What was found
IN LEVEL FLIGHT, CREW DETECTED ODOR IN AFT CABIN SPREADING FORWARD TO COCKPIT. NOT ELECTRICAL, BUT MORE OF SOMETHING GETTING VERY HOT BUT NOT QUITE BURNING. AIR CONDITIONING UNIT WAS SHUTOFF AND THE ODOR DISSIPATED. ACFT RETURNED TO BASE. MECHANIC NOTHING COULD BE FOUND AT THIS TIME. ON NEXT RUN UP A/C UNIT SHUTDOWN AFTER 10 MINUTE GROUND RUN COMPRESSER AND FAN UNITS BUT NO ODOR OCCCURED. FOUND FAULTY GENERATOR FAILED RELAYS. REPLACED RELAYS. A/C UNIT OPERATES CORRECTLY BUT ODOR RETURNED DURING 10 MINUTE MX FLIGHT. OPEN