Let's look at that wiring diagram.
![1724633372543.png 1724633372543.png](https://www.pilotsofamerica.com/community/data/attachments/124/124995-ae5e021b4e61e82ff1a8bc0bf470b556.jpg)
#6, way over on the right, is the external power jack. The bottom pin is the small one, and it is positive. Power runs though the top diode at #10 to the coil on the external power contactor, #11, closing the contactor. Now the + pin on the jack, the middle one, is the pin that carries the external power to one of the contacts in the external power contactor, though the contactor and through wire D-PA13 to another contactor's pin and then through wire D-PA11 to D-PC6 that takes it through diode #13, resistor #15 and fuse #14 to the "Bat" terminal on the master contactor. This allows the master to have the power it needs through its coil to turn the master on IF the master switch is turned on. The master contactor closes, allowing the big current to run through wire D-PA11 through the master contactor to the battery, charging it and making starting possible.
Now. The diodes at #13 and #10, resistor #15 and fuse #14 are all suspicious characters here. Any one of them will disable the external power if they're defective. They are found on small circuit boards. The first one here has fuses #14 and the clock fuse, and diode #13 and resistor #15:
And diodes at #10 will be on one of these two:
An open diode #10 (top) or #13, resistor #15 or fuse #14 will prevent external power operation. A
shorted diode #13 will drain the battery overnight by keeping the instrument cluster energized. I have seen it only once. I have seen burned-out resistors and fuses. The fuses are also subject to contact corrosion since they're close to the battery and its acidic fumes. That battery box is not airtight.