What was found
COCKPIT SMOKE: AFTER STARTING THE APU WE HAD R PACK FAULT EICAS MESSAGE. WE WERE UNABLE TO CLEAR IT AFTER RESTART. WHEN TURNING RPACK OFF AND BACK ON AS INDICATED BY MEL PROCEDURE ELECTRIC SMOKE LIKE ODOR AND ASH LIKE SUBSTANCE CAME OUT OF COCKPIT AIRCONDITIONING VENTS COVERING EVERYTHING WITH WHITE ASH/DUST LIKE SUBSTANCE. ALSO A STRANGE GRINDING NOISE & VIBRATION FELT ON FLOOR. AFTER SHUTTING PLANE DOWN, OPENED AFT EQUIPMENT BAY & COULD SMELL ELECTRIC SMOKE ODOR INSIDE. REMOVE RH ACM. P/N GG431-2010-3 S/N 01099. INSTALLED CUSTOMER SUPPLIED OVERHAULED UNIT. P/N GG431-2010-3 S/N 00648 WORK DONE IN REF TO GL-6000 AMM 21-52-21 21-52-09. OPS AND LEAK CHECKS GOOD.