What was found
DURING TAXI OUT, THE "APU DET INOP" ANNUNCIATOR LIGHT ILLUMINATED INTERMITTENTLY, SWITCHED APU TO OFF AND APPROXIMATELY 2 MINUTES LATER RECEIVED AN APU FIRE INDICATION. PULLED APU FIRE HANDLE AND DISCHARGED THE APU FIRE BOTTLE. MAINTENANCE TROUBLESHOT AND FOUND FIRE DETECTION WIRE BROKEN AT GROUND IN APU BAY UNDER TUB ISSUED MEL 26-8, COND. 1. "APU FIRE DETECTION SYSTEM INOP". WHEN AIRCRAFT ARRIVED IN PHX FIRE DETECTION WIRE WAS RETERMINATED, FIRE BOTTLE REPLACED, OPERATIONAL CHECK WAS GOOD.