As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTS, THAT THEY WERE ON THE RAMP WITH THE APU RUNNING, AND AFTER THEY SELECTED THE CABIN AIR ON, THEY GOT AN ACRID SMELL AND SMOKE IN THE CABIN. MX C/A: REMOVED AND REPLACED ACM WITH REPAIRED UNIT. PERFORMED SATISFACTORY OPERATIONAL CHECK. REFERENCED AMM 21-52-13.