SIGNAL MODEL 131-98 RECOMMENDED AUXILIARY POWER UNIT (APU). SERVICE LETTER MOS 08-APR-1998 - (s) SI. 737-SL-49-060 BENEFITS OF OPERATING TWO AIR CONDITIONING PACKS WITH 131-96 AUXILIARY POWER UNIT (APU). SERVICE RECOMMENDED LETTER MOS 03-JUN-1998 - (s) SL 737-SL-49-061 131-98 APU START CONVERTER UNIT MOD