What was found
UPON STARTING APU NORMALLY FOUND APU WOULD NOT ENGAGE TO ELECTRICAL BUSES OVERHEAD SELECTABLE DISPLAY SHOWED NO FREQS, VOLTS, AC AMPS. GROUNDED - YES. PERFORMED APU BITE TEST AND FOUND START CONVERTER UNIT SHOW GEN UNDERVOLTAGE. PARTIALLY RUN B737-800 FIM 49-40 TASK 807 E.(1) H. PER SEAMC SWAPPED GPCU. NO CHANGE. INSPECTED CONNECTOR AND HANDLE OF REMOTE APU PANEL IN THE WHEEL WELL. NO DAMAGE OR PROBLEM NOTED. REMOVED AND REPLACED APU STARTER GENERATOR PER B737-800 AMM 49-41-21. OPS CHECK GOOD.