What was found
ENGINE CONTROL LIGHT ON #1 ENGINE AFTER START. EXTERNAL POWER FAILED ON PUSH. RELIED ON BATTERY POWER FOR APPROX. 15 MINUTES PRIOR TO STARTING ENGINE #1 AND ESTABLISHING AC ELECTRICAL POWER. GROUNDED - YES. THE AIRPLANE VOLTAGE INPUT TO EEC IS OUT OF RANGE. ACCOMPLISHED #1 ENG EEC TEST, NO EEC FAULTS, REF B737-800 AMM TASK 73-21-00-700-804-F00. ENG RUN AT GATE, NO FAULTS. GROUND VERIFICATION GOOD. #1 ENG. EEC BITE, NO FAULTS, REF AMM TASK 73-00-00-810-801-F00.