A B737-300'S CABIN ALT WARNING HORN SOUNDED CLBING THROUGH FL180. WITH OTHER NON RELATED FAILURES THE CREW DECLARED AN EMER AND RETURNED TO LAND.

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AIRPLANE THOUGHT IT WAS STILL ON THE GND (AIR/GND SENSING SWITCH). ON CLBOUT; INITIALLY GOT 'CHK POS' ON FMC; SOON FOLLOWED BY NUMEROUS ISSUES SUCH AS TCAS FAIL; NO AUTOTHROTTLES; NO LNAV; NO VNAV; ETC; INCLUDING LOSS OF PRESSURIZATION. PASSING FL180; GOT ALT WARNING HORN WITH PRESSURE ALT READING (AND HOLDING) AT 10000 FT. CAPT LEVELED OFF; AND BEGAN A SLOW DSCNT WHILE I COORDINATED WITH ATC. WE ENTERED PRESENT POS HOLDING WHILE ATTEMPTING TO RUN APPROPRIATE CHKLIST. COMPLETED LOSS OF CABIN PRESSURIZATION CHKLIST; COORDINATED WITH MAINT; ATC; FLT ATTENDANTS; DISPATCH; ETC. RETURNED TO ZZZ WITH UNEVENTFUL; THOUGH OVERWT (119000 LBS) LNDG. LNDG WAS SMOOTH WITH MINIMUM VERT SPD. THRUST REVERSERS DEPLOYED NORMALLY; AS DID SPD BRAKES. LONG ROLLOUT WITH VERY MINIMUM BRAKING. RETURNED TO GATE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE REPORTER STATED THAT WHEN THE CABIN ALT WARNING HORN SOUNDED THE CREW IDENTIFIED THE HORN IMMEDIATELY AND LEVELED OFF. THEY SWITCHED THE CABIN ALT CONTROLLER TO STBY BUT THAT DID NOT SOLVE THE PROBLEM. THE REPORTER DOES NOT BELIEVE THE CABIN PRESSURIZED AT ALL AND THAT THE GND/AIR SENSE SWITCH FAILED ON TKOF. AN EMERGENCY WAS DECLARED AND THEY HEADED BACK TO THE DEPARTURE ARPT. THE SECONDARY INDICATIONS DID CONFUSE THEM AND WERE NOT CONNECTED TO A GROUND AIR SENSE SWITCH FAILURE UNTIL THEY WERE ON THE GROUND. THE CHECK LIST FOR LOSS OF PRESSURIZATION DID NOT ADDRESS THE FLT STATION LIGHTS THAT WERE ON AS WELL AS THE LOSS OF THE AUTO THROTTLES AND VNAV.