A C560 DIVERTED TO AN ALTERNATE FIELD FOR MAINT DUE TO THE MAIN CABIN DOOR UNLOCK WARNING LIGHT ILLUMINATED.

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WE DEPARTED PWK WITH 6 PAX TO MSP. JUST AFTER LIFTOFF; THE CABIN DOOR LIGHT CAME ON; WHICH INDICATES A MAIN CABIN DOOR UNLOCKED CONDITION. I TOLD THE FO TO ASK DEP FOR A LEVELOFF AT 3000 FT MSL. AFTER LEVELING; THE FO VISUALLY INSPECTED THE DOOR AND ALL INDICATIONS ON THE DOOR ITSELF SHOWED IT LOCKED PROPERLY. I HAD BRIEFED DURING THE TAXI CHKLIST THAT IN CASE OF ANY MAJOR EMER; IE; ENG FAILURE OR A FIRE; WE WOULD GO TO ORD; WHICH WAS 5 MI AWAY; WITH GREATER LENGTH AND WIDER RWYS DUE TO SHORT; NARROW RWYS WITH STRONG XWINDS AT PWK. IN CASE OF A LESS SEVERE PROB; I BRIEFED THE PLAN AS GOING TO MKE; ABOUT 30 MI N; WHICH HAS A CITATION SVC CTR. AFTER THE VISUAL INSPECTION BY THE FO; I WAS 99% SURE IT WAS A PROB WITH THE SWITCH AND NOT THE DOOR ITSELF. I INSTRUCTED THE FO TO CALL OUR COMPANY MAINT DEPT ON THE PHONE FOR GUIDANCE; WHILE I COORDINATED WITH ATC FOR A CLRNC TO MKE. I SAID THERE WAS A MAINT ITEM WE NEEDED TO HAVE LOOKED AT AT THE SVC CTR THERE. ATC ASKED IF WE NEEDED ANY ASSISTANCE OR IF WE WANTED TO DECLARE ANYTHING AND I SAID NO. WE ACCOMPLISHED THE CHKLIST FOR CABIN DOOR LIGHT ON IN FLT AND THERE WAS NO CHANGE. COMPANY MAINT SAID THEY WOULD CLR US TO MSP IF WE WANTED TO STAY LOW. I WAS STILL UNCOMFORTABLE WITH THE SIT AND ELECTED TO GO TO MKE. WE MADE A SLIGHTLY OVERWT (BY 400 LBS) LNDG AT MKE. THE SVC CTR THEN ACCOMPLISHED THE MAINT ON THE SWITCH AND THE OVERWT LNDG INSPECTION. WE THEN REBOARDED OUR PAX; RETURNED 2 TO PWK AND TOOK THE REST ON TO MSP.