ACR WDB UNABLE MAINTAIN PRESSURIZATION ON CLIMB STARTED RETURN TO DEP STATION AND DESCENDED; THEN FOUND SWITCH IN WRONG POSITION CAUSED PROBLEM AND RESUMED FLT.

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FLT DEPARTED LAX WITH #1 PACK INOP AND LISTED ON MEL AND FILED FOR A CRUISE ALT OF FL330. WE EXPECTED THE CABIN ALT THEREFORE TO CLB TO A HIGHER ALT THAN NORMAL IF BOTH PACKS HAD BEEN OPERABLE; SO WE WERE NOT CONCERNED WHEN IT INDICATED 8000'. AS IT CONTINUED TO CLB PAST 9000'; WE STOPPED OUR CLB AND REQUESTED 29000'. THE CABIN CONTINUED TO CLB; SO WE ASKED FOR 25000'. THE CABIN HAD THEN REACHED 11000'; SO WE ASKED FOR 10000' AND CLRNC BACK TO LAX. WE WERE GIVEN CLRNC TO 12000' AND DIRECT TNP. IN THE PROCESS OF RUNNING THE ABNORMAL CHKLIST; WE FOUND THE ENG START SWITCH IN THE ARM POS. WHEN IT WAS PLACED IN THE START ABORT POS; THE #2 PACK RETURNED TO OPERATION. ALL OF THE PACK INDICATIONS HAD BEEN NORMAL WITH THE EXCEPTION OF THE FLOW INDICATOR; WHICH WE ASSUMED WAS INOP (IT DID NOT CHANGE WHEN THE PACK CAME ON). PRESSURIZATION THEN STABILIZED WITH THE CABIN AT AN ALT OF 12500'. THE OXYGEN MASKS IN THE CABIN DID NOT DROP. CABIN THEN DSNDED TO 6000'. WE DID NOT DECLARE AN EMER; HOWEVER I DID NOT HAVE TIME TO RE-EMPHASIZE THAT FACT TO THE CTLR WHEN I HEARD HIM SAY HE WAS GOING TO TREAT US AS AN EMER IN ORDER TO GET US DOWN. NO EMER DSCNT WAS EXECUTED. ONCE EVERYTHING HAD STABILIZED; WE REQUESTED ENRTE CLRNC TO ATL AND CONTINUED W/O INCIDENT. NO ALTS WERE INFRINGED UPON AND ALL MANEUVERING WAS PERFORMED AFTER RECEIVING CLRNC TO DO SO; BUT I DID HEAR THE CTLR PUT AT LEAST 1 ACFT ON A VECTOR IN ORDER TO GET US DOWN THROUGH HIS ALT. (THE CTLRS DID A SUPER JOB AND WERE VERY HELPFUL.) THE CULPRIT SWITCH SHOULD HAVE BEEN PLACED TO START ABORT PRIOR TO TAXI (AND WAS ANSWERED FOR ON THE BEFORE TAXI CHKLIST). ITS POS; HOWEVER; WAS APPARENTLY MISREAD. THAT; ALONG WITH THE INOP PACK AND GAUGE; LED US INTO THE PRESSURIZATION PROB.