ACR LGT TKOF ABORT DUE WARNING HORN. APU HAD NOT BEEN SECURED NOR HAD THE CHECKLIST BEEN COMPLETED.

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ACFT ABORTED INITIAL TKOF ROLL DUE TO APU STILL RUNNING. ON TAXI OUT TO RWY 32R AT ORD THE #3 ENG WAS SHUT DOWN TO CONSERVE FUEL DUE TO DEP DELAYS. APCHING THE HOLD SHORT THE #3 ENG WAS RESTARTED. THE APU WAS LEFT RUNNING TO ALLOW FOR A 2 MIN COOL DOWN. THE SLIDE BAR FOR THE APU ON THE MECHANICAL CHKLIST WAS NOT SLID TO THE TKOF SIDE AS PER NORMAL OPERATING PROCS. THE ACFT WAS CLRED INTO POS AND HOLD AND THEN CLRED FOR TKOF. AS PWR WAS ADVANCED WHILE STILL COMPLETING THE LAST FEW ITEMS ON THE MECH CHKLIST; THE TKOF WARNING HORN SOUNDED. THE CAPT RETURNED THE THROTTLES TO IDLE. THE ACFT ROLLED FORWARD ONLY A FEW FT AND THE CAPT ELECTED TO TAXI THE ACFT UP TO THE FIRST TXWY ENTRANCE AND EXITED THE RWY. THE ACFT WAS CLRED FOR A SECOND TKOF WHICH WAS UNEVENTFUL. THE ABORTED TKOF WAS CAUSED BECAUSE THE APU WAS STILL RUNNING. THE CHKLIST WAS NOT YET COMPLETE BUT THE PWR WAS BEING ADVANCED. IF THE ADVANCE OF PWR HAD BEEN DELAYED UNTIL THE CHKLIST WAS CALLED COMPLETE; THE APU WOULD HAVE BEEN SECURED AND THERE WOULD HAVE BEEN NO REASON TO ABORT THE TKOF. THE APU SHOULD HAVE BEEN SECURED PRIOR TO TAKING THE RWY; BUT WAS OVERLOOKED WHILE ATTN WAS FOCUSED ON LNDG TFC TO RWY 27R AND HEAVY RADIO COMS OCCURRING AT THE TIME. FORTUNATELY THE MECHANICAL CHKLIST WORKS. I HAVE SINCE MODIFIED MY TECHNIQUE TO RUN MY FINGER DOWN THE CHKLIST PRIOR TO TAKING THE RWY TO INSURE ONLY RWY ITEMS ARE LEFT TO COMPLETE. I RUN MY FINGER DOWN THE CHKLIST AGAIN AFTER ALL ITEMS ARE COMPLETED AS IS NORMAL PROC. CAPTS CAN ALSO ASSIST THEIR CREWS BY ALLOWING THEM SUFFICIENT TIME TO COMPLETE THEIR CHKLISTS AND CALL THEM COMPLETE PRIOR TO ADVANCING PWR.