What was found
DURING DESCENT BELOW FL300 WITH THROTTLES PULLED BACK & PA HP VALVE OPEN L-R CAS MESSAGE ILLUMINATED, A LOUD BANG HEARD BY BOTH PAX & CREW. COMING FROM AFT OF PILOT SEATS BUT FWD OF PAX SEATS & ACRID BURNING ODOR FILLED ACFT FOR SEVERAL MINTUES THEN ODOR DISSIPATED. ONCE ON GROUND WITH APU RUNNING, ODOR & WHAT APPEARED TO BE SMOKE WOULD FILL ACFT IF ISOLATION VALVE WAS OPEN. WHEN ISOLATION VALVE SELECTED CLOSED, ODOR & SMOKINESS DISAPPEARED. FOUND INOP COCKPIT PAC/ACM. REPLACED WITH "REPAIRED" ACM. SERVICED OIL RESERVOIR & PERFORMED PRE-LUBE OF ACM BEARINGS. CHECKED COALESCER SOCKS. OPS CHECKS GOOD. MX IAW MM 21-00-00 & 21-52-00.