What was found
ON CLIMBOUT AT ABOUT 20,000 FT, WE GOT AN ACM OVERHEAT ANNUNCIATOR FOLLOWED BY A RISE IN CABIN ALTITUDE. WE COMPLETED APPROPRIATE CHECKLISTS AND RETURNED TO DEPARTURE. REMOVED OLD OVER TEMP SWITCH, INSTALLED NEW SWITCH. LEAK AND OPS CHECK GOOD. WORK DONE IAW MM CH. 21-51-00.