B737 CREW DEPARTED ORD WITH MEL ITEM INOP.

Narrative

PRIOR TO TKOF AT ORD; ELECTRIC LIGHT ILLUMINATED ON OVERHEAD PANEL. CONSULTED B737 PROC AND IT SAID LIGHT INDICATED A FAULT IN THE DC OR STANDBY ELECTRICAL SYS. NO FIX PROC OR GUIDANCE LISTED. CALLED MAINT AND REFERRED TO MEL PROC FOR ELECTRIC LIGHT. THROUGH TECHNICAL AND MAINT REPRESENTATIVES ON RADIO; PERFORMED PROC FOR STANDBY PWR CHK. PROC ACCOMPLISHED WITH SUCCESSFUL TEST. DECISION TO CREW PLACARD SYS WAS AGREED. TKOF AND REST OF FLT UNEVENTFUL. AFTER LNDG AT BOS; WAS TOLD BY FLEET MGR THAT MEL REFERRED ONLY TO INOP LIGHT; NOT SYS. MAINT RESET SYS AT BOS WITH NO PROBS ON RETURN TO ORD. THUS WE TKOF WITH AN ELECTRICAL FAULT OF SOME MAGNITUDE AND WERE LED DOWN THE PATH DUE TO CONFUSING MEL TERMINOLOGY AND NO GUIDANCE IN THE PROCS BOOK ON SYS FAILURE AFTER GATE DEP. IF RETURNING TO THE GATE WAS THE ANSWER; THEN IT SHOULD BE PRINTED IN BOLD LETTERS IN THE PROC. WE WERE NOT ALONE IN INTERPING THIS CONFUSING PROB. ALL TRAINING SAYS THAT THIS LIGHT IS A NO-GO ITEM AT THE GATE DURING PREFLT. NO REF TO OFF-GATE PLACARDING PROCS.