. DURING THE APCH; THE CHA LOC APPEARED TO BE INOP. AFTER COMPARING THE FREQ WITH THE FO'S SIDE; I GAVE THE CTLS TO THE FO FOR THE APCH. AFTER XING THE OM; I DID NOT FEEL THAT WE WERE IN GOOD POS TO CONTINUE THE APCH; SO A MISSED APCH WAS EXECUTED. THE FO SHOT THE 2ND APCH AND WHEN THE RWY WAS VISIBLE; I LANDED THE ACFT WITH NO FURTHER PROBS. MAINT LATER FOUND THE TORQUE FLUCTUATION WAS CAUSED BY A FAULTY AUTO COARSEN COMPUTER. THE CRM SKILLS BTWN MYSELF AND THE FO WERE GOOD. CHKLIST USAGE BECAME A BIT CONFUSING DUE TO THE EVER CHANGING PROBS WHICH REQUIRED DIFFERENT CHKLIST. ALSO; CHANGING CTL OF THE ACFT (DUE TO INOP LOC ON CAPT SIDE) PLAYED A ROLE AS FAR AS WHO AND WHAT CALLS SHOULD HAVE BEEN MADE. HOWEVER; AFTER WE GOT CAUGHT UP WITH THE CHKLISTS AND SETTLED DOWN; WE WERE ABLE TO STAY AHEAD OF THE PLANE AND PERFORM AS WE SHOULD. I FEEL THAT MY PERFORMANCE WOULD HAVE BEEN BETTER IF I WOULD HAVE RELAXED AND SLOWED MY BRAIN DOWN; IN ORDER TO PROPERLY THINK THINGS THROUGH AND CTL THE COCKPIT ATMOSPHERE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CAPT VERIFIED ACFT TYPE AS SAAB 340. THE AFTER TKOF PROC IS TO TURN ON THE YAW DAMPER AND AUTOPLT AT 400 FT. WHEN THAT WAS TURNED ON; THE COARSEN COMPUTER; WHICH IS THE AUTO FEATHERING COMPUTER; WAS FAULTY. CONSEQUENTLY; THE AUTO TRIM LIGHT CAME ON; BECAUSE WITH THE AUTOPLT ON; THE AUTO TRIM SIGNALS ARE LINKED TO THE RUDDER AND ELEVATORS THROUGH THE AUTOPLT; AND; BECAUSE THE 'COARSEN' COMPUTER WAS FAULTY; A WARNING ON THE CAUTION WARNING PANEL (CWP) SHOWED AN AUTO TRIM FAULT. WHEN THE YAW DAMPER WAS TURNED OFF; THE LIGHT WENT OUT. SHORTLY THEREAFTER; A TAIL PIPE HOT LIGHT ILLUMINATED. THE CAPT KNEW FROM PREVIOUS EXPERIENCE THAT THE SAAB 340 HAD A HISTORY OF TAIL PIPE HOT WARNINGS CAUSED BY MOISTURE IN THE CANNON PLUG. THINKING THAT MAY HAVE BEEN THE CAUSE; RPTR RESET THE CIRCUIT BREAKER TO SEE IF THAT WOULD CORRECT THE INDICATION. HOWEVER; WHEN THE PWR LEVER WAS PUSHED FORWARD; THE TORQUE DROPPED; AND THINKING THE ENG HAD FAILED; THEY SHUT DOWN THE ENG AND RETURNED TO LAND. SINCE POST FLT INSPECTION REVEALED THE FAULTY COMPUTER; HAD THE FLC TURNED OFF THE COMPUTER; WHICH IS SOP ANYWAY AT 1500 FT; IT PROBABLY WOULD HAVE MITIGATED THE FOLLOW ON PROBS. AT THAT POINT RPTR CAPT SAID HE FELT RUSHED; TRYING TO COMPLETE CHKLIST; RETURNING FOR AN APCH IN POOR WX CONDITIONS. CONSEQUENTLY; ON REFLECTION; HE THINKS HE MAY HAVE SELECTED THE WRONG ILS FREQ FOR THE APCH; BUT TURNED THE APCH OVER TO THE FO WHOSE INSTS WERE NORMAL. POST FLT INSPECTION REVEALED THAT THE COARSEN COMPUTER WAS BAD AND THE CAUSE OF THE PROBS. IT WAS TURNED OFF AND THE ACFT FERRIED BACK TO ITS BASE FOR REPAIR.