What was found
SUPPLY DUCT TEMP INDICATOR NEEDLE SWINGS AND MOVES ERRATICALLY, NEEDLE SWINGS 10DEG TO 60 DEG C, ONLY IN THE "CONT CAB" AIR TEMP SETTING. THERE ARE NO CAUTION LIGHTS ASSOCIATED WITH THE NEEDLE SWINGS. CONT CAB OUTPUT TEMP IS STABLE AND APPEARS TO BE OPERATING AT THE DESIRED/SELECTED TEMP SETTING. ACFT GROUNDED. VERIFIED AIR TEMP SELECTOR SWITCH S1 IS OPEN IN CONTROL CABIN POSITION REF SSM 21-61-51. REPLACED CABIN TEMP PANEL P5-17. OPS CK NORMAL, REF MM 21-61-15.