A B737-300 FLT CREW EXPERIENCED FAILURE OF THE AIR/GROUND SWITCH AFTER TAKEOFF; AFFECTING A NUMBER OF SYSTEMS. THEY DECLARED AN EMERGENCY AND RETURNED TO DEPARTURE AIRPORT.

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AFTER TKOF RWY XXL; COULD NOT SELECT VNAV AFTER FLAP RETRACTION. COULD NOT SELECT CLB MODE REDUCED THRUST. USED LEVEL CHANGE; FO FLYING. NOTICED TCAS FAIL AND ATC ASKED US TO SWITCH XPONDERS. THEY GOT CONTACT BRIEFLY WHEN I SELECTED L XPONDER. TALKING TO DEP; CLBING TO 13000 FT; GOT CHK POS MESSAGE. ACARS FLASHED DELAY CODE AND I SAW THAT WE WERE NOT INDICATING OFF TIME ON ACARS. I TOLD FO THAT I THOUGHT THE AIR/SENSOR SWITCH HAD FAILED IN GND MODE. WE CONTACTED DISPATCH AND MAINT CTL. FO TOLD ATC AFTER CLRING US TO FL230 THAT WE'D LIKE TO MAINTAIN FL200 AND WERE WORKING WITH MAINT ON A PROB. THEY WERE HAVING TROUBLE WITH OUR XPONDER AND WE SWITCHED OUT OF TA/RA TO XPONDER ONLY. ATC VECTORED US. MAINT CTL HAD ME PULL THE AIR/GND SENSOR BREAKER TO PUT US IN FLT MODE. WE WOULD NOT HAVE REVERSE ON LNDG AND WOULD RESET BREAKER AFTER TOUCHDOWN. DISPATCH; MAINT CTL AND I DECIDED TO RETURN TO ZZZ1. I DECLARED EMER TO ENSURE LONG FULL LENGTH RWY AT ZZZ. WE RETURNED AND LANDED ON RWY XY; MANUAL SPD BRAKES AND NO REVERSE; FULL LENGTH. EQUIP WAS STANDING BY AND LNDG WAS UNEVENTFUL. WE TAXIED TO GATE; BRIEFED MAINT AND PLANE WAS TAKEN OTS. 1 FLT ATTENDANT DID NOT FEEL EMOTIONALLY ABLE TO CONTINUE ON NEXT FLT. PAX WENT TO NEXT GATE AND WE FLEW IN OTHER ACFT TO ZZZ2.