B737 FLT CREW EXPERIENCES ELECTRICAL PROBLEMS WITH EXTERNAL THEN APU POWER DURING PREFLIGHT AND PUSH BACK. THE EXTENT OF THE PROBLEM GOES UNDETECTED UNTIL SHUTDOWN WHERE EXTERNAL POWER CANNOT BE ESTABLISHED AND THE APU WILL NOT START.

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THE FO AND I HAD ARRIVED AT THE ACFT EARLY AND HAD CONDUCTED OUR PREFLT DUTIES IN AN UNHURRIED MANNER. AS DEP TIME NEARED THE RAMP PERSONNEL WERE HEARD STRIKING THE SIDE OF THE ACFT WITH THEIR HANDS TO SIGNAL THEIR DESIRE TO DISCONNECT EXTERNAL ELECTRICAL PWR. THE FO REACHED UP TO BRING THE APU ON LINE TO CARRY THE ACFT'S ELECTRICAL REQUIREMENTS. AT THAT POINT THE AIRPLANE WENT MOSTLY DARK AND I HEARD HIM SAY 'THAT'S NOT RIGHT.' I HAD ASSUMED THAT THE RAMP PERSONNEL HAD PREMATURELY DISCONNECTED THE EXTERNAL PWR PRIOR TO HIM CONNECTING THE APU. I REACHED UP TO THE ELECTRICAL PANEL AND MOVED THE SWITCH TO BRING THE APU ON LINE ELECTRICALLY. AFTER A MOMENTARY HESITATION THE APU PICKED UP THE LOAD. WE VERIFIED THAT ALL OUR PROGRAMMED DATA WAS STILL VALID. THE DATA WAS VALID. WE COMPLETED THE CHKLISTS AND PUSHED BACK FOR ENG START. AT THIS POINT I HAD ALREADY MADE MY FIRST ERROR. WHEN THE FO STATED 'THAT'S NOT RIGHT' HE WAS TRYING TO COMMUNICATE TO ME THAT HE HAD DETECTED AN ERROR IN THE AIRPLANE'S ABILITY TO TRANSITION FROM GND ELECTRICAL PWR TO APU PROVIDED ELECTRICAL PWR. IT WAS OBVIOUS TO HIM THAT I HAD NOT CORRECTLY UNDERSTOOD HIM. I HAD ERRONEOUSLY THOUGHT THAT OUR RAMP PERSONNEL WERE RESPONSIBLE FOR PREMATURELY DISCONNECTING ELECTRICAL PWR BECAUSE OF MY PAST EXPERIENCE DEALING WITH OUR RAMP WORKERS AS WELL AS NUMEROUS OTHER DOCUMENTED INCIDENTS; WELL KNOWN; AND OFTEN DISCUSSED (AMONG PLTS) ERRORS AND OMISSIONS OF THIS WORK GROUP. IN THIS CASE THE RAMP HAD NOT DONE ANYTHING WRONG. I; NOT THEM; WAS MAKING MISTAKES BY NOT REALIZING MY ELECTRICAL SYS WAS NOT PERFORMING AS DESIGNED. OUR PUSHBACK AND ENG START WAS NORMAL. DURING THE AFTER START CHKLIST WE DISCOVERED THE YAW DAMPER HAD DISENGAGED. I FAULTED MYSELF FOR NOT HAVING SEEN THAT OVERHEAD AMBER LIGHT IMMEDIATELY AFTER STARTING THE ENGS AND ATTRIBUTED IT TO THE ELECTRICAL INTERRUPTION WE HAD EXPERIENCED EARLIER TURNING OFF THAT SYS. I TURNED THE YAW DAMPER SWITCH BACK ON AND WE COMPLETED THE AFTER START CHKLIST. DURING PUSHBACK AND ENG START THE APU WAS CARRYING THE ENTIRE ELECTRICAL LOAD OF THE AIRPLANE AND FUNCTIONED NORMALLY. DURING THE EXECUTION OF THE TAXI CHKLIST I MADE MY SECOND (AT LEAST) ERROR. THE CHKLIST DIRECTS THE CREW TO TURN OFF THE APU (MOST CASES). I NOTICED THAT THE BLUE LIGHT (INDICATING THE APU BEING POWERED AND READY TO ASSUME THE ELECTRICAL LOAD BUT NOT BEING ON LINE); THAT IS USUALLY ON AT THIS POINT IN THE CHKLIST; WAS OFF. HOWEVER I DID NOT CORRECTLY ASSESS THE RAMIFICATIONS OF THAT LIGHT BEING EXTINGUISHED. OFF WHEN IT SHOULD BE ON; BUT THEN OFF WHEN THE APU IS OFF; WHICH IS WHERE THE APU SHOULD BE AT THIS POINT. SO I HAVE THE LIGHT OFF WHEN IT SHOULD BE OFF BUT IS THAT A PROB? (ARE YOU CONFUSED YET?) ADDITIONALLY I WASN'T 100% SURE THAT I HADN'T ALREADY TURNED OFF THE APU; IN COMPLIANCE WITH THE CHKLIST; PRIOR TO LOOKING AT THE BLUE LIGHT. (THIS WAS MY 4TH; OF 5 SCHEDULED; EARLY MORNING DEPS NECESSITATING GETTING UP BTWN XA30 AND XB00 AM. 3 DAY TRIP ENDED YESTERDAY AFTERNOON; THIS 2 DAY TRIP JUST STARTING.) HAD I DONE THAT THEN THE LACK OF A BLUE LIGHT WOULD BE A NORMAL INDICATION. THE APU SWITCH WAS OFF AND THE BLUE LIGHT WAS EXTINGUISHED; NORMAL INDICATION; AND THE FO AND I HAD A DISCUSSION ABOUT WHETHER OR NOT THE BLUE LIGHT WAS ON PRIOR TO ME TURNING THE APU OFF. HE MENTIONED THAT HE THOUGHT THE APU MIGHT HAVE AUTO-SHUTDOWN. WHAT HAD STARTED OUT AS A GOOD MORNING WAS SLOWLY DETERIORATING IN MY ESTIMATION. THE FO AND I HAD A BRIEF DISCUSSION REGARDING THE STATUS OF THE APU'S ASSOCIATED BLUE LIGHT. AT THE END OF THE TAXI CHKLIST THE APU WAS OFF AND ALL ASSOCIATED LIGHTS WERE DISPLAYING CORRECTLY. WE HAD NO ABNORMAL WARNINGS IN THE FLT DECK AND I SPECIFICALLY CHKED THE ELECTRICAL LOADS AND VOLTAGE ON THE AIRPLANE WITH THE ENGS RUNNING AND THE APU OFF. ALL INDICATIONS WERE NORMAL. I DON'T REMEMBER ASKING THE FO IF HE HAD ANY RESERVATIONS ABOUT WHAT I WAS DOING OR MY DECISION TO DEPART. IF HE HAD ANY HE DID NOT BRING THEM TO MY ATTN IN A WAY THAT IUNDERSTOOD. WE CONTACTED THE TWR AND WERE GIVEN A TKOF CLRNC. OUR DEP AND CLBOUT TO 10000 FT WAS NORMAL. ABOVE 10000 FT WE BEGAN DISCUSSING WHAT HAD HAPPENED AND IT WAS AT THIS POINT THAT I FIRST UNDERSTOOD THAT THE FO HAD DETECTED A PROB ESTABLISHING ELECTRICAL PWR ON THE APU BACK AT THE GATE. PRIOR TO THIS REVELATION I STILL THOUGHT THE RAMP PERSONNEL HAD PREMATURELY DISCONNECTED PWR. REEVALUATING WHAT HAD HAPPENED IMMEDIATELY BROUGHT TO MIND THE APU BLUE LIGHT SITUATION AND IT WAS OBVIOUS TO ME THE APU ELECTRICAL SYS WAS VERY LIKELY EXPERIENCING DIFFICULTY FUNCTIONING AS DESIGNED. WE WERE SOMEWHERE AROUND 14000 FT AND CLBING WHEN I ASKED THE FO TO START THE APU. MY LOGIC WAS TO TRY AND DETERMINE WHETHER OR NOT THE APU'S ELECTRICAL SYS WAS FUNCTIONING NORMALLY. IF IT WAS THE START SHOULD BE NORMAL AND ALL THE NORMAL DISPLAY OF LIGHTS WOULD BE PRESENT. THE ANSWER WAS NOT LONG IN COMING. THE APU FAILED TO START AND A FAULT LIGHT ILLUMINATED ASSOCIATED WITH THE APU'S ATTEMPTED START. I NOTIFIED MAINT OVER THE RADIO. OTHER THAN A NOW NON FUNCTIONING APU AND THE ASSOCIATED FAULT LIGHT I HAD A COMPLETELY NORMAL AIRPLANE. OUR RTE AND DEST WERE EXPERIENCING EXCELLENT WX; IT WAS DAYLIGHT WITH NUMEROUS ARPTS ALONG THE RTE OF FLT. THE DECISION WAS MADE TO CONTINUE THE FLT. ENRTE I WROTE UP THE MECHANICAL DISCREPANCY ASSOCIATED WITH THE APU. THE PLAN WAS NOT TO START THE APU AFTER ARR AT OUR DEST AND HOOK UP TO EXTERNAL ELECTRICAL PWR. OUR RAMP FACILITIES AT DEST ARE CROWDED. AFTER I HAD MADE THE 90 DEG TURN INTO OUR PARKING SPOT I SHUT DOWN THE #2 ENG AND TAXIED THE REMAINING 75 FT PLUS OR MINUS ON 1 ENG. (IN HINDSIGHT ANOTHER MISTAKE AS I ONLY HAD 1 GENERATOR.) AFTER I BROUGHT THE AIRPLANE TO A STOP WE WERE UNABLE TO GET THE AIRPLANE TO ACCEPT THE EXTERNAL PWR BEING SUPPLIED BY THE RAMP. THE AIRSTAIRS BEGAN TO MOVE TOWARDS THE AIRPLANE. I WAS CONCERNED ABOUT PEOPLE APCHING THE L SIDE OF THE JET WITH AN ENG RUNNING. SO I SHUT IT DOWN AS WELL FULLY KNOWING THE AIRPLANE WOULD GO DARK SAVE FOR THE EMER LIGHTING SYS. AND THAT IS EXACTLY WHAT HAPPENED. I COULD NOT GET THE AIRPLANE TO ACCEPT EXTERNAL PWR; SO I ATTEMPTED TO START THE APU. THE APU FAILED TO START AND DISPLAYED THE SAME 'FAULT' LIGHT. OUR PAX DISEMBARKED INTO THE SUNSHINE OUTSIDE ASSISTED BY THE ILLUMINATION OF THE EMER LIGHTING SYS. WHEN THE LAST PAX WAS OFF WE COMPLETELY SHUT DOWN THE AIRPLANE. THERE WAS NO MAINT SUPPORT WAITING FOR US WHEN WE ARRIVED. THE ROOT OF THIS PROB WAS MY INABILITY TO CORRECTLY DETECT A FAILURE IN THE AIRPLANE'S ELECTRICAL SYS THOUGH MY FO HAD SUSPECTED DIFFICULTY. I TRY TO FOSTER AN OPEN AND COMMUNICATIVE ATMOSPHERE AMONG CREW MEMBERS WITH WHOM I FLY. I THOUGHT I UNDERSTOOD WHAT 'THAT'S NOT RIGHT' MEANT WHEN IN FACT I DID NOT. ALL THE CLUES WERE THERE THAT SHOULD HAVE ALERTED ME TO A PROB LONG BEFORE WE GOT IN THE AIR. I SHOULD HAVE PROJECTED INTO THE FUTURE MORE ACCURATELY TO INCLUDE THE SCENARIO OF US NOT BEING ABLE TO ESTABLISH GND PWR AT OUR DEST. THAT ONE CAUGHT ME BY SURPRISE AS I HAD INCORRECTLY ISOLATED THE APU AS BEING THE PROB. HAD I DONE THIS RIGHT WE WOULD HAVE NEVER GOTTEN OFF THE GATE. I WOULD'VE DETECTED THE PROB AND WRITTEN IT UP THERE AND THEN. ALTHOUGH I NEVER INTENDED TO GO FLYING WITH A DEFECTIVE SYS. THAT'S EXACTLY WHAT I ENDED UP DOING. AS IT WAS THIS FLT TERMINATED WITH A DARK AIRPLANE AT A DOWN-LINE STATION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: A LATER ATTEMPT TO ESTABLISH EXTERNAL POWER BY MAINTENANCE DURING TROUBLESHOOTING WAS SUCCESSFUL AND NO FAULTS COULD BE FOUND IN THE ELECTRICAL SYSTEM. IT IS NOT KNOWN WHETHER THE APU FAULT REAPPEARED DURING THIS TROUBLESHOOTING.