AN MD80 R PACK SUPPLY PRESSURE INDICATION WAS OUT OF LIMITS. TECHNICIANS SWAPPING INDICATORS FOR TROUBLESHOOTING FIXED R INDICATOR. L NOW FAILED. FOUND FAILED INDICATOR FROM R PACK INSTALLED IN L PACK.

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ON LOG PAGE X; THE PLT WROTE THAT THE R AIR CONDITIONING SUPPLY GAUGE INDICATED 33 PSI AND THE L GAUGE INDICATED 27 PSI. PER MD80 MAINT MANUAL; THE R INDICATION WAS OUT OF LIMITS. WE DECIDED TO REPLACE THE R SUPPLY PRESSURE INDICATOR AND THE R XMITTER. WHEN WE TURNED THE PACKS ON TO DO AN OPS CHK; THE R-HAND INDICATION WAS WITHIN LIMITS; BUT NOW THE L PACK PRESSURE INDICATION WAS ERRATIC. I SIGNED OFF LOGBOOK PAGE X AS HAVING FIXED THE R-HAND INDICATION PROB AND CREATED A NEW LOGBOOK ENTRY PAGE Y; STATING THAT THE L PACK INDICATION WAS ERRATIC. THE AIRPLANE WAS SUPPOSED TO BE AT THE GATE SO WE WERE UNABLE TO DO ANY FURTHER TROUBLESHOOTING AND DEFERRED THE PROB PER MEL 21-3. WHEN THE NEXT MECH ADDRESSED THE WRITE-UP FOR THE L PACK PRESSURE INDICATION PROB ON LOG PAGE Y; HE REMOVED A PRESSURE INDICATOR FROM THE L SIDE THAT HAD THE SAME SERIAL NUMBER AS THE PRESSURE INDICATOR THAT WE HAD; SUPPOSEDLY; INSTALLED ON THE R SIDE. THE ONLY THING I CAN THINK THAT COULD HAVE HAPPENED IS THAT MY CO-WORKER SWAPPED THE INDICATORS FROM L TO R TO DO SOME TROUBLESHOOTING AND THEN FORGOT TO SWAP THEM BACK. SO THE PRESSURE INDICATOR SERIAL NUMBER X THAT WAS SUPPOSED TO GO ON THE R SIDE ENDED UP GOING ON THE L SIDE.