What was found
DURING PREFLIGHT, ENGINE #2 FIRE LOOP WILL NOT TEST. GROUNDED - YES. PERFORMED B737-900 FIM 26-10 801 AND FOUND "#2 ENGINE LOOP "B" LOW DET RESISTANCE CLEAN CONNECTIONS" FAULT. ALL DETECTOR RESISTANCE VALUES ARE WITHIN LIMITS. UPON INSPECTION FOUND ISOLATOR CHAFFED THROUGH ON UPPER CORE B LOOP. REPLACED WITH SERVICEABLE FROM STOCK REF B737-900 AMM 26-10-01-400-801. REMOVED AND REPLACED #2 ENG UPPER CORE LOOP B HARNESS (MW0326) IAW AMM 26-11-01. INSTALLATION TEST GOOD. ENG FIRE DETECTION OPS TEST GOOD PER AMM 26-11-00. FIRE DET CTL MODULE BITE TEST NO FAULTS. CLEARS MEL 26-2B.