A C402 ON TKOF CLB AT 4200 FT DIVERTED DUE TO REDUCED CLB PERFORMANCE. FOUND FLAPS FULLY EXTENDED WITH SELECTED KNOB IN 'UP' POS.

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UPON TKOF AT ZZZ; THE FLAP SELECT KNOB WAS IN THE UP POS. WHEN THE AIRPLANE WAS TAKING OFF; I NOTICED THE RATE OF CLB WAS GREATLY REDUCED 300-500 FPM. AFTER TROUBLESHOOTING THE PROB (CHKING FLAPS UP; GEAR UP; MIXTURE RICH; WING LOCKER DOORS CLOSED); I DECIDED TO COME BACK AND LAND. WHEN I LANDED I FOUND THE FLAPS WERE DOWN WITH THE FLAP SELECT KNOB IN THE 'UP' POS. THEN I NOTICED THAT THE FLAP POS POINTER WAS AT 45 DEGS. BECAUSE IT WAS NIGHT; I WAS UNABLE TO SEE THE POS POINTER DUE TO POOR LIGHTING. I CYCLED THE FLAPS AND THEY WORKED FINE. THE REDUCED PERFORMANCE OF THE AIRPLANE WAS THE REASON FOR RETURNING TO ZZZ ARPT. ONE LAST NOTE: THE FLAPS ARE NOT VISIBLE DUE TO BEING SPLIT FLAPS. THEREFORE; WHEN LOOKING OUTSIDE THERE WOULD BE NO WAY TO DETERMINE THE FLAPS TO BE UP OR DOWN. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE AIRPLANE WAS A C402 AND IS OPERATED IN A PART 135 OP. THE RPTR STATED THE TRIP DEPARTED AND THE FLAP OP WAS NORMAL AND A LOG RPT WAS ENTERED FOR MAINT. THE RPTR STATED MAINT FOUND A LOOSE WIRE FOR THE ELECTRICAL DRIVEN FLAPS BUT IS UNAWARE OF THE FAULT LOCATION.